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Old 12-22-2009, 06:29 AM   #1
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Cold air in duct in furnace room?


Our basement is typically much colder than the main or second floors of our newly purchased (built '97) townhouse. Yesterday I noticed a 4" duct in the furnace room that came down from the ceiling and pointed toward the air supply duct (about 6"-8" away) and has cold air coming out of it. I wasn't around to test it while the furnace was running, however. (i can check that tonight).

I wasn't able to see where the duct originated, but it runs parallel in between the big square air return duct and air supply duct for at least 4'. After that, the ceiling is enclosed in the rec-room area.

Any ideas? Could it just be cold air 'falling' from cold windows upstairs somewhere through the ducts to the basement? It's just weird that the 4" duct is much colder to the touch than the air return and supply ducts on either side of it.

The furnace is vented with a PVC exhaust stack.
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Old 12-22-2009, 06:40 AM   #2
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Cold air in duct in furnace room?


Is this duct metal or PVC? My house has a 4" PVC pipe that runs to the attic in case radon remediation is needed. Perhaps similar for you?
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Old 12-22-2009, 06:58 AM   #3
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Cold air in duct in furnace room?


it's a standard 4" metal duct. It even has 3 or 4 elbows in it to make it face toward the air supply duct.

Don't know much about radon, we live in Ontario.

I'll have to see if warm air comes out when the furnace is on. Wish I had checked it last night.
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Old 12-22-2009, 06:54 PM   #4
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Cold air in duct in furnace room?


Post some pics of it. Do you have a gas or electric water heater? That pipe may be a combustion air pipe if you have a gas tank and chimney.
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Old 12-22-2009, 07:53 PM   #5
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Cold air in duct in furnace room?


It is probably a fresh air supply required by codes for proper combustion.

You can duct it into a 5 gallon pail to prevent the continuous siphoning of cold air in-flow, but will draw the air into the utility room when it is needed. - A very common situation.
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Old 12-23-2009, 06:28 AM   #6
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Cold air in duct in furnace room?


Thanks concretemasonry..

The furnace is gas.

While the furnace was running last night, I placed my hand over the same duct. Same small amount of cold air coming in.

I'll have to snap a couple pictures when I get home tonight. I'll also have a peek outside for any vents. Would it be on the side of the house or roof? (or does it not matter?)

To make things even more confusing, I'm trying to figure out where one of my air supply branches from the supply main goes to! In our laundry room I count 7 supply branches. One goes to the laundry room (basement), two go to main floor bathroom and front hallway, two to master bedroom (2nd floor), and one to 2nd floor bathroom. I can't figure out where the 7th goes to as all other registers/vents are fed from the other side of the house. I immediately thought of bob22's post about a duct to the attic for radon remediation, and wondered if the unaccounted-for supply duct branch goes to the attic, and the cold air is brought down from the attic in that cold air duct in the furnace room, but figured that doesn't make sense. I'll have to do a bit more investigation I guess.
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Old 12-23-2009, 10:42 AM   #7
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Cold air in duct in furnace room?


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Originally Posted by concretemasonry View Post
It is probably a fresh air supply required by codes for proper combustion.
That would be my guess as well.
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