fresh air intake into cold air return
Greetings everyone:
I am looking at new 80% furnaces. I heard about new air quality recommendations towards increasing fresh air intake into HVAC systems. I have looked into ERV and HRV but I was discouraged from tying them into the existing HVAC system and that it would be better if they had their own dedicated duct work. Therefore, I found another potential alternative I want to run by the group. It is a motorized, dampered fresh air intake made by Honeywell. A penetration to an outside wall is obviously needed and when there is a call for heat or cold, the damper will open and fresh air will correspondingly be drawn into the cold air return of the furnace. I think it can also be programmed to open as specified intervals (when one is using a furnace with variable speed motor and the fan is recirculating the air constantly). I realize that this puts a slight load on the furnace or AC since in winter one is dumping cold air in the air return return and during summer one is dumping hot air into the cold air return. However, the device is much cheaper than an ERV or HRV.
Does anyone have input on using a device like this? I would have the furnace company install it as part of the job.
Here is a description of the unit:
http://www.zonecontrolblog.com/the_zone_control_blog/2008/10/honeywell-8150.html
Thank you.
Edited 11/6/2008 11:40 pm ET by Will92
Edited 11/6/2008 11:42 pm ET by Will92
Replies
I can remember doing this 25 or so years ago w/o the damper. The purpose was to prevent backdrafting of gravity vent furnaces. It was also done to prevent the structure from going "negative" and was often part of a radon mitigation process.
I don't know your location but the real benefit of the damper is probably in the summer if you have humidity issues at that time with airconditioning. Whenever a gravity vent burner is firing, there must be a source of air to replace the air going up the chimney. You would be providing a source from the whole house instead of the furnace room where the burner is located.
In years past, before buildings became sealed and insulated, this air came from around the doors, windows, lights, outlets and other openings.
Sending the air into the RA of the furnace allows the air to also be used to replace exhaust air such as bath fans, range hoods, fireplaces, etc. provided that the damper is open. The thing to remember, is that any air exhausted or vented, from a building will be replaced. In the intrest of energy conservation, it is best to control this in some way. They tell the siding guys to think like water, you need to think like air.
Chris
The purpose was to prevent backdrafting of gravity vent furnaces. It was also done to prevent the structure from going "negative" and was often part of a radon mitigation process.
I disagree. When you put ventilation air into the return air, it is not for combustion air ... that should be a separate inlet ... and is actually required in certain situations. The fresh air intake being referred to will not prevent backdrafting if the furnace is in a tight room ... this assumes of course that the OP meant to connect the vent duct/damper to the return air of the furnace.
Never heard of this strategy for radon. Radon mitigation normally occurs sub slab w/ negative pressure system, not with ventilation air. Certainly ventilation will lower radon levels, but that is not the way to mitigate radon.
The basic idea is to provide positive pressure in the house. This prevents back-drafting on any open combustion appliances (or from leaks in closed combustion appliances), helps with radon mitigation, and eliminates drafts from minor air leaks through the building envelope. In addition, of course, it provides a positive way to achieve air changes in the structure.
Conscience is the still, small voice which tells a candidate that what he is doing is likely to lose him votes. --Anonymous
It will only provide backdraft prevention if it is actually on when the appliance fires up. So if it is not interconnected, you can't rely on it to provide that function and certainly not for the furnace it is connected to, as many are in a "confined space" and would require separate combustion air anyway.
As I indicated in another posting ... ventilation is no way to mitigate radon. You'd have to have it on 24/7.
The only purpose that it has ... is your last reason: ventilation air, air changes, fresh air to replace stale, overly moist, odors, or otherwise polluted air that is exhausted or otherwise unwanted.
>>ventilation is no way to mitigate radon.Mitigating radon isn't what we're discussing - we're talking about "mitigating" to a degree conditions which contribute to higher radon levels in the home.
"Ask not what the world needs. Ask what makes you come alive... then go do it. Because what the world needs is people who have come alive."
Howard Thurman
>>[A return air supply] It will only provide backdraft prevention if it is actually on when the appliance fires up.Why do you think that?Are you thinking that the flow or air in a proporly drafting gas appliance can't be reversed?Stick a draft gauge in a flue -fire the unit up and then traverse the house turning on ventilation fans -- it doesn't happen mcuh, but I've done plenty of testing where a properly drafting appliance went negative in that sort of testing.
"Ask not what the world needs. Ask what makes you come alive... then go do it. Because what the world needs is people who have come alive."
Howard Thurman
The set up mentioned would not put a positive pressure on the house. A fan would have to be added inline to accomplish that. Since he is only using the normal blower in the furnace, the system would only intake fresh air as a result. The guys question seems to have been forgotten though. "Will this replace an ERV or HRV?". The answer to that question is simply no. The HRV can pre-heat the fresh air being dumped into the return plenum of the furnace with heat energy pulled out of the houses stale/humid exhaust air.
It will too create positive pressure. Think about it.
The only function of economic forecasting is to make astrology look respectable. --John Kenneth Galbraith
No it will not. The furnace has return air ducts throughout the house. These return air ducts are the primary source of air for the blower in the furnace. (Picture a circle here) The only time you would conceivably pressurize the house by adding a 3" fresh air intake is if the return air system is not balanced with the supply side, the furnace would then make up the air from outside. Now I will give up the fact that a lot of forced hot air systems are in fact not balanced properly and therefore could in fact lead to the condition, but the system in question can not be marketed or used in that fashion. Side note, the draft on most fuel burning appliance is designed to go up the chimney at or near atmospheric pressure, pressurizing a house with a natural draft appliance such as a conventional gas furnace, could in fact cause the appliance to burn or draft improperly.
