Cold air spilling back in from attic unit

[MLaborde]

My return unit is in the attic above my return vent. The return vent runs down through a wall and sits at floor level in the wall.

If I am not running the heat I get cold air from the attic spilling down and out of the vent. Of course, if the heater is running or has recently stopped running, the cold air does not spill in.

I can not find a break in the duct and the insulation, while old, is intact.

There is no damper of any kind that I can find on/in the duct. Considering that the unit is in my attic and cold air sinks, I understand why the air would rush in if the system is off but I don't understand why there isn't some sort of preventive system in place.

Should there, in fact, be some sort of damper behind the return vent somewhere?

Thanks in advance!

[Shophound]

Does the return air duct in question run along an exterior wall?

You may also have duct leakage concerns in the attic that can contribute to cold air entering the home through the return ducting and grill.

[MLaborde]

Does the return air duct in question run along an exterior wall?

You may also have duct leakage concerns in the attic that can contribute to cold air entering the home through the return ducting and grill.

No, the return duct runs inside an interior wall. Surrounded by a closet, a bathroom, a bedroom, & a hallway.

I have checked the ducts in the attic and can not find an obvious break. The duct is insulated and has blown insulation around/over it also so any break in the dust could be hard to find. However, the amount of cold airflow back through the return vent is substantial and should point to a traceable break.

Is there some way to check for breaks that are hard to find with a visual inspection?

[rihvactech1]

we know cold air drops so there has to be i leek in the duct work. it might be realy tuff to find but it could be comming from an open filter door or somthing eben on the supply if you can not find this and you need to do somthing about it you can have the retern taking out ant put a high retern in. this would not be the best thing for heating but grate for cooling.then you will not get the cold air on your feet. well good luck

[Shophound]

Is there some way to check for breaks that are hard to find with a visual inspection?

Yes, it's called a "duct blaster" test. The ducts are pressurized and rate of air leakage is then determined. It's even more useful if while the ducts are pressurized, the tech conducting the test goes around and finds the leaks.

[tipsrfine]

No, the return duct runs inside an interior wall. Surrounded by a closet, a bathroom, a bedroom, & a hallway.

I have checked the ducts in the attic and can not find an obvious break. The duct is insulated and has blown insulation around/over it also so any break in the dust could be hard to find. However, the amount of cold airflow back through the return vent is substantial and should point to a traceable break.

Is there some way to check for breaks that are hard to find with a visual inspection?

Is the return duct inside the wall using metal ductwork directly, or is just using the wall chase as a return to the attic ductwork?

[MLaborde]

Is the return duct inside the wall using metal ductwork directly, or is just using the wall chase as a return to the attic ductwork?


There is metal ductwork.

[tipsrfine]

There is metal ductwork.

Is the supply boot air sealed? In other words, take the return grill off and check to make sure the air IS comming from the ducts, rather than out of the wall through gaps between the return duct and the wall.

[MLaborde]

Yes, it's called a "duct blaster" test. The ducts are pressurized and rate of air leakage is then determined. It's even more useful if while the ducts are pressurized, the tech conducting the test goes around and finds the leaks.

I was thinking about closing the vents then using a smoke bomb & a fan while looking in the attic for a stream of smoke. The odor has prevented me from doing this so far. :D

We've had this problem for 4 winters now (ever since we bought the home). It is also hard to cool that end of the house in the summers. We had the ductwork checked by a pro and also had a new return unit installed ($$$) by the same pro. I asked the owner about the issue of cold return air and he said that was only because the until was in the uninsulated attic. He's been the only guy that we let work on either HVAC systems in the house until now. Recent events plus this ongoing issue have caused us to lose confidence in him & his company.

But we are in the boonies & choices are limited... My other choices have bad reputations, unfortunately. I was hoping to find a way to ID the problem myself before calling in someone else to take my money & not help me.

[MLaborde]

Is the supply boot air sealed? In other words, take the return grill off and check to make sure the air IS comming from the ducts, rather than out of the wall through gaps between the return duct and the wall.

