Return Flex Duct in Mobile Home
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I work for a weatherization company that work on low-income family housing. At work there has been a debate about if the return flex duct that runs from the top of the mobile home furnace to the attic should be removed and sealed or left as is. A blower door is performed to see what the mobile home’s air infiltration rate is and we adhere to the ASHRAE 2016 standards (installing exhaust fans etc.). Should it be removed or should it stay and for what purpose was this flex duct installed to the return coming from the attic?
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Replies
We'll help you get it straightened out, but we're going to need a little more information.
I can't tell much from the picture -- there's a flexible duct that goes up into the attic? And it's attached to the return for the furnace? Is it the only source of return air? Is the attic part of the conditioned space of the home-- ie, is the roof insulation above the attic? Or is the insulation between the ceiling and the attic?
The attic isn't part of the conditioned area. The insulation is on the ceiling. The return is also from the front of the furnace that pulls from the hallway. Yes the return flex duct is attached to the return and goes into the attic (could possibly to a roof vent). In my experience this is found on mobile homes of the 1980s and 1990s.
That sound like the outside fresh air feed to the air handler. If that is the case, I would leave it as. If you have the equipment, it would be worth while to check the airflow through the flex and make sure you are not getting either too much or not enough fresh air into the system.
Is this combustion makeup air?
No this is not combustion air, it goes directly into the return system.
What would be "too much" or "not enough"? Other than combustion makeup air I don't see the need for outside air anyway. Typically mobile homes are so leaking that there's no need for supplemental ventilation.
wouldn't bring direct cold air from outside be bad for the furnace's heat exchanger?
I own a single wide and all the supply air returns under doors down the hallway and back to the furnace with a couple of filters mounted inside the door. Do you have a double wide?
Blower door test on a mobile home? lol!
Mines like trying to heat a bird cage..
I have seen this done on both single wide and double mobile homes.
If it is the fresh air feed for the air handler, then no, this air should not come from the attic as it will be polluted with insulation dust. I have seen instances where the installer simply draped the flex hose over a truss chord and the hose end was directly into the insulation, which resulted in insulation being spread around the house through the furnace ducts.
So the phrase "fresh air feed" is puzzling me, it's not normal for an HVAC system to pull in outside air. To the extent a house needs ventilation except in super-tight construction that's handled with point-of-use vent fans and infiltration. Am I missing something?
In BC, the fresh air intake has to do with the code requirement of mechanical ventilation. A prescriptive solution path is to have a forced air furnace provide continuous supply air and a bath fan exhausting continuously. This can be done instead of a HRV or ERV.
Not sure where the OP is but seems similar to what I have seen.