Edited 11/11/2008 8:18 pm ET by losh
Will too!
The only function of economic forecasting is to make astrology look respectable. --John Kenneth Galbraith
Dan, The only time this could happen is from the venturi effect of air passing by the fresh air inlet. Is this what you are referring to??
I haven't installed a scorched air furnace in a home in 25 years. In homes without a ducting system, these systems do nothing but prevent a negative pressure situation in the house.
The only time air ever passes through them is when it actually is needed as makeup air and that will never cause positive pressure in a home.
No.And I'm referring to a standard forced air system with full supply and return duct systems, the more balanced the better.A duct leading from outside to the return air plenum will produce a slight positive pressure in the house.
The only function of economic forecasting is to make astrology look respectable. --John Kenneth Galbraith
Yes, that's what meant. Only when the fan is running. The problem is that there are so many variables in controlling how much air actually enters the house...and that's where the problems start.
There may be problems, but a positive pressure is produced.
The only function of economic forecasting is to make astrology look respectable. --John Kenneth Galbraith
>>Never heard of this strategy for radon. Radon mitigation normally occurs sub slab w/ negative pressure system, not with ventilation air. Certainly ventilation will lower radon levels, but that is not the way to mitigate radon.Radon levels can be increased by the negative pressure zone that naturally occurs in a basement, and, given the lack of good return sealing adding such a supply can help reduce the negative pressure in the basement.
"Ask not what the world needs. Ask what makes you come alive... then go do it. Because what the world needs is people who have come alive."
Howard Thurman
You are right ... negative pressure can have a tendency to increase radon levels ... but I've never heard of mitigating it with ventilation air. I've had a lot of training in radon and worked with a number of mitigation systems and NEVER heard of anyone ever suggesting mitigation through ventilation. While ventilating may mitigate, that means you would have to overventilate ... as you would have to do it 24/7.
I believe the state of Ohio includes this as one mitigation technique.And don't forget, the low pressure is caused, in part, by the gas appliances in that zone, and, I believe, is seasonally on the same cycle as when we use our gas appliances most
"Ask not what the world needs. Ask what makes you come alive... then go do it. Because what the world needs is people who have come alive."
Howard Thurman
You're kidding, right. I'm guessing the state of Ohio knows zip about radon mitigation if that is the case. Maybe this would be true if the system guaranteed 24/7 ventilation ... which would be taking this discussion out of context. Even so, that is likely a poor strategy to use as it would be energy intensive (ventilating and using the larger furnace fan to do it).
Your second statement doesn't make any sense to me (I'm say I have no idea what you are trying to say).
Edited 11/8/2008 10:05 am ET by Clewless1
Edited 11/8/2008 10:06 am ET by Clewless1
Seems it would be easier/better to seal your return, then. Ventilation is not mitigation. I think any radon expert would laugh at using ventilation for mitigation. I'd hate to see the OP walk away w/ the notion that it might improve his radon outlook ... or any other reader getting some ideas for himself. While your notion has theoretical merit ... the practicality of it is just not there. Sorry to oppose this, but anyone planning to mitigate radon this way would be making a mistake, I think. I'd rather take up smoking and take my chances than rely on occasional/intermitent ventilation to mitigate a radon problem.
>>Seems it would be easier/better to seal your return, then. Ever try sealing a panned joist bay return?>>Ventilation is not mitigation. You're right, but as noted, that's not what we're discussing.
"Ask not what the world needs. Ask what makes you come alive... then go do it. Because what the world needs is people who have come alive."
Howard Thurman
If the panned bay is accessible ... should be relatively easy to seal. Pull the pan off and have at it. Then seal the pan when reinstalling.
I thought there was clear implication that your statement was one of mitigating radon. You imply by supplying ventilation that the condition for higher levels of radon are minimized ... sounds like intended mitigation strategy to me. I understand what you are saying, but you can't eliminate the condition a small percentage of the time and then call it good (i.e. the condition for higher radon levels is now gone 'all the time'). It's like ventilating a smoke filled room for short periods and declaring it good to occupy all the time. While I or you may CHOOSE to expose ourselves to negative conditions for whatever periods of time, you really can't imply that you can rely on this approach to control the condition.
>>If the panned bay is accessible ... should be relatively easy to seal. Pull the pan off and have at it. Then seal the pan when reinstalling.Easy in theory, a bit tougher in practice, in my experience.Sorry if I misspoke on the radon side of things - minor "mitigation" of the radon effects of negative pressure in the lowest level of a house is merely one minor part of the effects of such a systems- it is no way intended to be a significant system or approach to deal with radon issues.Sorry I wasn't clear on that.
"Ask not what the world needs. Ask what makes you come alive... then go do it. Because what the world needs is people who have come alive."
Howard Thurman
Yeah ... its practical applications that can make or break a 'simple concept' (referring to the pan joist duct return).
No prob ... just trying to round out the discussion since the radon thing was brought up ... didn't want widespread confusion to result in widespread rumors of ways to mitigate radon as opposed to simply affecting it to some degree. Good discussions.
Clew,
You are correct in that it is not the proper way to provide combustion air to a furnace. If the 80% furnace is in a tight room it won't meet code. However, it was often done for combustion air and could still be done for other appliances, think large gas stoves, remote waterheaters, fireplaces, gas logs, fart fans... There are, however, much better ways of getting to the goal with current technology, the OP wanted to know more about it.
Some houses are negative for a variety of reasons. It keeps the structure from behaving like a giant vacuum cleaner for the radon gas located in the dirt under/around the foundation. Like I said, part of the process, especially with dirt floor basements, old open chimneys, unsealed crawl spaces, etc.