The air is coming through the duct, at least it is coming down through the duct. When I remove the grill & filter I can put my hand up inside and feel the flow above the opening.

I've resorted to covering the return vent with a towel when I turn that unit off on that end of the house & only leaving it open at night when we must turn on the heat.

[tipsrfine]

The air is coming through the duct, at least it is coming down through the duct. When I remove the grill & filter I can put my hand up inside and feel the flow above the opening.

I've resorted to covering the return vent with a towel when I turn that unit off on that end of the house & only leaving it open at night when we must turn on the heat.

Maybe check with your utility company to see if they have any energy audit programs that are free or low cost. Having someone use an infra-red camera would spot leaks in your ducts even under all the insulation. Also remember air in means air out somewhere else. Having a blower test performed in conjunction with the infra-red scan can spot big air leaks in your home. Hot air rises and exits through high leaks in the home, and cold air is drawn into the home in the lower areas. I don't think that cold air if "falling" so much as it is being "drawn" in due to these air leaks.

[rihvactech1]

do you have both high and low reterns in your house.

all the air might be migrating from the walls if not proply inslated also. so having ducked reterns might help both your cooling and heating problms but it is hard to say not seeing the sys in person. if it is not inslated good you can tell buy checking if you get cold air comming from a light swithc or outlet

[MLaborde]

Maybe check with your utility company to see if they have any energy audit programs that are free or low cost. Having someone use an infra-red camera would spot leaks in your ducts even under all the insulation. Also remember air in means air out somewhere else. Having a blower test performed in conjunction with the infra-red scan can spot big air leaks in your home. Hot air rises and exits through high leaks in the home, and cold air is drawn into the home in the lower areas. I don't think that cold air if "falling" so much as it is being "drawn" in due to these air leaks.

Calling the power company is a good idea. Thanks!

I know there are drafts other places in the house. I have weatherstripped the exterior doors but the windows need to be replaced. I have plans to replace the ancient single-pane aluminum windows and crappy storm windows with insulated aluminum windows this spring.

[tipsrfine]

Calling the power company is a good idea. Thanks!

I know there are drafts other places in the house. I have weatherstripped the exterior doors but the windows need to be replaced. I have plans to replace the ancient single-pane aluminum windows and crappy storm windows with insulated aluminum windows this spring.

Focus on attic leaks first. Your home is not unlike a hot-air ballon. A hole at the bottom is not a big deal, but a big hole at the top and you lose all your hot air. Remember insulation in the attic on top of an air leak is not going to stop the air from flowing through it, and when it does it reduces the R-value of that insulation to next to nothing.

Google "building science" and follow those 2 words with anything you want info on, like "building science attics", or "buildings science crawlspaces" etc...
You will find much good info to read.

[MLaborde]

Focus on attic leaks first. Your home is not unlike a hot-air ballon. A hole at the bottom is not a big deal, but a big hole at the top and you lose all your hot air. Remember insulation in the attic on top of an air leak is not going to stop the air from flowing through it, and when it does it reduces the R-value of that insulation to next to nothing.

Google "building science" and follow those 2 words with anything you want info on, like "building science attics", or "buildings science crawlspaces" etc...
You will find much good info to read.

Thanks for the info. Will do.

[Shophound]

Hot air rises and exits through high leaks in the home, and cold air is drawn into the home in the lower areas. I don't think that cold air if "falling" so much as it is being "drawn" in due to these air leaks.

As for how this could cause cold air to fall down a return duct from the attic, even with the duct being away from exterior walls, is largely due to this air exchange between house and attic. The return grill, being by the floor, is in the negative air pressure zone. Penetrations in the ceiling directly below the attic are in the positive air pressure zone. The air handler cabinet in the attic is not airtight, nor are the plenum seams, duct connections, or ceiling supply boots. Combined, this is the most probable reason why cold air is spilling down the duct and into the house.

One of the first areas to focus on to reduce this problem is to stop all air leaks at the ceiling into the attic. The less air can be made to escape the house, the less makeup air is needed to replace it.