outside combustion air for furnace
Guidance please. We are building a new “week-end” (soon to be retirement) home. The furnace is a 92.1%AFUE Condensing Furnace Updraft unit located in a walkout basement. Our furnace appears to have been installed with correct exhaust venting but we had expected a direct vent furnace that draws combustion air from outside. Our dealer and general contractor (now) say that such systems (with external air intakes) don’t work well, decrease efficiency and frequently cause the furnaces to malfunction. Both have also expressed open skepticism of “tight houses” causing mold problems. Our combustion intake is through a vent on top of the furnace. (and according to the users manual, the intake CAN be vented to the outside) We are putting a fairly significant ammount of “sweat equity” into this and have made this house fairly tight: well sealed and insulated. Is this (interior) venting system a potential problem? We have a Panasonic Energy Exchange ventilator centarally installed in the living space.
Second related question: for the next 2-3 years this will primarily be a weekend home: our plan had been to set the temperature at 45-degrees through the (winter) week-days, using a programmable thermostat (wi-fi accessible) to warm the house prior to our arrival. This was discussed with our contractor. But the users manual describes a minimum recommended return air temp of 55 degrees. Our contractor says it’s nothing to worry about. (My sister hasa similar furnace (but direct vented): she & her husband recently began spending several months each winter in southern Texas with their furnace here (in Missouri) set at 45, what kinds of problems, if any, are we creating? Or are we being overly cautious?
Gary
Replies
Make up air
I'm not very well versed in furnaces but I'll say this - you seem uneasy with your contractor.
Tight houses can, and do have issues far more complex than standard building practices of even 20 years ago. You either need to do necessary research to be comfortable dealing with those issues or have total confidance in your contractor to understand dealing with them.
Seems like you have at least some knowledge, and your contractor is more skeptical. That doesn't sound like either one of you is steering the ship. Be careful.
I appreciate the observations. My post here is part of the research you are advising. The manual for the furnace says that the furnace does not necessarily need to draw from outside air. BUT if drawing inside air it cautions that the ROOM in which the furnace is located should not be "tight." It doesn't say anything about a tight HOUSE though.
I've posted this question to the manufacturer as well, but have not heard back.
Actually we have been very pleased with the general contractor and have been very happy with the construction process. The furnace deal (with his prefered sub-contractor) is the first time we've really doubted ANY part of the process. The furnace installation, including duct work, seems more than a little sloppy & rushed which is uncharacteristic of anything else on this job. (it doesn't help that friends who have used this HVAC guy in the past have not been satisfied with his work or responsiveness: of course we hadn't heard these stories until after the work was done.) In truth, the contractors response to my questions about the furnace installation represent the very first hint even of defensiveness or disagreement. (and I honestly don't think of this as simply the straw that broke the camels back)
We have confidence that our house is very well built. The contractor has been on or ahead of schedule with every portion of the job done by his crew. (even under budget) Communications have been open and straight forward and he has been imediately responsive to small adjustments we have requested. (As we have also been responsive too - and pleased with- recomendations and changes he has proposed.) Early on he acknowledged that he was more "traditional" in his style and a bit skeptical (but open to learning) of some of the more modern super efficient techniques. That said, he has incorporated and seemingly embraced a couple of new ideas that we have shared with him. (usually from Finehomebilding) He has even commented that he liked some of the new ideas and we know he has actually incorporated a couple of those ideas in other projects he has going on.
When this comes full circle, I may be very comforatable with the furnaces air intake arrangement, but I need some guidance to get there.
Gary
It's hard to fix ducting and furance mistakes later
I'm just a DIYer, not an hvac guy. But here's an opinion for you:
My personal experience:
I own two houses with sealed combustion furnaces. Neither have any issues. Both work great. I would never buy another furnace or water heater that used internal combustion air. The water heater market is moving (slowly) towards external combustion air water heaters. In my air, I think new furnace permits require external combustion air.
I'd say, first, and I think you know this now, that you needed to specify that you wanted an external combustion air furnace up front. IMHO, installing anything else in 2014 is strange.
Second, more than anything else, it bothers me that the contractor is trying to discourage you from getting the furnace you want
You need to be aware that adding an external combustion air intake, depending on the location of your furance room and other issues, could be very difficult or even impossible at a later date. There are code requirements related to location of air intake and exhaust in relationship to doors and windows and other items like bathroom exhaust fans and such, as well as having drywall on the ceiling will likely make the task destructive. I would worry that that it will be very difficult to fix this problem later. Undoubtedly, your next furnace will be sealed combustion. Much better to have that setup now.
This is merely my own uneducated opinion but I would worry about using internal combustion air in a tight house. As I understand it, under supplying combustion air can cause excess carbon monoxide product. There are some case studies on-line that also suggest this. Here's one. http://www.extension.iastate.edu/pages/communications/co/co_cases.html
i second...
...jim's comments-and i am well versed in furnaces/venting/tight house construction and associated issues.
