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Old 02-07-2013, 07:27   #1
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Gas furnace flame sensor question.

We moved into a house in March of last year, and the house is at/around the 20 year mark. During that time the furnace kept kicking on and would then kick off again a few moments later, but I didn’t think much of it as we were coming up on spring and I had no experience with a gas furnace. Fast forward to October when we turned the heat back on, and the furnace kept doing the same thing. One of our friends who does heating and air came out and replaced the flame sensor, and we’ve had no problems with the furnace for the rest of the fall and winter until a few weeks ago. Then the furnace started doing the same thing where it would run for a little bit and then the flame would kick off. My friend came out and showed me how to pull the flame sensor and kind of scrape it off and then “wiggle” the connections to the circuit board in case it did it again. In the past week I’ve had to do this about four times, and I notice there is a kind of carbon build up on the sensor every time I pull it. My question is this; shouldn’t a flame sensor last longer than a season, or is this a part that needs to be replaced yearly? Or, could it be possible that the replacement sensor he installed last fall was a dud?

Thanks in advance.
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Old 02-07-2013, 07:35   #2
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If you are having that much carbon buildup with a gas furnace then something is wrong with the mixture or possibly your flue needs cleaning/adjustment as it may be causing a puff-back due to improper draft or blockage.

If that is the case it may be more serious as to Carbon Monoxide buildup. I would have a pro look at for proper burn and ventilation IMO.

BTW Don't let my Fl location fool you, I am originally from NY and had a gas heating system for over 15 years

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Old 02-07-2013, 07:35   #3
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Hmmm, carbon buildup on the flame sensor doesn't sound good at all. Typically you'll get a bit of whitish residue that builds up there, not black stuff. I'm not sure what that means but I can tell you that at 20 years that furnace is probably about shot, or at the very least due for a thorough going-over by a pro. One thing you should know: A modern unit is not only much more efficient, not just with heat but with AC too, but with a variable speed fan it is also more quiet and can circulate air 24/7 if you want it to.
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Old 02-07-2013, 07:53   #4
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Is this propane gas by chance? It sounds like you have a rich fuel/air mixture at your flame. Why, I don't know. Maybe you need more air at ignition, or as stated your furnace is not drafting properly. Somewhere I heard this sensor buildup can be a problem in a home where candles are burned.
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Old 02-07-2013, 08:31   #5
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My furnace is about 8 years old and I just started having a problem with the flame sensor. There was surprisingly little buildup on it but it was enough. Wiped it down and it's been perfect since. Yeah, you have something else going on.

Last edited by cowboy1964; 02-07-2013 at 08:32..
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Old 02-07-2013, 08:42   #6
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I don't have that problem with my gas furnace. I don't think I've replaced a flame sensor. I've had mine for 13 years now.

So they shouldn't wear out that fast. Something is causing your issue.
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Old 02-07-2013, 08:54   #7
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I agree, you should have the whole system looked at by someone who knows what they're doing.

Black carbon buildup on the thermocoupler sounds like bad juju to me.
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Old 02-07-2013, 20:30   #8
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Thank you for the advice, I appreciate it – so far being a home owner has been more of a kick in the tool-bag than the blissful euphoria I was led to believe it would be.

I talked it over with The Warden and we’re going to have the unit replaced here in the next week or two. Seeing how well the former occupants of this home took care of things, it’s probably a no brainer to just go ahead and start replacing all the appliances anyway.

I was concerned about the buildup on the sensor too (a kind of brownish / blackish color), and it seemed ridiculous that I would haveto keep pulling the sensor out and scraping it off just to keep the unit running. My friend checked the serial numbers on the unit and thinks it is anywhere from 17 to 22 years old, so it’s time to replace it anyway.

Thank you again for all the help.
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Old 02-07-2013, 22:46   #9
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Sounds like bad fuel/air mix. Cause could be a couple of things.
You need a really good HVAC person to check it out. may cause real problems if not corrected/repaired.
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Nov 11, 2013 at 11:42