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:
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…