Concerns about backdrafting and makeup air
We’re in the process of air sealing a mid-Atlantic house built in the mid-80s. (Today is a humid 94 degrees and in the winter we go below freezing.) We had low CFM-rated (OKNA-brand) windows installed recently. After we seal the rim joists and the attic, I am concerned about what make up air and backdraft situation will be. We will be installing a 300 CFM range hood soon, too. Our water heater was just replaced with a power vented model, and we expect to replace the furnace very soon. We do have a wood burning fireplace. I have read for hours on end, and still can’t figure out if I should be concerned with these issues. If anyone can provide pointers, I would really appreciate it.
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Forum User,
The tighter your house, the stronger the argument that you should avoid atmospherically vented appliances (including fireplaces and wood stoves) and stick with sealed combustion appliances.
Wood burning fireplaces aren't compatible with a tight home.
Do wood stoves include "pellet" stoves?
How about gas stoves? Should those also be avoided?
Also, is there any instrument that can measure if my home is too tight for me to be using a wood burning fireplace or a powerful range hood?
Forum user,
Most jurisdictions mandate that all combustion appliances, including those you mention, be supplied with outside air, either directly or via wall vents.
How are gas stoves usually supplied with outside air? Just being located next to a window?
Sealed combustion hot water tanks are extremely rare, i need to replace my atmospheric version but have not found an atmospheric or power vent that is sealed combustion
Alan,
You wrote, "Sealed combustion hot water tanks are extremely rare, i need to replace my atmospheric version but have not found an atmospheric or power vent that is sealed combustion."
The reason that you have not found an "atmospheric" water heater that is sealed-combustion is that these are two different types of water heaters. The kind you want is a sealed-combustion water heater. Atmospheric water heaters, by definition, aren't sealed-combustion.
Here are links to tank-type water heaters that are sealed-combustion:
Lochinvar water heater
Phoenix water heater
Here are links to tankless sealed-combustion water heaters:
Bosch water heater
A.O. Smith water heater
Forum User,
For more information on providing combustion air to wood stoves and pellet stoves, see this article: All About Wood Stoves.
There are many reasons why people with a fairly tight house might prefer an electric kitchen range to a gas kitchen range. Gas ranges are polluting. If you insist on a gas range, the usual solution to the pollution problem is to operate a range hood fan that exhausts to the outdoors. Be careful, however: large exhaust fans come with their own problems, and may require dedicated makeup air systems. For more information on this issue, see Makeup Air for Range Hoods.
Forum User,
The instrument used to measure the air leakage rate of a home's envelope is called a blower door. Here is more information on blower doors: Blower Door Basics.
You seem to be implying that it's a good idea to leave a house a little bit leaky, so that you can continue to use atmospherically vented appliances. That's a bad strategy. There are a great many reasons why you don't really want a leaky house. You want to make your home as tight as you possibly can make it.
If you accept this approach -- the approach advocated by Green Building Advisor -- then you should also install a mechanical ventilation system (to provide occupants with fresh air) and you should make sure that your combustion appliances are all sealed-combustion appliances.
Martin, thanks for the educational information. I guess I could always open the windows a little when running the wood fireplace (only once or twice a year anyway.)
Aren't "power vent" water heaters by definition sealed combustion?
Forum User,
Most power-vented water heaters use indoor air for combustion air.
If you aren't sure which type you are looking at, choose a water heater with two pipes -- one pipe for incoming combustion air, and another (the flue) for combustion gases.
Yes, if the fireplace is only a special occasion thing, you can just open windows. Just be sure you have a good way to seal off the chimney the rest of the time--a regular damper doesn't really seal. A chimney balloon is advertised as doing much better, although I can't say I've tried one.
http://www.chimneyballoon.us/chimneyballoon.html
Note that the two pipes in a sealed-combustion appliance are often concentric, making it a little trickier to tell by looking.
Our furnace is sealed (concentric), but power vent is not. So what specifically does this mean? It won't be able to light up?
Gasketed top sealing dampers for fireplaces can seal reasonably well, but only if the flue liner and fireplace are fairly tight. But too often the masonry is surprisingly (abyssmally?) air-leaky, even on homes only a few decades old. The lousy seal of the steel flap damper at the top of the firebox is sometimes the least of it.
Cracking a window can help when starting the fireplace, and once it's really going it'll usually draw quite well even with the windows closed. The danger zone is when the thing hasmostly died out but still has burning embers in the ash. In a tight house it really needs to be fully out before closing off the windows or closing the damper.
Forum User,
Most homes have enough random air leaks to allow power-vented water heaters to operate. If your house is tight, though, the power-vented water heater (along with other exhaust appliances like bath exhaust fans and clothes dryers) might pull air down your fireplace flue, bringing a sooty smell inside your house.
Say there isn't enough indoor air for combustion, is this merely an operational issue or is it a health hazard?
Forum User,
I have never heard of a house that was so tight that a power-vented water heater wouldn't work.
As I said, I don't think there is a risk that the power-vented water heater won't work. The risk is that the power-vented water heater, in conjunction with your other exhaust appliances, can introduce unwanted smells into your house by pulling air down your chimney.
The power vented water heater might backdraft the fireplace if you followed Dana's advice to close the window when the fire is going well. In addition to smelling bad, that could result in a carbon monoxide hazard. It's a good idea to have a good carbon monoxide monitor in the house if you have any combustion appliances, or an attached garage. Then you needn't worry.
Our power vent water heater installer was over today because there was a leak with the expansion tank. He told me he actually once witnessed a case where a power vent water heater wouldn't light because it was in a well-sealed room.
Thanks everyone for helping me understand the concepts. Most of the useful articles online were too advanced for me. I couldn't find reading material explaining all the terms and concepts.
@ Martin That Phoenix water heater looks mighty impressive, i wish it was available around here, even direct vent tanks are extremely hard to find. I wish i lived in the US, you have many more options for products (i speak of far beyond water tanks).
Alan,
Where do you live?
Sometimes an ordinary electric-resistance water heater is the best solution -- coupled with a few PV modules, if you live somewhere with a good net-metering agreement.
I live in Ontario Canada, i can't afford any panels though, but i need a replacement water heater, i wish there was a 2 pipe version easily available but at the moment i may just go with the chimney vent unless a decent priced alternative comes along before i can afford to replace mine (its rusting inside, the anode rod is gone, long story).
I have a 15ACH50 house and a very shallow crawlspace i can't easily fit into to put down plastic and seal the gaps to outside, so its a major work in progress.
On the plus side i have reduced heating load 30% last winter from the winter before (lived here 2 years), i insulated the walls with cellulose and replaced the furnace from 80% eff (and dying, 23 years old) with a two pipe 95% eff.
We have a great solar policy around here, but i can't afford to buy any and if i could i would sell the energy to the grid for better pricing.
Are tankless typically two piped? The higher price would wipe out any savings i would guess, and it would be even longer before i could buy one, my goal was for the one i have to last another 5 years (its 8 years old) then i would have had more money to spend and probably more options to choose from.