I agree with many of the other posters that sealed combustion and an HRV is probably the best long-term answer with the current energy forecast. Then the home can be properly sealed and humidity, fresh air etc. can be properly controlled and energy use minimized.
Chris
This setup is common in this part of the country, and often is done without a damper -- just a relatively small port with 2-3 inch duct.
Note that the duct should be insulated to prevent sweating in cold weather.
If you are replacing your furnace, why not replace it with a 90%+ efficiency model that draws its combustion air from outside? I can't imagine why anyone would replace a furnace with anything else in this day and age.
I don't know where you are located, but sucking freezing cold air into your house ducting system is a very bad idea. If you need fresh air in the house, you should go with a heat exchange system.
If you just want combustion air, then go with a new furnace with a sealed combustion chamber using outside air.
"If you are replacing your furnace, why not replace it with a 90%+ efficiency model that draws its combustion air from outside?"I agree that the sealed combustion units are by far the best, but combustion air is not the problem the OP is trying to solve."I don't know where you are located, but sucking freezing cold air into your house ducting system is a very bad idea. If you need fresh air in the house, you should go with a heat exchange system."That all depends on your climate. What's right for Tulsa may not be right for Duluth.
Jon Blakemore RappahannockINC.com Fredericksburg, VA
The only logical use for a system like he's talking about is to supply makeup air for a furnace that uses air from inside the house for combustion. Many localities require these to be installed. That doesn't make it a good idea!
In his case, he would be far better off installing a 90+ furnace that uses outside air for combustion.
If he wants fresh air, he can open the window or use an air exchanger if that suits the purpose better.
If you have ever seen the effect of sucking below zero air into your air ducts in the winter, you would not be a fan of such a system.
It makes no sense in the summer or the winter unless you need makeup air for the furnace and in that case you should provide it to the combustion intake area, not the ductwork.
If it's such a bad idea, why do I see the technique used in Minnesota?
Conscience is the still, small voice which tells a candidate that what he is doing is likely to lose him votes. --Anonymous
They use it in many states . The sole purpose is to prevent negative pressure in houses that have been sealed up better than they used to be. This was a shotgun approach at best. They cause problems when it's cold in the winter or hot in the summer.
The primary culprit in creating negative pressure was the furnace or boiler sucking air out of the house. With the new appliances, most of the cities we work in have dropped the requirement for these systems because they were so inefficient. We have to install air exchangers in every new home we build now. More costly, but certainly a better way to handle the problem.
In the OP's case, he would probably eliminate the need for such a crude system simply by installing a more energy efficient furnace instead of the one he is planning to install.
Most people that were required to install these systems have long since plugged them up so they don't work anymore because they caused them so much trouble when it got really cold outside.
They did work, but certainly cost a lot of heat $$ to the homeowner.
The system is designed to work with high-efficiency furnace systems. A standard open combustion system needs far more makeup air that the system would supply, with its typical 2-3" duct.
Conscience is the still, small voice which tells a candidate that what he is doing is likely to lose him votes. --Anonymous
What kinds of "air exchangers" are you installing and what has been the general experience with them? Could you give me a general price guesstimate for just the unit without duct work or installation?
There are dozens of companies that make these units. They are commonly called HRV's or Heat Recovery Ventilators.
It is best to install their ducting system separately from the ducts in the HVAC system if possible. In new homes, we never use the same ducting system.
You can get as elaborate as you want with the ducting, but it can also be very simple. It's really not that expensive. In a new home, it costs us about $1500 on average to install these.
I was not a fan of these units when they were first required, but I have become convinced that they are worth the investment.
"The only logical use for a system like he's talking about is to supply makeup air for a furnace that uses air from inside the house for combustion. Many localities require these to be installed. That doesn't make it a good idea!:
No, that is not true. Indoor air quality is far bigger than just preventing backdrafting and the potential for CO poisioning. The system that the OP is referring to can be and is used on systems that are 100% electric, with no combustion appliances. It is to introduce fresh air into a house that is too tight to allow for natural air exchanges.
Make up air for combustion appliances is entirely different.
"In his case, he would be far better off installing a 90+ furnace that uses outside air for combustion."
I agree that a sealed combustion unit is far superior to atmospheric venting.
"If he wants fresh air, he can open the window or use an air exchanger if that suits the purpose better."
It is an air exchanger, just not an HRV or ERV.
"If you have ever seen the effect of sucking below zero air into your air ducts in the winter, you would not be a fan of such a system."
What if you don't live in an area where the temperature dips below zero? Building science and HVAC design cannot (CANNOT) be effective if the climate of the building site is not considered.
"It makes no sense in the summer or the winter unless you need makeup air for the furnace and in that case you should provide it to the combustion intake area, not the ductwork."
Unless you're concerned about indoor air quality and don't want to open a window.
Jon Blakemore RappahannockINC.com Fredericksburg, VA
Hi everyone and thanks for the input.
I want to clarify that I'm not looking to maintain or improve combusion air make up with a device such as the Honeywell 8150. I already have a lower combustion air makeup inlet (into the basement) for my existing (circa 1990) furnace and understand that a new 80% furnace might require an additional "high" combustion air makeup inlet (again into the basement) at best. My intent with a device like the Honeywell was to simply draw in some fresh air once in a while to be inserted directly into the cold air return of the furnace (before it hits the filter and is conditioned).