Dealer AND Contractor?
Direct vented furnces work very well when combustion air is properly ducted directly to them. I would challenge the dealer/contractor that says otherwise. Size the intake as per the installation manual/mfr instructions and look for a new dealer/contractor if possible.
Taking combustion air from indoors is not a problem. The amount of air consumed is very small compared to the volume of the house and normal contruction practices are not so tight that leakage won't easily supply all the combustion air required. The only concern is that the room in which the furnace is installed is not sealed off from the rest of the house, such that combustuion air can be drawn from a larger volume.
The low temperature return air recommendation by the furnace manufacturer is based on extending the life of the heat exhanger. The lower the return air temperatures, the more condensation is formed during the intial heat up. Failing to comply will most likely void your warranty, though in reality, cause very little harm to the unit, especially a high efficiency unit which is designed to condense flue gases under all operating conditions.
and i second...
...quite strongly-tim's comments here!
thanks tim-nice to have a poster who knows what they are talking about!
dump the contractor!
Heating, ventilation, mold = remote control
A modern condensing furnace is designed as a balanced flue - and logic says that any other form of installation is wrong.
You do not want to be in a situation where the furnace is fighting for air, struggling for air means it is not working properly, and it is possibly burning warm air, which is a waste of your money.
Who made it? Which model? You have the fitting instructions. Is it a twin pipe model? Can you not fit the other incoming air pipe yourself?
Mold is caused by people. Cooking. washing, drying, breathing, sweating, indoor flowers, fish tanks, children, animals. Putting water vapor into the home.
Mold spores are all over the world, but you don't see mold growing on everything!
Mold needs food and water to grow, food is mainly wood, paper, leather.
If you have a tight home, then mold will grow on cold walls and ceilings, in the right (for it) conditions.
Opening a window lets the water vapor out.
Running an exhaust fan in the kitchen while cooking, ditto bathroom while washing lets the water vapor out. Running your Panasonic will no doubt work; does it have a humidistat?
Having a Weather Station connected to an always on computer, enables you to keep a check on the temperature and humidity - this shows when there is too much water vapor in the air.
You will have a problem! The home will be cold!
You will use it for a short time, it will probably not warm up properly - then it will be cold again.
Probably, having a phone/wireless controlled dehumidifier will be the best solution. Cheaper to run, as you will not be putting any water vapor into the home, you merely have the typical night-time fall in temperature with the corresponding rise in humidity.
All furnaces have to start up - regardless of the existing room temperature. Look up the average monthly temperatures for the area, especially overnight, you will probably find that the heating won't be on very much at 55.
I find that after switching my heating from 60F to 72F on heading home, it takes about five hours to reach 72F and about twenty nine hours to get back to feeling normal. I simply phone home in the morning when I start homewards and its basically sorted by the time I get home about seven hours later. I hate a cold bathroom.
A modern condensing furnace is designed as a balanced flue - and logic says that any other form of installation is wrong.
You do not want to be in a situation where the furnace is fighting for air, struggling for air means it is not working properly, and it is possibly burning warm air, which is a waste of your money.
Who made it? Which model? You have the fitting instructions. Is it a twin pipe model? Can you not fit the other incoming air pipe yourself?
Mold is caused by people. Cooking. washing, drying, breathing, sweating, indoor flowers, fish tanks, children, animals. Putting water vapor into the home.
Mold spores are all over the world, but you don't see mold growing on everything!
Mold needs food and water to grow, food is mainly wood, paper, leather.
If you have a tight home, then mold will grow on cold walls and ceilings, in the right (for it) conditions.
Opening a window lets the water vapor out.
Running an exhaust fan in the kitchen while cooking, ditto bathroom while washing lets the water vapor out. Running your Panasonic will no doubt work; does it have a humidistat?
Having a Weather Station connected to an always on computer, enables you to keep a check on the temperature and humidity - this shows when there is too much water vapor in the air.
You will have a problem! The home will be cold!
You will use it for a short time, it will probably not warm up properly - then it will be cold again.
Probably, having a phone/wireless controlled dehumidifier will be the best solution. Cheaper to run, as you will not be putting any water vapor into the home, you merely have the typical night-time fall in temperature with the corresponding rise in humidity.
All furnaces have to start up - regardless of the existing room temperature. Look up the average monthly temperatures for the area, especially overnight, you will probably find that the heating won't be on very much at 55.
I find that after switching my heating from 60F to 72F on heading home, it takes about five hours to reach 72F and about twenty nine hours to get back to feeling normal. I simply phone home in the morning when I start homewards and its basically sorted by the time I get home about seven hours later. I hate a cold bathroom.
It's unclear -- is this unit a "sealed combustion" unit?
Most sealed combustion furnaces strongly prefer to have the intake and exhaust relatively near each other so that any wind pressure acting on one will act equally on the other, avoiding the case where wind might block the exhaust. Failure to conform to manufacturer's specs here can easily cause the furnace to "trip", because it can't get a draft going.