Looking for whole house ventilation fix
Bainbridgebud
| Posted in Energy, Heating & Insulation on
Hi, we have a nine year old house that is too tight. I have trouble with chimneys drafting “backward” when not in use and I feel sure the house is always at slight negative pressure. I have cut several heater vent size holes into the crawl space and they draft well but it is still not enough and at some point the crawl space air is going to not smell that great. I do not want to open a window everytime I want to build a fire. Is there a simple mechanical through the wall device that anyone has tried? We do have a whole house fan but this just makes it worse as it pulls air out with no accomodation for inflow other than leaks etc,
thanks
Replies
You might have a couple of different things going on here.
If your chimney doesn't draw well, it might be too short. That's often a problem where there is not much to be done except opening a window when you build a fire.
If you have negative air pressure in the interior of the house, one possibility is that your heating system is not very well sealed. Think about it: if you don't have any windows or doors open, then your heater is just moving air back and forth. Pressure will be relatively negative near the cold air returns, and relatively positive near the heating ducts. But overall, there shouldn't be an imbalance. However, if some of that hot pressurized air gets lost into your crawl space due to a leaky heat duct, then you have to make up for that somewhere. That would be negative pressure for the overall house.
I'm not clear on what you meant by cutting holes into your crawl space. Do you mean you put holes in your floor so that crawl space air could flow into your house? Unless you have a sealed crawl space, that's not a real good idea.
You can get HRV (heat recovery ventilators) that bring clean fresh air into the house and exchange the heat from the conditioned air blowing back out. Here's an example:
http://www.rewci.com/faheenreve.html
Sounds more like you need a good HVAC guy...
good luck,
-t
An HRV or an ERV is a whole-house ventilation system. Neither appliance is intended as a source of combustion makeup air; in fact, HRV manufacturers specifically note that these appliances should NOT be considered sources of combustion makeup air.
The problem with the fireplace is a combustion makeup air problem. Three possible solutions:
1. Plug the fireplace flue and stop using the fireplace.
2. Open a window whenever you light a fire.
3. Install a dedicated combustion makeup air vent from the exterior to a location near the hearth.
Martin Holladay, senior editor
http://www.greenbuildingadvisor.com
Edited 4/3/2009 7:40 am ET by MartinHolladay
"If you have negative air pressure in the interior of the house, one possibility is that your heating system is not very well sealed."
Heating system won't have anything to do with it, however, as prev post suggested may have to do with the wrong type of fpl. There are basically 3 types of fpl, low efficiency, essentially a fireproof alcove to burn stuff in, mid efficiency, same as low, but air flow from the room (combustion air) can be regulated/controlled (some folks call these air tights) and high efficiency, a separate supply of air from the o/s of the house to inside the fpl. In modern tight construction only the latter will meet with your requirements.
To make the best of your situation you must provide make up air to the house, else it will be pulled in through the kitchen/bathroom/dryer. That means opening a window, or as prev poster suggested, perhaps a grill on the wall beside the fpl ducted to the o/s Verify as well that the chimney size, pipe lengths, elbows, clearance on the roof, appropriate cap (high wind?) etc are within design limits, all this is in your instal manual.
"If your chimney doesn't draw well it might be too short"Did you post this in another, earlier thread, or it it "common knowledge"--'cause I don't see the physics backing it up?
Personally, I consider it "common knowledge" although that's a pretty sketchy thing. Like they say in the intellectual property arena "everything is obvious to the inventor"!If you want to dig into the physics, here's a clip from a wikipedia entry that covers the gist of the point, along with the equation for determining flue gas draft rates:-------------------------
http://en.wikipedia.org/wiki/Chimney"The combustion flue gases inside the chimneys or stacks are much hotter than the ambient outside air and therefore less dense than the ambient air. That causes the bottom of the vertical column of hot flue gas to have a lower pressure than the pressure at the bottom of a corresponding column of outside air. That higher pressure outside the chimney is the driving force that moves the required combustion air into the combustion zone and also moves the flue gas up and out of the chimney. That movement or flow of combustion air and flue gas is called "natural draught/draft", "natural ventilation", "chimney effect", or "stack effect". The taller the stack, the more draught or draft is created."-------------------------In the equation for flue gas effluent rate, the height and cross sectional area of the chimney are proportional to the effluent rate. Cross sectional area is directly proportional, whereas only the square root of the height influences the rate. However, floor space, materials and aesthetics limits the cross sectional area of most chimneys.I've lived in a variety of houses with chimneys, and I've had problems getting a reliable steady draft in the three different single story homes. No problem in probably a dozen+ two story or higher places I've lived in or stayed at.-t
Sure seems like there would be some sort of limit to the length effect....after all, anyone in this business has some understanding of the length limits on various vents....
An HRV (or ERV) like Webted mentioned is the way to go.
You should also consider installing a chimney-top damper to seal it when its not in use and providing make-up air to it when it is in use if your house is really that tight.
An example of the damper (not a brand recommendation): http://www.dynamitebuys.com/store/cart.php?m=product_detail&p=564
I tend to agree w/ Mr Holladay(SP??). If your only/primary concern is the chimney drafting wrong, you do need to in a sense 'open a window' when you use it. In the Northwest, they were actually requiring combustion air for wood stoves and fireplaces on newer (tight) homes. This can have a motorized damper in it ... flip a switch or otherwise interlock w/ your appliance in whatever fashion (I am NOT implying a jerry rig, here, guys ... just painting a concept) ... this would provide air when you need it and closed when you don't.
