Forced hot air return has cold air coming through it

[skidude]

Hi guys,

I'm wondering if anyone here is in the same boat as me... I purchased a P43 this fall and it has been keeping my house comfortable (one story Ranch in VT). I have used very minimal oil (7 gallons when the auto delivery truck came). The issue I am running into is the cold air return for the furnace (on the front side of the house) has cold air coming out of it, presumably from the duct work in the attic coming down and the room the furnace is in is only about 45-50 degrees (not directly heated by pellet stove).

I have the thermostat for the furnace set to 60 degrees and every once in a while if the stove runs out the furnace will kick on. Because of this, I do not want to block the cold air intake. Has anyone else had this issue, and is there anything that can be done. I'm assuming that switching the filter to a different kind would not do much to help...??

 

[Danno77]

So you are saying that the cold air in the ductwork up in the attic is falling down and right out that cold air return when the furnace isn't running? If that's the case, then maybe you need to insulate the ductwork up in the attic a little better?

 

[skidude]

That is what I am trying to say. The ductwork is insulated, (not extremely well, but has rolled insulation and then a space blanket looking thing over it. I think even with more insulation eventually it would cause the same issue (ductwork is in the attic which is cold). Perhaps the cold air is mainly coming from the furnace (and the room the furnace is in), and I don't think I can insulate the whole furnace...

I have tried switching the fan on (not that this would solve the issue, but it would negate the effect. I do not think the furnace was wired to have the fan on when it is off (switch is on "on" not "auto" but still off unless the furnace is running.

 

[heat seeker]

I have a similar situation, but with AC ducting in the attic. The air in the ducts gets cold, then drops down to the lowest point because it's more dense than the warmer air. I solved our problem by Velcro-ing insulated cloth covers over the vents, and sealing off the return duct. You may have to insulate the ductwork a lot better, but I think that you're going to have this problem unless you block the duct off, or install a motorized damper.

 

[skidude]

I do not want to seal off the ducts as I use the furnace as a backup for the pellet stove. I guess I can look into a motorized damper...

 

[seige101]

That is what I am trying to say. The ductwork is insulated, (not extremely well, but has rolled insulation and then a space blanket looking thing over it. I think even with more insulation eventually it would cause the same issue (ductwork is in the attic which is cold). Perhaps the cold air is mainly coming from the furnace (and the room the furnace is in), and I don't think I can insulate the whole furnace...

I have tried switching the fan on (not that this would solve the issue, but it would negate the effect. I do not think the furnace was wired to have the fan on when it is off (switch is on "on" not "auto" but still off unless the furnace is running.

Sounds like the wiring to the t-stat to have the fan run independently of the heat setting was never hooked up.

 

[skidude]

Sounds like the wiring to the t-stat to have the fan run independently of the heat setting was never hooked up.

Seige,
This appears to be correct. I looked today and there is only a red/white wire going to the t-stat. I suppose I could look into adding a green, but this is probably a summer project just in case I break anything...

 

[heat seeker]

this is probably a summer project just in case I break anything...

You think like I do!

 

[Highbeam]

This is another reason why ducted systems are such energy hogs. They are always loosing heat. Last I checked, the requirement is only R-8 for ductwork.

 

[Highbeam]

Seige,
This appears to be correct. I looked today and there is only a red/white wire going to the t-stat. I suppose I could look into adding a green, but this is probably a summer project just in case I break anything...

Funny how we wait until the heat of summer to go crawling around in the attic messing with fiberglass.

 

[begreen]

Seige,
This appears to be correct. I looked today and there is only a red/white wire going to the t-stat. I suppose I could look into adding a green, but this is probably a summer project just in case I break anything...

One possibility could be that the pellet stove is creating negative pressure. If so, you might need an outside air kit for the stove. You could try putting a thermometer at the return air duct. Then crack open a window close to the stove about 1/2". See it the return air temp goes up. If so the stove is pulling makeup air through the ductwork.

 

[DexterDay]

One possibility could be that the pellet stove is creating negative pressure. If so, you might need an outside air kit for the stove. You could try putting a thermometer at the return air duct. Then crack open a window close to the stove about 1/2". See it the return air temp goes up. If so the stove is pulling makeup air through the ductwork.

I was also thinking an OAK would help this. If your stove doesn't have an Outside Air Kit on it? Then that stove is throwing 80 CFM out the exhaust vent, and that air has to be replaced somehow? Installing an OAK will reduce the negative pressure in your home and keep it warmer to boot.

Without an OAK, your throwing all that air outside (you already paid to heat) and pulling cold air in through all the leaky/drafty points of your home. With an OAK, the stove is a sealed system and all the warm air you paid to heat? Stays inside Win/Win

 

[Highbeam]

The humidity stays in the house too since the outside air leaking in will be very dry once you spend more money to heat it up.

 

[woodgeek]

Duct airsealing could also help. The guys who put in my central return plenum made a hash of it...a 'panned joist plenum' install. Just threw a metal plate with a duct connection over the ceiling joists, a grille under, and screwed a bunch of sheet metal pieces between the joists to try to close off the plenum. Leaked like a sieve and the leaks whistled. Solved (by me) with a lot of silicone.

So, if your ducts are R-8 it is hard to believe that they would be that cold. Returns only need to be R-4 according to code, and installers often skimp in the return side (thinking it ok since the air is closer to outside temp). My guys left the return plenum **uninsulated**, while making the rest of the system R-8 to satisfy code.

To summarize, if your return is well insulated, I think you are drawing cold attic air in through leaks in your duct system, and that is carrying cold air into the house. Ties into BG's negative pressure comment above. If true, indicates a major $$ energy loss when the furnace or AC is running.