I live in Denver so can appreciate the concern that dumping outside, very cold air in the winter is not necessarily energy efficient, but as pointed out, the cost of an ERV is not cheap either with a potentially long pay-off period. In fact, this is also the reason I am not leaning towards a 90%+ efficient furnace. I am pretty frugal with heat in the winter and my calculations showed at least a 12 year payoff for the additional expense of a high eff. furnace. This is, of course, predicated on natural gas prices remaining stable. I checked some DOE projections which predict flat pricing over the next decade. I'm not sure if I believe that, but that is what their report said.
One manufacturer of ERV's and HRV's told me it was not ideal to tie in such a system with the HVAC ducting in a typical home and that it is better to have ductwork dedicated to the ERV and HRV. It was based on this advice I began considering devices such as the Honeywell 8150. Do folks here find it acceptable to use an ERV or HRV with existing HVAC ductwork? If so, perhaps I should reconsider this route.
Regarding radon, my level is just over the action limit (4.8 in non-finished, unoccupied basment and below limit on first and second floors - 2.6) and I had a sub slab system installed (and have yet to test the new level). I must confess that part of my interest in fresh air intake would be to further lower radon by dilution. Although below action limit in the living quarters, I believe it is prudent to lower it as much as possible. Again, this would depend on findings of the post mitigation test. And, although not a primary radon reduction technique, the EPA does recognize ERV's and HRV's as secondary means to reduce radon.
Will92,
Here is a quick experiment to try, It will tell you if there would even be any air coming in with the duct you are talking about.
Close the door to the basement.
Close all other doors and windows in thew house.
Make sure all bath fans, furnace, range hood etc. are off.
Take a tissue, hold it by a corner in front of the edge of an outside door by the lockset. Now SLOWLY open the door. Keep the tissue about 2 inches away from the crack and see which way it waves.
This simple test will show if the house is positive or negative to the outdoors. Just because the duct is installed, doesn't automatically mean that air will be traveling down it when you want it to. Fans can be thought of as pumps for air, air exhausted out means air comes in from somewhere.
Chris
>>This simple test will show if the house is positive or negative to the outdoors. In those particular weather conditions at a that particular pane in the house.Houses with basements typically have neg pressure in the basement and pos in the attic.And factors such as direction of wind can affect the pressure zones and levels in the house.
"Ask not what the world needs. Ask what makes you come alive... then go do it. Because what the world needs is people who have come alive."
Howard Thurman
Not to beat a dead horse, because I think I made my point ( at least for your question) but think of the mechanics of how these systems work.
In the winter when your (not very efficient) furnace is running, you will have this cold outside air ducted into your cold air return. This will indirectly provide combustion air for your furnace (which will be sucking combustion air from inside your house) This in itself can create all kinds of uncomfortable drafts in the home. You should not be installing a furnace that uses combustion air from the house or if you do, it should be in a room with its own separate combustion air supply.
When the furnace starts to circulate air through the ducting system, cold air is drawn through the outside vents and dumped into the furnace, not because of any particular need for makeup air, but simply because it is being sucked into the house by the air circulation in the ductwork. I have seen furnaces unable to heat air hot enough to heat the house because of the influx of frigid air from outside. Most of these systems have some sort of damper, but they usually leak like crazy and are very ineffective in actually controlling how much air gets in the house. I am not familiar with your Honeywell unit, maybe it could control this better than most.
When the furnace is not circulating air, any disturbance in the rest of the house....doors opening, bath fans, range vents etc. tends to suck this outside air directly into the room through the cold air returns. This also can make for uncomfortable conditions when its cold outside.
Around here the difference in cost between installing a 90+ furnace with an air exchanger compared to what you are considering, is about $3000. Money well spent in my opinion! You are still thinking back to the 80s.
BTW, if you think you have a radon problem, you should have a dedicated venting system, not rely on something like you're considering to mitigate the problem.
Remember...you basically have 3 choices for letting fresh air into your house...open a window, use a system like you are considering ( you have to heat up every cubic foot of intake air to room temperature ) or use a heat exchanger ( which recycles 80% or so of the heat ) or I suppose you could rely on the leaks the original builder installed for you.
Edited 11/8/2008 9:45 am ET by BoJangles
I want to clarify that I'm not looking to maintain or improve combusion air make up with a device such as the Honeywell 8150. I already have a lower combustion air makeup inlet (into the basement) for my existing (circa 1990) furnace and understand that a new 80% furnace might require an additional "high" combustion air makeup inlet (again into the basement) at best. My intent with a device like the Honeywell was to simply draw in some fresh air once in a while to be inserted directly into the cold air return of the furnace (before it hits the filter and is conditioned).
I live in Denver so can appreciate the concern that dumping outside, very cold air in the winter is not necessarily energy efficient, but as pointed out, the cost of an ERV is not cheap either with a potentially long pay-off period. In fact, this is also the reason I am not leaning towards a 90%+ efficient furnace. I am pretty frugal with heat in the winter and my calculations showed at least a 12 year payoff for the additional expense of a high eff. furnace. This is, of course, predicated on natural gas prices remaining stable. I checked some DOE projections which predict flat pricing over the next decade. I'm not sure if I believe that, but that is what their report said.
The 2003 IRC for single family residential dwellings calls for a high and low combustion air intake. The furnace vintage has nothing to do with it.
Reading here: http://forums.taunton.com/tp-breaktime/messages?msg=109397.4
Your calculations do not take into account the added infiltration air when using outdoor air for combustion.
Yes, you have combustion air intake(s), but these do not work as well as a dedicated combustion air intake pipe and a sealed burner assembly.
Yes, the pay-off is longer, but if you believe that energy prices will stay "stable" over the next decade, I have a bridge to sell. It will not be less expensive in the future to upgrade.
