Thread: Furnace Pulling Return Air from Attic

I saw a new construction house today where the 2nd zone furnace (Lennox gas 90+ efficiency) inside a closet was pulling return air from the attic. The duct was tapped right off the back of the return plenum. I questioned a rep for the builder and he said the system calls for it as the house is very tight from an air infiltration standpoint. I asked him about the effect on the system efficiency in AC mode when it will have to cool and dehumudify the hot attic air. He said the duct has a damper that is open sometimes and he wasn't even entirely sure himself. Sounded odd to me.

Anybody ever see an installation like this?

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Originally Posted by Nick Ostrowski

I saw a new construction house today where the 2nd zone furnace (Lennox gas 90+ efficiency) inside a closet was pulling return air from the attic. The duct was tapped right off the back of the return plenum. I questioned a rep for the builder and he said the system calls for it as the house is very tight from an air infiltration standpoint. I asked him about the effect on the system efficiency in AC mode when it will have to cool and dehumudify the hot attic air. He said the duct has a damper that is open sometimes and he wasn't even entirely sure himself. Sounded odd to me.

Anybody ever see an installation like this?

I think they have gotten confused with "Make-up combustion air" and the supply air. This is a link to an install manual for a Lennox 90+ unit. You will not find anywhere in it what you found.
http://www.thecomfortzone.com/heatin...ers_Manual.pdf

Thanks Scott. It really didn't make sense the way the builder rep was explaining it. Pulling return air from the attic just seems that it will make the system work harder to heat up cold air in heat mode and cool hotter air in AC mode. And why would anybody want to circulate attic air through the house?

Sounds like a load of BS and make-up air. Around here, builders reps are usually somebody's idiot kid or nephew that can't hold a regular job.
There are a lot of potential issues with pulling return air from the attic. For something like this I would write it up as a defective install and bullet point some of the potential problems. Let them work out the details.

Originally Posted by Markus Keller

Sounds like a load of BS and make-up air. Around here, builders reps are usually somebody's idiot kid or nephew that can't hold a regular job.
There are a lot of potential issues with pulling return air from the attic. For something like this I would write it up as a defective install and bullet point some of the potential problems. Let them work out the details.

Markus - good advice.
I agree with you fully.

Verrrrrrry strange. Sounds like an evaluation by an independent HVAC tech may be appropriate.

And the PVC exhaust pipe for this furnace had 3 separate leaks.

Originally Posted by Nick Ostrowski

I saw a new construction house today where the 2nd zone furnace (Lennox gas 90+ efficiency) inside a closet was pulling return air from the attic. The duct was tapped right off the back of the return plenum. I questioned a rep for the builder and he said the system calls for it as the house is very tight from an air infiltration standpoint. I asked him about the effect on the system efficiency in AC mode when it will have to cool and dehumudify the hot attic air. He said the duct has a damper that is open sometimes and he wasn't even entirely sure himself. Sounded odd to me.

Anybody ever see an installation like this?

I'm not sure what your picture shows.

I occasionally see a duct running outdoors hooked up to the return duct on the furnace with a damper so you can use outdoor air to cool and ventilate the inside of the house in the evening or whenever it makes sense which is fine.

But pulling air from the attic is asking for IAQ problems. Attics are dirty, dusty, sometimes critter infested and have floating insulation particles that are now being mixed in with the indoor air.

I attended a seminar where the speaker said when he is called in to evaluate sick building syndrome on commercial properties he would first take a look at where the air return for the HVAC was, and often it was located above the suspended ceiling in the office, and that was causing people to get sick.

Once, he said the return air was being pulled in from outside but the duct was next to a loading dock which constantly had trucks and traffic in and out. The people inside were being CO poisoned.

Originally Posted by Mike Lamb

I'm not sure what your picture shows.
.

The pic is taken from inside the attic looking straight on at the duct as it comes through the wall into the attic. I didn't get a good pic of the pipe coming off the back of the return plenum.

I just posted a note from the Energy Star people concerning makeup air on another thread----Message #8 on "Makeup Air In Energy Star Home" You might want to look at that.

Are they referring to combustion makeup air or allowing fresh air into the supply air in the house?

