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80% efficiency furnace in a foamed / sealed attic space

MattJF | Posted in General Questions on

We have an 80% furnace that is in a walk up attic space we are planning on foaming, insulating and sealing up. I am trying to determine what steps we to take to determine if there is sufficient combustion air or if we will need to provide outside air. The space will be a conditioned storage space. We are planning to replace the furnace in a year or two.

The furnace is an American Standard Silver 80 100,000 BTU.

From the manual and ANSI Z223.1 the furnace is a class 1 fan assisted combustion device.

From ANSI Z223.1 with an unknown ACH (I assume ACH50, but the standard copy I have doesn’t clarify), you need 50ft^3 per 1000BTU. The space contained within the drywall is 5370 ft^3, so we technically have enough space. Should I worry any further? This formula assumes a .33 ACH (from ft^3=15ft^3/ACH*(BTU Device/1000 BTU)), which seems unlikely to achieve with interior spray foam, caulking, and drywall.

Should we have a CAZ test performed? I can DIY this to know I we are safe, but the inspector is going to want someone professional. Who is most cost effective to perform this test?

I can have a blower door test done, but that is generally whole house.

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  1. GBA Editor
    Martin Holladay | | #1

    Here's the advice I gave in my 2010 article on the topic ("Creating a Conditioned Attic"):

    "You have to come up with a plan to provide your appliance burners with combustion air. The best solution is to install only sealed-combustion appliances in a conditioned attic. ... If you hope to convert an existing unconditioned attic to a conditioned attic, the presence of any atmospherically vented appliances (for example, a gas water heater or a gas furnace) complicates the retrofit work. If you can’t afford to buy new sealed-combustion appliances, you’ll probably be better off leaving your attic unconditioned."

    If you are planning to replace your old furnace "in a year or two," the obvious solution is to get this job over with now, and do it right. It's time to install a sealed combustion furnace.

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