Combustion Make up Air from Attic

[Bruce Ramsey]

Todays inspection has a utility closet about 3'x6' on the left front corner of the building. Distribution panel and gas water heater. Exterior door and window in the closet. In the ceiling is a duct open to the attic for make up combustion air.

They did a fine job of fire stopping around the water heater exhaust flue up through the ceiling. The make up air duct, not so much.

Is there any kind of flap, valve or whatever that would make this open duct more fire safe?

Or should they just put a louvered vent in the exterior door? Or something else?

[Ted Menelly]

Todays inspection has a utility closet about 3'x6' on the left front corner of the building. Distribution panel and gas water heater. Exterior door and window in the closet. In the ceiling is a duct open to the attic for make up combustion air.

They did a fine job of fire stopping around the water heater exhaust flue up through the ceiling. The make up air duct, not so much.

Is there any kind of flap, valve or whatever that would make this open duct more fire safe?

Or should they just put a louvered vent in the exterior door? Or something else?

Make up air from the attic is fine and how it is done in most cases where the Heating unit is in the home. No, you do not need a fire damper. This is one of the reason I feel all shafts having to be stopped at the ceiling height. Supply air and ventilation is brought from attics all the time. The basics is that the utility closet or mechanical closet is a shaft and there can be 2 holes in the ceiling and those holes open directly to the attic.

Most shafts are not any where near the fire hazard and these shafts usually house flue pipes that go out thru the roof and not just open into the attic.

Anyway. enough of that. I don't want to get the strict code guys and shaft guys upset. But it is allowed and I do not personally find anything wrong with it.

[Jerry Peck]

Is there any kind of flap, valve or whatever that would make this open duct more fire safe?


Yes, but then the combustion make air duct would be the same as not being there ... you do not want to the be the one who suggests doing anything to that combustion make air - other than telling them it is likely too small and needs to be a larger hole through the ceiling, and that it needs a similarly sized (i.e., larger) hole and duct from the the attic, down through the ceiling, to within 12" of the floor.

You might be able to use a louvered door in place of the second combustion make up air duct, but then that would make that room not an interior room and that room would need to be treated as being outdoors, and consideration given to all that would mean, besides, a louvered door might not even provide enough net free vent space to even work. (I.e., recommend a duct through the wall, and within 12" of the floor, for the second make up air duct if they do not want another duct down from the ceiling.

[James Duffin]

Accoridng to the NCFGC you can not even have a screen over the duct.


5. Ducts shall not be screened where terminating in an attic space.

[Bob Harper]

Passive MUA does not work, from a functional standpoint

The gas code prescribes several means of drawing MUA including the appropriately sized duct from the attic with no screening or dampers. However, the IECC effectively nullifies this method. Your best method is to provide adequate powered MUA into the home such as introduced into return ducts, and provide large louvered doors then test to prove it works including worst case depressurization test and combustion analysis.

Good luck changing out that equipment and being able to meet the code requirements for duct sealing.

BTW, they have until 31 Dec to get the Fed. tax credit if they change out that clunker draft hood equipped furnace with a 95% Cat IV plus there may be State level tax incentives.

[James Duffin]

NC 2009 Energy Conservation Code is based on the 2006 International Energy Conservation Code. It seems NC still uses the Mechanical Code for duct sealing. If the water heater typical (40K BTU's) and the duct is a 6" the installation should be OK in NC.

[DavidR]

Passive MUA does not work, from a functional standpoint

The gas code prescribes several means of drawing MUA including the appropriately sized duct from the attic with no screening or dampers. However, the IECC effectively nullifies this method. Your best method is to provide adequate powered MUA into the home such as introduced into return ducts, and provide large louvered doors then test to prove it works including worst case depressurization test and combustion analysis.

Good luck changing out that equipment and being able to meet the code requirements for duct sealing.

BTW, they have until 31 Dec to get the Fed. tax credit if they change out that clunker draft hood equipped furnace with a 95% Cat IV plus there may be State level tax incentives.

Exactly!

No one told the air which direction to flow. :D

[Jerry Peck]

No one told the air which direction to flow. :D

David,

We have that same problem with wind speed lines on the map ... there is no cop there to issue the wind a ticket when the wind ignores the 90 mph line and blows through at 120 mph! :D