The Wrong Way to Retrofit a Furnace in a Spray Foam Insulated Attic


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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 some of them do.

Combustion air retrofits

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The photo above 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:

  1. Inadequate duct size. They used a 4″ flex duct that you usually see on bath fans (which have their own problems), and it ran at least 20′ to the place where it exited the attic. This would not meet the code requirement for combustion air inlets.
  2. Poor duct installation. The duct wasn’t pulled tight, further reducing the air flow through the duct.
  3. Tape over furnace louvers. The louver area is designed to allow the proper amount of combustion air to enter the furnace. By covering some of them with tape, the installer of this retrofit may be guaranteeing the opposite of what they were aiming for: less combustion air, not more.

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

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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.

What should you do instead?

In my opinion, atmospheric combustion appliances have no place in sealed attics. Yeah, building codes allow it if there’s enough volume, but if you’re spending the money on a such a significant improvement to the building enclosure, you should also budget for sealed combustion equipment or go all electric.

If you can’t do it all at once, at least install low-level carbon monoxide monitors so the homeowners will be alerted if a CO problem develops.

Allison Bailes, III, PhD
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Posted In: Building Performance, Residential Buildings

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