It's no problem dumping the fresh air from an HRV into the return plenum of your furnace. The pre-heated air (recovered from the exhausted air) flows over the heat exchanger causing your furnace to work less. This is the heat recovery portion working for you as this newly conditioned air is fed into your living spaces via existing ductwork. Why would you want to install separate duct work for this. You do need new ductwork from your wet spaces (Basement, Baths, and kitchens) in order to suck the old stale air out of the house. Hope this helps
>>If you have ever seen the effect of sucking below zero air into your air ducts in the winter, you would not be a fan of such a system.We're sucking below zero ir into our home whether we like it or not.I'd suggest it makes some sense to do so in a controlled way where that air is being immediately diluted by the warm air in the return.
"Ask not what the world needs. Ask what makes you come alive... then go do it. Because what the world needs is people who have come alive."
Howard Thurman
You are under the misconception that the only time these operate is when the furnace is circulating air. That is not so. When you have them connected to your cold air returns it is just like running the duct to any room in the house. The cold air will come into the rooms through the returns when the furnace is not running.
They are notorious leakers and you will have an almost constant supply being drawn into the ducts. The problem is compounded if you have an old house with a lot of penetrations into the attic.
That's why I like heat exchangers. These other systems function, but certainly not efficiently. With the heat exchangers, you can control when you want them to run.
>>You are under the misconception that the only time these operate is when the furnace is circulating air. That is not so. When you have them connected to your cold air returns it is just like running the duct to any room in the house. The cold air will come into the rooms through the returns when the furnace is not running.Depends upon th installation technique - and if the eternal cold air ris in the basement (most common in my area) - well, cold air doesn't rise - ans a cold ar "trap" cn be use to prevent infiltration when the lower isn't running.>>They are notorious leakers Of corse - that's what they are there for - what's better, the leaks you know and an control or the leaks you don't know?>>and you will have an almost constant supply being drawn into the ducts. The problem is compounded if you have an old house with a lot of penetrations into the attic.The whole house and all of the conditions have to be considered - there are no magic bullets - and if the installer doesn't understand the dynamics of houses -especially the varying pressure planes and how they are cased will end up with a poor "solution.">>That's why I like heat exchangers. These other systems function, but certainly not efficiently. With the heat exchangers, you can control when you want them to run.There are arguments both for and against all approaches - e.g., active solar v passive solar.Economic analyses generally favor passive approaches, I believe.Technological/active systems are most favorably viewed without suc economic considerations.As 'they' say: "your mileage may vary"
"Ask not what the world needs. Ask what makes you come alive... then go do it. Because what the world needs is people who have come alive."
Howard Thurman
I have one installed in my house. I initially thought that I was going to have to install an HRV (our natural air leakage rate is very low, too low for appropriate indoor air quality) but the cost is pretty high.
My HERS rater (Home Energy Rating System) was able to plug some figures in for me that conclusively (as far as I'm concerned) supported my conclusions.
In my climate, the payback period on an HRV (even a cheap one) would be over 10 years. You have the significant cost of the unit plus the additional KWH needed to run the unit. One should also keep in mind that the best (Honeywell make a very nice HRV with a nice price tag to boot) system is only ~80% efficient. So you're still losing some energy to the unconditioned air that you're bringing and the conditioned air that you're dumping outside.
Plus, if you've invested some big bucks in an efficient HVAC system, the penalty of the unconditioned air will not be as severe as if you had an old SEER 8 AC unit.
I just went with the Honeywell Y8150A1009 and called it a day.
Jon Blakemore
RappahannockINC.com Fredericksburg, VA
There is absolutely and never will be a payback on an HRV, the principle is to recover part of your energy while ventilating your house (A good thing). It can never pay itself back except by saving your insulation, drywall, framing, windows etc.
"There is absolutely and never will be a payback on an HRV, the principle is to recover part of your energy while ventilating your house (A good thing)."I don't understand what you mean. To me, energy means, among other things, cost. If I have to exhaust conditioned air AND pull in unconditioned air to replace it, there will be a cost associated with that extra work.So, if it's not to save some money, what is the benefit of saving "energy"?
Jon Blakemore RappahannockINC.com Fredericksburg, VA
Jon, What he means is exactly what you said. The purpose of this unit is to draw fresh air into your home and exhaust stale air.
There is a cost to this....that's why we disagree with Tim about this.
Edited 11/12/2008 8:31 am ET by BoJangles
If you need to ventilate your home due to tight construction (Most older homes have plenty of cracks), and it may be code like it is in Nova Scotia, rather then just exchange the air itself with a simple air exchanger, you can mitigate the energy loss by using a heat recovery ventilator instead. This allows you to recover some of the heat you have already paid for before it is hurled outside the house. Win Win. Of course it is costing you money to exhaust conditioned air outside but the less the better. Remember if you do not need to exchange the air in your house, do not buy any type of ventilator. (Of course I strongly recommend all houses have them).
I see. I thought you were comparing the costs of an HRV to a simple fresh air intake, not the costs of ventilating vs. the cost not ventilating.
Jon Blakemore RappahannockINC.com Fredericksburg, VA
In michigan, several years ago they called for a barometric damper on such an air inlet - then they went to just a straight opening - worked fine.
Such fresh air supplies can help prevent the basement from going negative pressure and help prevent back drafting - in theory, of course, not necessary but the reality is that returns are rarely sealed well, and the fresh air supplies also help counteract the general chimney effect that naturally occurs in house and the negative basement pressure zone.
It doesn't take much to cause problems with the draft on a gas appliance - the 'force' of acceptable draft is about the same as if you were gently blowing out a match from a few inches.
