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The furnace started blowing cold air so I checked the furnace and the burners weren't lit only the pilot was on. This happened last year and repairman said that if it got too hot would trip a sensor and needed to be reset. I pushed the reset button and turned the furnace back on another burners lit but the flame was kind of wild and you could smell smoke from the vents so I shut it back off. The filter is a little dirty is this the problem?

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Dirty filter could defiantly contribute to overheating. Not sure what a "wild" flame looks like though. –  Tester101 Dec 29 '11 at 16:00
    
the flame wasn't steady it would get bigger and smaller more like a campfire than a gas flame –  furnace question Dec 29 '11 at 16:04
    
Does the exhaust go up the chimney, or through a PVC pipe to the outside? –  Tester101 Dec 29 '11 at 16:58

2 Answers 2

Time to call a serviceman again. If you are smelling smoke from the heat vents, there is most likely something seriously wrong with your furnace. Combustion air and the ambient air should NEVER be mixing and this presents a dangerous situation where CO (Carbon Monoxide) could be entering your living area.

This is potentially fatal if not fixed properly. I wouldn't mess around with this one on a DIY level.

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Extra points for Carbon Monoxide detectors. Seriously - it can even manifest as mental illness, or worse, death. Buy one.

I agree about calling the service guy 100%. If your furnace is the old school 80% efficiency kind, there are only a few things that go wrong... it is in fact pretty much a great big stove burner heating up a manifold.

I'm assuming it's not your filter (so much) as it is the dirt, but more importantly it sounds like the gas flow regulator isn't working correctly. If he really fixed it last year, this shouldn't be happening again so soon. Is it an oil furnace? I can see this happening. Is it gas? You have a serious problem.

Another possibility is that your burner manifold is cracked or broken, or rusted. If your furnace is new - or ironically, very old - you should get those two things inspected. If you hear repeated "booming" when it turns on, it's time to replace it.

A word of caution: Furnace repairmen are not all created equally. Some of them don't know anything other than "you should let us replace your furnace", which will be said after they screw around for 15 minutes pretending to fix things. If you're lucky, they'll at least wirebrush your manifold and ignitor.

Regardless of how you select one - Years in business, size of yellow pages ad, family owned, big chain, fly by night - there's almost no guarantee that your tech will even be able to repair your unit unless he's been at it for some time. Age means nothing.

Finally, if your furnace is fairly old, give some thought to simply buying a replacement unit if you can afford it.

And check with your gas company - they may have some deals available.

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