In 'the old days' (back in the early 90s), I saw a through the wall, pop open vent used for ventilation air in new homes. Some windows have a sliding vent in the bottom. All designed to allow makeup air into the house anytime you used e.g. an exhaust fan, fireplace, combustion appliance.
What type of heating system do you have?
I also was confused by your description of holes in your crawlspace.
Hi, thanks for all the input, the heating system in the house is oil furnace, forced air. The problem is exactly what you refer to as make up air. We had cut a couple of holes in a closet floor and put heat registers and screening over them and they draft air from the crawl space to "make up" some of the air needed, but not enough. I need to find a simple through an exterior wall type vent possibly with louvers that just opens anytime there is too little air in the house. I have always wondered if there is something inexpensive, compared to a heat recovery system add on to the furnace. Opening a window does work but an always on system would seem better. Thanks
Is your crawlspace closed up tight and insulated?
I think there are other air intakes that were like bladders that varied in size depending on pressure ... I may be thinking of something else here. If you want them on ALL the time (don't know why you would), just design a louvered vent.
You could do a standard louvered vent and motorized damper and put it on a switch or some other control (even air pressure control!). Use it only when you need/want it ... off the rest of the time. Put it wherever you want. There is a fix.
If your crawl is vented, your 'fix' isn't very good IMO. If it is not, you've done nothing ... other than a possible direct avenue for e.g. Radon to get into the house.
Thanks for the thoughts, the crawl is vented, so I am assuming that the air I get is just basically outside air, but it is not enough as it is now, what is IMO? The idea of a louvered opening is basically the way I think I will go, I figure the louvers will open when the pressure is lower in the house than out and then close when pressure is equal.
Call me crazy but I think it is possible that with your arrangement of cut in floor vents to your vented (still don't know if insulated) crawlspace may be drawing air from your house, not bringing it in.
I could be way wrong.
In My Opinion (IMO).
A gravity damper in a louvered opening will not work BTW (by the way). It will work, but it won't work when you want it to ... it will work when the wind is right. Best to have some type of control over it.
I wonder if you could install a vent from the exterior into the return air trunk on your furnace. Don't know if it's kosher, but it's occasionally done around here...
I think that's what some of us suggested as an option ... however, that only works when the furnace is running and if he wants to use the fireplace w/out the furnace he has no vent/comb air coming in for it unless the furnace is running as well (which probably defeats the purpose).
I there's an opening into the return air, it would work all the time - Not just when the furnace was running.
Not if it has a motorized damper in it ... which was required at one point in the PacNW ... and which I would recommend in such an application. You certainly wouldn't want e.g. -20 F air dumping into a cold furnace and then having the furnace kick on.
A motorized damper was never mentioned before. I've never seen one installed around here.
Energy code in the NW required it at one point if you used the return air duct to provide the ventilation/makeup air for the exhaust fans. Don't know the current status. They were nice dampers ... simple, tight. You could also use them for combustion air to a 'tight' furnace room ... then you interlock w/ the burner to ensure it is open before the furnace fires.
As far as the furnace ... a couple of choices ... dampered ventilation air into the RA side of the system. It opens w/ the furnace firing; closes when off. In the Northwest for a while, they required combustion air to the furnace room be dampered w/ motorized damper connected to the furnace controls. Furnace can't fire until damper is open (using a 'make' switch). Both would provide combustion air for the furnace ... one indirectly, the other directly (i.e. into the mechanical room).
Just to clarify, my recommendation for a ERV was based on the the OP's description of how tight his house was. I was not proposing it as a source of make-up air. I believe anyone having a tight house should strongly consider an ERV.
I also recommended an outside air source for the fireplace, something that would provide make-up air. Both my fireplace and wood stove have outside air sources, something I consider critical in any tight house installation.
My experience with backdrafting of all types (water heaters, gas fireplaces, wood-burning fireplaces) , in almost every case, has proved to be a problem with pressure imbalances caused by the forced-air system.
You can easily test this out by running the furnace blower and then starting a fire in the fireplace. If you start the fire first, before running the blower, you will not get as accurate results, because the fire will get a chance to start a draft, which could be enough to overcome the de-pressurization cause by the blower and duct system. If the chimney backdrafts when you operate the furnace blower, you've got a problem with the duct system. If so, post again for the fix.
If the backdrafting happens without the furnace blower running, your problem could be that the chimney is too short in relation to the house height. This happens if the house is a two-story with the fireplace on the main floor, and the chimney-top lower than the second-story roof.
To understand this, think of your house as a hot air balloon that is too heavy to lift off the ground; but the warm air is still trying to rise. Its greatest pressure is at the top, of course, so that's where the warm air escapes. The upper part of the house is somewhat pressurized. The lower floor is somewhat de-pressurized, so any openings on the lower floor, like the chimney, become a pathway for what amounts to make-up air that is replacing the air leaking out the top. The chimney top would have to be higher than the highest part of the house to keep this from happening.
Since you have cut openings into the crawlspace, which I assume is a ventilated crawl space, I doubt that you lack combustion air, as some posts have suggested.
Since you have an oil-burning forced-air furnace, you may be at risk of carbon monoxide from it. If your house is depressurized enough that your fireplace won't draw, then your furnace may backdraft also. You can buy an inexpensive carbon monoxide detector or you can hope you and your family wake up every morning.
Cheers,
tightwad
not to worry, we have a working carbon monoxide detector, never want to be in a house with any fuel burning appliance without one, thanks for the concern