"Ask not what the world needs. Ask what makes you come alive... then go do it. Because what the world needs is people who have come alive."
Howard Thurman
That outside air duct tied into the return ducting will put more than a "slight" load on your residential HVAC system unless you are located in a mild climate area.
Even 4" round undampered duct tied into a return system in the midwest can cause problems with the AC in the summer. If the furnace is sized tight, you will have problems on design days.
If you do a load calc and add say 100 cfm of forced ventilation, the numbers change quite a bit... 100 cfm through a 4" round tied to a retun duct is not impossible... that 100 cfm of forced ventilation can easily turn into 4000 btu of cooling.
The closer the OA intake is to the furnace, the more air it can draw. If it is further away, air may not be pulled from the next return in the trunkline.
You could install a single new opening in a wall or floor for your ERV to remove indoor air to be exhausted and tie the tempered outdoor air to the return ducting. There are controls available that sense CO2 and activate the unit for ventilation purposes.
Conditioning outside air requirements in commercial applications is a big expense, which is why CO2 sensors are more common- leave the OA dampers at minimum until needed.
If you go with the Honeywell OA damper, you must ensure that your HVAC system is sized properly to take on the additional load. When you factor those additional costs for upsized equipment (initial purchase and operational costs), the ERV may actually be cheaper or a wash in the long run.
"100 cfm through a 4" round tied to a retun duct is not impossible"
Not impossible, true, but very unlikely. You should know better. 50 cfm being pulled through a 4" duct (with a wall jack, a backdraft damper, a volume damper and xx ft of pipe) is a stretch but more likely.
The truth is, for Code ventilation in a residence (0.35 ACH or 15 cfm/pers), say for 4 people in 2500 sf house (avg 8.5 ft room height). 0.35 ACH = 124 cfm. In the winter at 75 deg temperature difference (say -10 to 65) the "cost" is 10,000 (less than 13% of the average furnace) btuh at the worst case design conditions, which occur 0.4% of the time. For cooling in a warm, humid environment, 124 cfm could potentially cost 6200 btuh, but again, only on the worst day of the year, and even at that, these are slightly exagerated numbers. Cost is minimal
Several points here. 1) seldom do heat/energy recovery ventilators pay for themselves in energy savings alone for comfort HVAC applications, especially in residential applications, in less than 20 years. 2)A very simple, flexible and cost effective way to provide fresh air (i.e. ventilation) and slight positive pressurization is a gravity damper (with lockable volume damper) ducted to the return side of the FA system is a very inexpensive and wise investment.
Fresh air and positive pressurization are two very highly recommended HVAC "improvements" for all.
Several points here. 1) seldom do heat/energy recovery ventilators pay for themselves in energy savings alone for comfort HVAC applications, especially in residential applications, in less than 20 years. 2)A very simple, flexible and cost effective way to provide fresh air (i.e. ventilation) and slight positive pressurization is a gravity damper (with lockable volume damper) ducted to the return side of the FA system is a very inexpensive and wise investment
What????? HRV or ERV systems are not supposed to pay for themselves. They are installed for the purpose of providing substantial exchange of air in a home for reasons of health and comfort. There is a cost involved like there is for any other appliance.
Your tube and damper system is mandated in some areas to prevent negative pressure situations in a tightly sealed home. There is no comparison between the two when it comes to providing fresh air to a home.
There is no way that you will provide positive pressure to a tightly sealed home with one of those systems unless you have a hurricane blowing up the ductwork.
Do not underestimate the effect of dumping frigid air into your heating system. No, it may not cause a big problem on a moderate day, but it sure can cause problems on a very cold day...just the time when you want your furnace cranking out maximum btu's
You do not understand how adding fresh air to the return side of a forced air system will increase the relative pressure in the space? Actually, if installed properly, there is no way that a barometric damper controlled ventilation system will NOT provide positive pressure, especially in a tightly sealed home/building. You would have to understand the relative pressure in a system and in a space. More air in than out = positve pressure.
1 in 1000 houses count as tightly sealed ("unusually tight construction" by ASHRAEs definition), maybe. All buildings, even those that are supposedly "tightly sealed", leak, in and out, and they all "consume" air. Dryers, all non-seal combustion appliances, fireplaces, bath fans, kitchen exhaust, etc. You get the picture. These all contribute to negative pressure, i.e. more air out than in. This would be the opposite of what I explained above.
If an ERV is used, providing an equal amount of air in as air out (as, I believe most do), the effect is neutral and the avergage, negative house is still negative. The level of envelope sealing does not affect this fact. They (ERVs) can be balanced to provide more in than out and they can be utilized as aprt of the exhaust system instead of simply providing "air changes". In residential applications, this is a rarity.
Most homes today are very lightly occupied, i.e. fewer occupants in a larger space, making air changes, except for large well engineered systems with a very specific purpose, relatively insignificant. Point exhaust is all that is required. That and a source of makeup air for the air that is combusted and exhausted. This is where the "more in than out" concept is important. The air "consumers" will get the air somewhere. IF its not provided in a controlled and specific way, it will come in through constrction joints (yes, even in a "tightly sealed" home), widows, doors, unused vents and exhausts. What goes out, must first come in. This is one of those widley understood and indisputable laws of physics.
Providing controlled ventilation in a system causes problems only if it was done by an unknowledgable individual or as an after thought. Even in cheeseland, that works. It works well and is done in just about every commercial system installed in Wisconsin (and every other state) for the last 50 years. If the system is so poorly sized that 10% of system flow in outside air cannot be accomodated without "causing a big problem", I would expect a great deal of other problems will be experienced as well. Poor air distribution, inadequate duct work, poor humidity control, ...the list is long.
