Thread: Return Air

This furnace is newer 2009, but the town home was built in 1984. A portion of the return air is being pulled from the exterior, with another return in the living room. Is the acceptable and any ideas why it has been installed like this.

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Hi Mathew,

Read M1602 in the '06 IRC.

M1602.1 Return Air:

Return air shall be taken from inside the dwelling. Dilution of return air with outdoor air shall be permitted.

M1602.2 Prohibited Sources.........................xxxxxxxxxxxxxx

They are missing a sediment trap in the gas line, so you can call for a repair and an overall check of the installation by a gas heating tech.
That much freezing air being drawn in is not right, IMO. Can there be a damper in there somewhere?
The sloppy wiring is also a heads-up, unprofessional installation.

Those icicles shouldn't be there either.

In the trades we call that 'make-up air'. Make-up air is required in commercial/mercantile type occupancies. 30% is typical around here.
If it were a professional install, I wouldn't have any problem with it. That however doesn't look too Pro, more DIY. Based on the pic I would guess the perpetrator read an article,saw a TV show or something.
Make-up air can be a really good idea for some people and homes. A tight house, occupant that has allergies or a house with pets, etc.
A good install should include a powered damper. I sure wouldn't want the damper open with real cold temps.
I'd write it up as an atypical install for RES, mention the pros and cons and tell the client unless they feel the need to leave it, to remove it. Also I'd tell the client to ask the Seller if there is a specific reason for it being there.
What did the interior return duct look like? Only one in a hallway or a few scattered around?

MUA is a delicate subject. You need it but only in the right way and place.

First of all, ASHRAE proved passive MUA supplies are unreliable at best. The only reliable MUA is powered delivery. However, the introduction of cold air into a return before it has a chance to mix with the rest of the return air (RA), you can crack a heat exchanger. Therefore, cold MUA needs to be tempered either with enough warm RA far enough back for adequate mixing (usually at least 10feet from the return plenum), warm the MUA such as with electrical resistance duct heaters, or a combination. The cold air intake needs to be suitable for moisture condensing, have a damper to close it off in summer during periods of high humidity (unless for combustion safety in which case it remains open), and be properly sized.

Let's say you install a huge, powerful MUA system. Depending upon how it is connected to the RA trunk, it may overpower the system causing air stagnation in the rest of the house that does not reflect when measuring External Static Pressure or Delta T (temp. difference btw RA and Supply Air or SA). Now, if the delta T or ESP are out of whack, it can adversely affect combustion efficiency. Therefore, ANY duct sealing, filtration or MUA must be accompanied by combustion analysis.

Aside from the fact that the installation could be a little neater, I see no real problem. A damper would be nice, even a manual damper but in reality the pressure drop thru that piece of flex is much higher than a thru the wall return.

I have done that on a couple of homes where high humidity seemed to be a concern. I recommend closing the damper during the summer months because infiltration is higher as a result of people going in and out more often.

I think Mr. Keller is correct. The home owner probably felt that it was something they needed. I had a conversation with one of my customers about air infiltration he felt coming in his home from around where the kitchen exhaust went up into the attic. I told him that he probably had some leakage in his supply duct causing more air to be pulled from the home than is being put back in. This puts the home in a negative pressure. Air will then come from wherever possible to balance out the pressure. Now depending on the situation, if the ducts cannot be sealed, for whatever reason, I would rather bring OSA in through the filter and the hvac equipment than have it come from the attic space, floorboards, or wherever else.

With the non-condensing furnace pictured here, the minimum return air temperature back to the furnace is specified, by the manufacturer, to be 60 degrees. When fresh air is brought directly into the return in colder climates, it often creates return air temperatures colder than this specified 60 degrees. This can create a multitude of issues. First and formost is that it can reduce combustion and cell temperatures far enough that flue gas will condense in the vent, and if low enough, will condense in the HX of the furnace.

Obviously, this furnace is not designed to create or handle condensation of flue gasses, and it's creation will cause heat exchanger and vent system reliabilty issues.

A close eye should be kept on the vent system for corrosion and condensing vent gas.

We had about 5 days this winter where temps were -15 to -20. That might cause an issue