Odor from gas furnance

jonandabbyJanuary 17, 2007

We have a carrier gas furnace. Starting about 2 months ago, VERY occasionally vents on one side of the house start emitting a very strong odor that I would describe as a smokey rubber odor. The smell happens at all times of the day, is expelled from the vents for about an hour, and within and hour or two it is gone. It seems very clearly to come from vents in one part of the house, and if you walk by the heater when it is coming out from the vents the heater does smell.

We have had the heater checked out, and he could find nothing wrong. The gas company has checked for gas and CO, and they found nothing. We've had the vents cleaned, and it still happens.



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When you smell the odor, go to the furnace, open the blower and filter door and remove the filter. If you smell the odor, it is comming from the furnace. Otherwise something is in the duct branch for those rooms.

    Bookmark   January 17, 2007 at 2:49PM
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Sometimes when blower motors go bad the winding heat up and it will put out a smell like you are describing....Did the Tech check the blower motor? Amp draw etc?

    Bookmark   January 17, 2007 at 3:12PM
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I happened to be in the basement yesterday for the start of one of the "incidents." Here is the chronology:

Our heater (a Carrier combination gas furnace/A/C) is about 17 years old. When the heat kicks on, it goes in phases. There is a minute or so cycle where it sounds like there is a small fan kicking on and the pilot light and burners are starting to heat up. After about a minute, the blower starts.

Last night, I heard the sound of an electrical circuit turning on, and after about 20 seconds it turned off. The blower never kicked on, and it sounded like the "startup" phase didnÂt finish. A few other things I noticed - we have a humidifier that turns on ONLY when the heater is on. After that circuit/pilot sound, the humidifier started to run and didn't turn off although the blower never started. (coincidentally or not, the carpet near the heater was soaking wet. I think that the humidifier was running when these things keep happening, and overflowed onto the carpet. But thatÂs just my theory.)

I went up to the thermostat, a digital Honeywell connected to the power from the heater. The thermostat was blank - the time and temperature was off. I tried to turn the switch from auto to "on" to manually switch on the heater, and there was no response (We'd typically hear the sound of the heater's initial cycle turning on).

I went upstairs, AND THE SMELL (like metallic burning rubber for those new to thisÂ.) HAD STARTED. It was very strong.

I went down to the basement to see if the heater was smoking or anything, and it wasn't. The smell wasnÂt particularly noticeable next to the heater. I was down there for about 5 minutes, and went back to the thermostat, which had switched back on. I played around with the temperature and fan (which went on). Initially, the system was only blowing air that wasnÂt being heated. After a period of about 20 minutes, I was able to get the heater to start blowing hot air. The odor coming from the vents had traces of the smell for about an hour, and then the smell had mostly gone away.

It apparently happened again this morning  my wife called and told me she detected the odor again.

Does the new information help? Between the increasing frequency (at least once a day, where it used to be every week or so) and the quirkiness with the humidifier and the blank thermostat, this seems less like a duct problem and more like an electrical problem to me. But I know nothingÂ. J

    Bookmark   January 19, 2007 at 9:29AM
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You need to get that tech back out there before something worse happens.... Sounds like overheating of wiring....It is hard to say what it could be over the internet, be save and get someone competant to help figure it out.

    Bookmark   January 19, 2007 at 11:01AM
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I have American Home Shield, so we have a tech there today. It only costs $55 including labor and parts to fix anything. I'll report back in case people are curious and/or he says something that doesn't make sense.

    Bookmark   January 19, 2007 at 12:05PM
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Update and answer for anyone interested -
It appears to finally be fixed. Apparently the "capacitor" that was supposed to jumpstart the blower was faulty. It didn't charge up enough, so sometimes the blower never started. The odor was caused by the burners firing up and frying all the dust on the elements, which burned because of the lack of airflow.

    Bookmark   February 8, 2007 at 2:53PM
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When the furnace starts and the blower does not, it will run for a few minutes and then the limit switch will turn it off by turning off power to the thermostat, which is why your thermostat went blank.

    Bookmark   February 8, 2007 at 6:26PM
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We also have been experiencing exactly same behavior with our heating furnace ( By BDP COMPANY a division of CARRIER CORPORATION. model 39GAD048125 ABEA, it is a GAS furnace) I guess about 20 years or so old.
The odor has become unbearable especailly in upsatirs bed room since yesterday.
My concern was that the blower motor coils might be getting over heated and hence the odor. More than the odor we were concerned about fire hazard...

But when I opened the furnace door and observed the clicking of the relays, the behavior was exactly the same as you described.

Do you know the faulty capacitor, was one of theose in the furnace control circuit board or else where, may be with the motor? I am ready to change the board myself, but then again I have to find out the right one to go with this model( They are any where from $150 - 250 )
Any info on this appreciated. If you can provide the contact info of the mechanic fixed the problem, it will be handy as well.


    Bookmark   February 9, 2007 at 12:19AM
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vsrajg, are you aware that electronic boards are not returnable? If the board is not the problem, and it doesn't appear to be in your case, you own it no matter what. Don't even think of trying to return it! Might I suggest before you go spending 250 dollars for something you probably don't need you invest that money instead in a service call to find out what the actual problem is? Even if you order the wrong board, once you slap down the cash and bring it home, its yours for keeps. It would be a shame if after all that you find out it was just a 5 dollar capacitor. good luck

    Bookmark   February 9, 2007 at 10:35PM
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I was looking from the side of the heater when he was fixing it - it did NOT look like he was fishing around anywhere near the circuit board (which they had earlier replaced, accomplishing nothing). It looked like a 9 volt battery, only 4 times as big. It was located sort of near the blower on the left side (in mine, the circuit board is on the upper right).

Hope this helps.

    Bookmark   February 12, 2007 at 8:12AM
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Sounds like maybe the capacitor, way, way, waaaayyyy cheaper then a PC board.

    Bookmark   February 12, 2007 at 3:57PM
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The motor run capacitor has nothing to do with control boards, it's mounted on or near the blower motor - the on the side types look a little like a red bull can or an old auto ignition coil.

I suppose if you like to tinker, you could read the values off it, and buy a replacement and see what happens, but unless you're absolutely sure it's solved it, avoid running the system due to the risk of fire and/or smoke damage, it could be very dangerous.

    Bookmark   February 12, 2007 at 5:41PM
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You can check a cap easily with a decent analog ohmmeter, the needle ahould swing to infinite ohms slowly as the leads contact the cap. terminals, reverse the leads and it should do it again.

    Bookmark   February 12, 2007 at 5:53PM
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