I'm not saying that ERVs and HRVs are useless, I design systems with them very frequently. These systems ARE sold specifcally to pay for themselves. I've never seen one pitched that didn't include that line and I've seen ost of them. More often they are correctly used (one that understands psychrometrics) to allow a system to accomodate a higher percentage of ventilation without compromising humidity control.
The furnace, BTW, operates the same whether the outside air temperature is 20 above or 20 below. It has fixed capacity and unless compromised in some way, always provide the same output, regardless of what you may want. If you have a 120,000, nominally 5 ton furnace, moving maybe 1400 cfm in heating mode, RA of 70 and OA at -10, the mixed air temperature is 62 degrees. If the OA is -20 the mixed air temperature is 61. Not exactly "dumping frigid air in heating system" is it? Even for a 80M input furnace, the supply temperature (at 90% efficiency) is over 100 degrees on that -20 degree day. Not a problem any where at any time.
I agree that you will get a temporary positive pressure (when the furnace blower is running) and sucking in air from the outside and I agree that you will get some fresh air distributed within the house.
This in no way compares with the performance of an HRV. Surely you don't believe they offer comparable performance? I'm not saying all homes require them, but I would say most new homes really benefit from such a system. I don't think new houses are built as badly as you think they are. The ones we build are very tight and I think most of the builders you hear from on this site build the same way.
I'm not aware of any manufacturer that advertises their HRV as a product that is going to save a homeowner money by installing one. They may advertise that their unit is more efficient than the other guy's, but if you are pitching them as a moneymaker, you are misleading your customers. They obviously cost money to operate. You are dumping some percentage of your heated air outside every time the unit runs. If they were going to save everyone money, people would be lined up for blocks trying to buy one.
As for your furnace example...That may be true in some situations, but I can tell you that people definitely have problems with furnaces not being able to heat the house where some of these systems are installed. Your example requires a precise mix of air to come up with those figures. In the real world, things don't always work that way. A previous poster ( who you scolded ) pretty much described dead on how these things actually operate most of the time.
As I told the OP, maybe Honeywell has a better invention than most of the setups I see. The key to these things working is very good control of the amount of air that enters the house. If you can't accomplish that, you are going to have a problem!
"A previous poster ( who you scolded ) pretty much described dead on how these things actually operate most of the time. "
Danski did not acurately describe how these work. He understands more than you do, but not much more. I "scolded" him as you say for using BS numbers to exaggerate a point. Using examples based on something close to reality and/or actual performance. He posted an anal extraction.
"Your example requires a precise mix of air to come up with those figures. In the real world, things don't always work that way. "
Actually, my example was very basic and very representative of 100's of forced air systems in operation. The real world works that way. Whether the OA fraction is 5%, 10% or 15% is immaterial. I don't believe that you have any real world experience of a functioning HVAC system that is set up as I describe.
Edited 11/12/2008 11:18 am by Tim
Huh?
My real world experience tells me that adding an untempered outdoor fresh air intake to a residential return duct system is not good. The equipment is not sized for the additional load to start with, and typical slipshod residential HVAC installation in general makes equipment perform badly. I have witnessed a 30% loss of airflow to poor duct design- without even addressing duct sealing.
Generally speaking, there are more problems on the cooling side than heating. I have witnessed insufficient cooling capacity caused by a single 4" untempered air intake to the return ducting. Yes, there were duct problems, but that intake threw the system off.
Anal extraction?
You can plug CFM values for forced mechanical ventilation into a load calculation program. You get a number, a real one, in return. That is where my number came from- I plugged 100 cfm of forced ventilation into an existing load calc... the number will vary based on the design conditions of the application.
The number of typical residential HVAC contractors capable of properly measuring the actual airflow delivered by that hypothetical 4" pipe is small. The number that will do it is smaller. The number of potential clients willing to pay for those services is smaller yet.
Depending on how close the hypothetical 4" pipe is to the furnace, I don't see 100 cfm being an impossibility... doubly so if there is a shortage of return air to start with. If I run across one again, I can measure the airflow.
Also, these ERV's and HRV's need to be balanced if they are connected to an existing duct system, so the actual amount of air being exchanged is known... and the unit performance changes based upon CFM and FPM (yes, you still may need an inline duct heater if the OA is cold enough on an ERV)... Of course if you are exhausting more air than you are bringing in, you will have all kinds of uncontrollable air leaks due to infiltration.
I wonder how many people do that... you know, actually balance an ERV installation?
I do understand that a ERV will not pay for itself, but it is a way to get tempered outdoor air into a tightly sealed home... and I also know that most homes do not fit into the "tightly sealed" catergory.
The OP can try the modulating Honeywell damper and see if it works in that application. Nothing wrong with that, but the possibilty for problems is still there. I wonder if the installing contractor will measure the airflow?
" I have witnessed a 30% loss of airflow to poor duct design- without even addressing duct sealing."
Please explain to me what you witnessed and how you determined 1) what the design air flow was and 2) how you witness a 30% "loss".
"I have witnessed insufficient cooling capacity caused by a single 4" untempered air intake to the return ducting. Yes, there were duct problems, but that intake threw the system off."
So again, please explain, in detail, like with real numbers, how you witnessed insufficient cooling capacity. How much of a sensible and latent load were added by the untempered outside air, at what (OA) conditions and at what flow rate? How did the "duct problems" affect the cooling capacity of the system? What were the heat creating duct problems? What was the refrigerant pressures? What the coil entrering and leaving conditions? What was the dP accross the coil?
