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Building Science

Can Atmospheric Combustion Work in a Spray-Foam-Insulated Attic?

In an attempt to reduce costs, some contractors do questionable retrofits on attic furnaces

This horizontal (low-boy) furnace in a sealed attic has a combustion air duct taped to the furnace cabinet. Not only does it violate the building code, it may not work and makes the sealed attic not so sealed after all.
Image Credit: Unless otherwise noted, all photos are by Energy Vanguard

A while back I wrote about the incompatibility of putting an atmospheric combustion furnace in a sealed attic. Most often the attic is sealed by installing spray foam insulation at the roofline, thus bringing the attic inside the building enclosure and turning it into conditioned space (directly or indirectly). The good news is that some installers understand this problem and seek to address it. The bad news is what a few of them do.

Combustion air retrofits

The photo at right is a case in point. The furnace was up in the attic before the spray foam was installed. The homeowner hired a spray foam contractor to improve the building enclosure but the budget didn’t include enough money to change out the furnace at the same time.

I don’t know if the combustion air retrofit you see above was done by the spray foam installer or the HVAC contractor, but in either case, this one’s almost certainly not going to work. Here are the main problems:

[Photo credit: Nikki Krueger]

The image at left shows a better installation. The duct looks like it’s 6 inches or 8 inches in diameter, and it’s made of rigid metal. Both of those things will allow more air to move through.

And that air might even move toward the furnace instead of away from it. Of course, there’s no guarantee of that. As my friend David Richardson likes to say, combustion air doesn’t care which way we show the arrows pointing on our diagram. Air flows from areas of higher pressure to areas of lower pressure. Under some circumstances, air might flow out through that inlet rather than in.

The need for air

Now let’s go a little further. Let’s look at what…

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  1. Eric Habegger | | #1

    "Bringing combustion air in from outdoors can waste energy and lead to comfort problems." I'm a little confused about this comment. Or perhaps I'm reading this out of context. Isn't this only true if the outdoor air displaces indoor living space air before it reaches the unsealed combustion site? If the outdoor air goes directly to a sealed combustion site without displacing living space air that seems ideal to me.

    EDIT: I guess the key is how you define the sealed space and how you define the living space. For instance, in your example of a sealed attic with insulation beneath the roof how you define the attic ceiling boundary is important. If that boundary is totally air sealed the you must bring in air from an attic vent to outside air for the open burner. But in this case the cold air from outside will cool off the attic and negate the whole idea of having a conditioned space attic. If you vent to the living space below then you depend on air leaks to the living space and then have the same problem, only more so.

    However it seems helpful to not define the problem using the attic as the sealed combustion space. In the end you have to localize the sealing to the burner combustion area and always have remote ducting for both exhaust and inlet air if the fire is anywhere within the house. So your conclusion is perfect.

  2. Eric Habegger | | #2

    If a foolish consistency is the hobgoblin of small minds I guess I have to plead guilty, at least in part. It seems to me that there is no good way to seal a gas range and so one would have the same problems discussed here. Just smaller. Actually, I've seen people comment that their house CO^2 measurements go up when using a gas oven or range so it may be that the combined effect of cool (or hot) displacement air along with combustion byproducts is not a good thing in an otherwise healthy house.

    Something to consider although a lot of people will just hate having to take this idea seriously. It will be interesting to see if eventually gas ranges also become a relic of the past. I would guess "yes", though perhaps not in my lifetime.

  3. Pascal Dornier | | #3

    atmospheric furnaces - just say no
    For both air quality (NOx) and energy efficiency reasons, they're not allowed in Switzerland any more. Germany mandates replacing furnaces that are over 30 years old.

    A concentric exhaust system (intake air on the outside, exhaust on the inside) takes care of the venting, and improves efficiency by preheating the combustion air with waste heat from the exhaust.

    Another regulation here mandates a system temperature of no higher than 50 C to allow future upgrades to heat pumps, and keep the furnace in the more efficient condensing mode.

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