WE have a forced air oil fired furnace (and water heater if that matters) and while it works well most of the time, sometimes on a windy day we'll get cool air blowing through the registers when the furnace isn't even running. What could be the cause of that? The furnace is vented out through a chimney from the basement to the roof, there are no gaps that we can see. It's been serviced regulary and no problems, but still this cooler air coming in.

Thanks.

It could be caused by a few items. If someone installed a direct outside air make up into the return air. When was the last time you have your heat exchanger checked?

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Originally Posted by hvac1000

It could be caused by a few items. If someone installed a direct outside air make up into the return air. When was the last time you have your heat exchanger checked?

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We've lived here over 7 years and I'm sure there's no outside air return. This is the first year we've noticed it. It was fully inspected/serviced last year at the beginning of the heating season by our regular furnace guy.

I would carefully look over your duct work especially any running in a crawl space or attic. If you don't find a problem there, likely it is the heat exchanger. A leaky heat exchanger is a serious, life threatening problem. Better have the guy take a look at it.

We have a fully finished basement (where the furnace is located) so there are no UNheated areas, although the basement is chillier. I'll have my husband have a look. What exactly is the heat exchanger? And why is a leaky one dangerous? We do have two carbon monoxide detectors in the house as well, in case it's it's a CO leak you're meaning?

The CO detectors are very important in this case. Make sure they have good batteries. The heat exchanger in a modern furnace is several tubes the fuel burns in. The combustion products then are collected and pas out the vent. The air from the main blower is heated as it passes over the outside of the tubes and on to your house. A leaky heat exchanger allows the combustion products including CO to mix with the air going to your vents. It would also allow outside wind to leak out your vents. You really need to find where the cold air is coming from and stop it.

Thank you very much for the explanation. Now I'm even more concerned, but we'll have someone come take a look at it ASAP!