I agree that 100 cfm passing through a very short, straight and unrestricted 4" galvanized duct is possible even under the low pressures created by a residential furnace operating in the high speed fan cooling mode. Possible but very unlikely and you know this, or at least should. You know how much pressure drop there is in 20 equivalent feet of 4" with 100 cfm? Do you know how much pressure drop there is through a wall cap, a skuttle, a volume damper and 20 feet of 4" duct at 100 cfm? It is more than most res fans can produce on one side. How are you going to measure the air passing through a 4" duct? How are you going to measure the air flowing through any duct?
1) Poor air flow... rooms not heating or cooling, Changed fittings at the furnace... 30% more air in the system as measured at the registers. Works fine now... more than one application, same results.
2) AC didn't cool enough with the OA open. AC worked fine with OA dampered shut. Simple solution was chosen, eliminating things that should have been looked at, but not "in the budget", as they say.
No numbers needed.
You can measure airflow with a pitot tube and a manometer...and do some math. It can also be measured with an anemometer.
Trial and error is sometimes the best approach. I don't often deal with what passes for HVAC syems installed in houses. I designed and installed mine. I accomodated the OA in the selection setup of the unit. It works well.
It isn't always trial and error, but rather a balance of knowing what will work or make an improvement. While a complete Manual D on a residential system is nice, most familiar with the basic principles can make a few well placed changes to make major improvements.
Real world field conditions... square throat ells without turning vanes... lots of flex duct... poor duct sealing... gaps in metal round pipe connections from needle nose pliers being used to crimp the pipe... all show that many contractors are more concerned with time on a job than a job done right- but that leads to the lowest bidder mentality.
Knowing how to measure all of the parameters of a HVAC system is great... and I certainly will admit to not knowing everything... but the reality is that the clear majority of the residential market from builders to homeowners competes on price... and there are plenty of contractors that have a low price in big print with a big gotcha asterisk in teeny tiny print.
Yes, there are exceptions.
A HVAC company offering an air conditioner "clean and check" at $69.95 does not have the time in that price to even check the charge correctly, yet that company gets the calls from the price shoppers, and the customer gets the repair ticket with "Added 1# of Freon: $40".
Unfortunately, HVAC equipment will provide hot or cold air under a wide variety of installed conditions. As long as the hot or cold air comes out of the machine, most do not care until it breaks. Convincing people that it will work better and cost less when the equipment and ducting is installed correctly is difficult at best.
"Unfortunately, HVAC equipment will provide hot or cold air under a wide variety of installed conditions. As long as the hot or cold air comes out of the machine, most do not care until it breaks. Convincing people that it will work better and cost less when the equipment and ducting is installed correctly is difficult at best."
And that is a universal rule no matter where you live.
I think you spend too much time in the office.
Packers 24.....Bears 10
Thats what they pay me for. I get my hands dirty from time to time but I don't mess with houses, other than my own.
BTW Cowboys 27....Packers 16 at Lamblow Field.
I'm no Bears fan. But thanks for the thoughts ;p
Butttttttttttttttttttt.......................
A return temp of 70 and a mixed temp of 61 may be ok, but now drop the return temp to 60 and a mixed of 51. Now you are in a no go zone for at least Carrier and probably a lot of others.
Obviously you have seen the problem!
The return air temperature is taken as the space setpoint. While its is possible that someone might be keeping their house at 60, pretty unlikely. As is -20 outside air temperature in most of the lower 48. In extreme environments I would not advocate untempered ventilation.
BTW, the Trane installation guide for for condensing furnaces requires the EAT to be at 50 degF or above. Carrier is obviously a weaker product. ;>
Do you (or anyone you know personally) set your thermostat to 60? Really, Why not sat what if the return air is 40 and -50 outside? While we making up unrealistic numbers, just pick any that suit you or Bo.
The point being, is that I have system set up exactly as I describe. It works and it works well. This is in northern IL and I regularly see -10 and -15 degrees outside.
Those who do not have the equipment, system and/or technical knowledge to understand air movement are obviously baffled and confused by this concept. I understand that I cannot remedy their ignorance in a few short paragraphs.
Edited 11/12/2008 10:59 am by Tim
Lots of customers up here who head south for the winter. Something about escaping -40. Not uncommon for the stat to be set between 50 and 60.
Personally, I recomend they set it higher, like 75.
If the furnace goes out, assuming a heat loss of 1 degree an hour, the difference between 60 and 75 would be 15 hours. Cheap insurance against freezing.
Your in the US suburb of Canada, right?
1) an unccupied house is not what were discussing, and I would recommend closing of the ventilation supply in such a case.
2) if the furnace dies during the winter and no one is home, 10 hours, or 100, its going to freeze.
1) an unccupied house is not what were discussing, and I would recommend closing of the ventilation supply in such a case.
I recognize that, but you can bet the vent probably wouldn't get closed.
2) if the furnace dies during the winter and no one is home, 10 hours, or 100, its going to freeze.
Most insurance requires 24 hour checks, still think it is cheap insurance.
I HATE fixing frozen houses.
I like debating real life situations.
Rule of thumb for adding outside air to return air is a minimum of 10 feet before the furnace.
Still prefer not to do it up here. Condensation can also be an issue.
Compared to the midwest, Canada could be considered "extreme" and as such, I would not recommend untempered ventilation during the colder months, either.
Due north of North Dakota.
That's what is interesting about some of these "discussions".
Some have no idea what the enviromental conditions of an area are.
I won't even discuss ERV, they are non existant here. Ventilation air is mandated by code here, so we see lots of HRV.