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I'm in a rental house for the next few months which hasn't been particularly well maintained. It does have a newish looking Goodman gas furnace, yet the gas bills seem outrageous to me ($250 per month when I'm used to $60 in my own house). It's an old house, so that could be chalked up to lack of insulation.

However, the ducts in the bedrooms actively blow only cold air (somewhat below room temperature). The duct in the bathroom on the same level of the house (there are only three rooms up there) blows extremely warm air, as one would expect.

I went down to the basement and checked, and the two ducts labeled "bedroom" are definitely warm — as they should be. So whatever is happening is happening somewhere between the basement and the second floor (first floor, perhaps?).

It snowed quite a bit this last week, and one thing we noticed is that one side of the house has extremely large icicles forming — larger than any other house in the neighborhood. This suggests to me that we are heating the outside of the house instead of the inside.

The only thing I can think of is that the ducting is totally disconnected inside the wall or between the first level ceiling and second level floor. But I can't see how it would both (1) pressurize that area enough to blow cold air and simultaneously (2) never heat it up enough to blow hot air.

To be totally clear, this is not a cold draft from the vent I'm talking about. We do get that as well, when the heater is turned off. But when the furnace is on, the thing actively blows cold air through the bedroom registers.

Anyone see this before? Any thoughts? If not, can we together concoct some risk threatening enough that my landlord would actually hire a contractor to fix this? He doesn't seem like the eager-to-fix-things type, nor is he the type to let me do it myself — but if he won't, I damn well will.

Update: The previous occupant informs me that the vents blowing cold air are the returns. I can't imagine why they'd be blowing instead of pulling, though.

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It definitely sounds like that section of duct has a leak or travels through an un-insulated part of the structure. Sometimes the cavities between walls or under floor joists are used as duct or returns and they are not enclosed. Leaks in the building envelope, especially with such cold weather, could keep that area cold all the time and any warm air blowing through it will get cooled down right away.

If you are feeling a draft out of that vent when the heat is not blowing, that is concerning and may indicate a large air leak. There are a variety of methods for testing for duct leaks. One common method is by using a blower door on an external door to get a baseline leakage number for the home. Then, all of the vents are covered and another leakage number is obtained. The difference indicates the leakage of just the duct system. A service that does this is usually inexpensive (under $100) and can tell you just how much air the ducts are leaking in additional to helping you find other sources of cold air leaks into your home.

If the landlord isn't willing to pay for investigating it, you may want to pay for it yourself since you could then bring him/her hard numbers and ultimately save on your heating bill once repairs are made. If the landlord still will not budge, I'd make the claim that warm building air mixing with cold duct or external air could cause mold growth and make you sick.

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Well, I'm going to check your answer as correct because it's a good one. But the landlord has decided he's not going to renew my lease (for no reason that he's communicated to me), so it looks like it won't be my problem for much longer. Thanks. –  mohawkjohn Jan 26 at 1:03

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