Return jumper duct

[Tim H]

This house is one year old, unvented attic with Icynene applied to the sheathing and roof framing, and covering up the soffit vents.

What is different about this from the other unvented attic/Icynene applications that I have seen, are jumper ducts from the return side of the heat pumps, going to dampered roof vents.

The 4 ton unit also has a supply register installed in the ductboard return (the 1.5 ton unit does not have a register).

Any ideas what is going on? Should the supply register be opened, closed, or removed (or changed seasonally)?

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Thanks

Tim

[Garet]

As houses are constructed tighter against air leakage we're going to see more systems with make-up air drawn from the outdoors. Normally I see a motorized damper that closes the make-up duct when the system is not running.

Are you trying to say they are drawing return air from the attic as well as from the living space and exterior? This would be a return register, not a supply register.

[hausdok]

The house is tight. The jumper duct is a fresh air intake. I agree with Garet, those usually have an actuator wired to a 24-hour timer that opens/closes a damper to bring in fresh outside air.

Was there a 24-hour timer on the HVAC unit, in the laundry or in a utility closet somewhere?

ONE TEAM - ONE FIGHT!!!

Mike

[Tim H]

As houses are constructed tighter against air leakage we're going to see more systems with make-up air drawn from the outdoors. Normally I see a motorized damper that closes the make-up duct when the system is not running.

Are you trying to say they are drawing return air from the attic as well as from the living space and exterior? This would be a return register, not a supply register.

Yeah, I guess it would be a return register in function. Should have just said register.

No motorized damper or timer that I could see. Just a mechanical damper on the roof vent-the same type used for a roof surface dryer exhaust.

It (the register) would be drawing make-up air from the attic, but it was closed.

If it is drawing make up air from 2" - 3" above the roof cover, it is introducing 130 degree plus air into the system in the summer. That can't be the best way to go about supplying make-up air, can it?

Thanks for the help, Guys.

Tim

[Garet]

No motorized damper or timer that I could see. Just a mechanical damper on the roof vent-the same type used for a roof surface dryer exhaust.

The mechanical damper for dryer exhaust is intended to let air out, but not in. That would not work for a make-up air intake on the return. Something does not make sense.

[Tim H]

No motorized damper or timer that I could see. Just a mechanical damper on the roof vent-the same type used for a roof surface dryer exhaust.

The mechanical damper for dryer exhaust is intended to let air out, but not in. That would not work for a make-up air intake on the return. Something does not make sense.

Exactly. My original question was not very clear. There is what looks like a normal backdraft damper type vent hood at the roof. Looking into the return from the filter slot, I could not see any damper or conductors. I have seen a pretty good handful of these unvented attic-Icynene insulated designs, and have never seen this.

This was a warranty inspection, and the homeowner had the HVAC contractor coming in to do an annual inspection anyway. The homeowner is going to ask for an explanation, and if/when I hear the explanation, I will share it here.

The picture shows the vent hoods for the two heat pump ducts and the kitchen fan duct. They are identical.

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Thanks,

Tim