Nick, I went to Rich's other post and was thinking, is this duct into attic performing what was talked about in that thread? I have found both of these threads to be quite interesting. Thanks!!

Originally Posted by Tom Rees

Nick, I went to Rich's other post and was thinking, is this duct into attic performing what was talked about in that thread? I have found both of these threads to be quite interesting. Thanks!!

Not sure Tom. All I know is the setup I saw was pulling return air directly from the attic. I documented it in the report and listed some of the pitfalls associated with that setup. It's up to the builder now to convince the buyer of the purpose of that arrangement. I don't like it.

Rich
Please post the proper link to your reference..

Originally Posted by Raymond Wand

Rich
Please post the proper link to your reference..

Not sure what you want. The message is here and others found it. It was a quote from an email to me from Energy Star...

Thanks Rich, okay I got it, I thought you were referring to another outside link.

A forced air system must be balanced. If you aren't pulling air from a space, conditioning it and returning the same volume through the supply registers, you are not "ventilating. HVAC without the 'V' makes one a HAC(k).

If you are not removing the same volume from a space as you're trying to pump it, you pressurize the space and the designed flow will not enter the space. You will also have air stagnation since it is not ventilating.

You can bring in makeup air from outdoors for IAQ and combustion safety but at a price. Unless you dehumidify and pre-heat it, you will add to the latent load of the house. If an outdoor/attic intake is not screened, insulation, lint, critters, etc. can enter the return system. If there is something stinky in the attic, such as animal poop or mold, it will get recirculated back into the home.

Originally Posted by Bob Harper

If an outdoor/attic intake is not screened, insulation, lint, critters, etc. can enter the return system. If there is something stinky in the attic, such as animal poop or mold, it will get recirculated back into the home.

G2407.11 Combusion Air Ducts
- Exceptions
5. Ducts shall not be screened where terminating in an attic space.

I believe the reasoning for #5 is that insulation could cover the screened opening and block it. In theory the combustion air duct is under negative pressure and would suck any falling debris or insulation tight against the screen reducing flow.

Originally Posted by Bruce Ramsey

G2407.11 Combusion Air Ducts
- Exceptions
5. Ducts shall not be screened where terminating in an attic space.

I believe the reasoning for #5 is that insulation could cover the screened opening and block it. In theory the combustion air duct is under negative pressure and would suck any falling debris or insulation tight against the screen reducing flow.

Apples and oranges, Bob is talking about ventilation air, not combustion air.
I think this has been somewhat ambiguous throughout this thread.

Originally Posted by Jim Luttrall

Apples and oranges, Bob is talking about ventilation air, not combustion air.
I think this has been somewhat ambiguous throughout this thread.

Agreed. I'm having trouble keeping track of whether or not people are talking about combustion make-up air or return air that will be conditioned and recirculated throughout the house.

I’m guessing the builder was referring to a make up air duct. He said there was a damper on it (guessing barometric), which would open when, for example, a bath fan or kitchen hood were used to allow air in to relieve negative pressure and be closed the rest of the time. The HVAC system should be a balanced system and the damper would not open during its operation. However, around here the make up air needs to come from outside the building.

Combustion air is not tapped into a return plenum as Nick described. This is probably someone's attempt to bring "fresh" air into the home for IAQ purposes or perhaps to address some depressurization issues. However, it is not good bringing cold air directly into a return plenum. At the very least, it should be mixed with the rest of the stale return air about 10 ft. min. from the return plenum so it has a chance to temper a bit. This close can skew the Delta T or temp. difference across the heat exchanger and Total External Static Pressure (TESP). That can skew a combustion analysis which might reveal enough of a problem to cause flue gas condensation or damage to the heat exchanger. In summer, you would add to the latent load by introducing hot moist air directly into the unit again, whacking and jacking the parameters and performance.

The problem with fresh air introduction is cost. Nobody wants to spend the money on the hardware and certainly don't want to pay one penny for the increased cost of comfort resulting from either tempering the outdoor air or paying for a machine to temper it (heat/ cool/ filter/ dehumidify/ odor removal) for you.

I think it is just a cheap and stupid way of assuring some level of air exchange in a tight house. If that is their answer to controlled ventilation then I doubt that the house is very tight anyway.