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Old 10-03-2008, 07:02 PM   #1
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Furnace coming on, then going off


My furnace will heat up, the blower motor will come on and blow warm air for about 1 or two minutes, sometimes a little longer, and then the blower will shut off. it will almost immediately come back on and do it all over again. also, the self diagnostic light is not blinking any codes at all. any suggestions on what this could be? possibly my thermostat going bad?

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Old 10-03-2008, 07:47 PM   #2
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Furnace coming on, then going off


It could be a number of things. What kind of furnace is it? Model #? How old is it?

Check for a plugged air filter and blocked air return vent grills. Those are things that are easy to do. If the filter is plugged, obviously change it, but if you don't have another handy, try running the furnace for about 5 minutes without it, and see if that helps.

If that doesn't help, it could be many things, like a loose wire(s)/connection(s), broken wire (making intermittent contact), slightly plugged exhaust vent/chimney, debris (hair, dirt, etc.) built up on the A-coil (adequate air can't get through), a short list of parts going bad could be....t-stat, pressure switch, gas valve, limit switch(s), control board, relay(s) [usually on older models...relays on newer units are part of the board], blower motor, loose blower fan, etc. Hard to tell at the moment. Start with checking the filter and return grills for blockage, and let us know the outcome.

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Old 10-03-2008, 08:05 PM   #3
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Furnace coming on, then going off


thanks for the reply! I will check all those things and get back. hopefully something easy.
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Old 10-03-2008, 08:06 PM   #4
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hi boulder, listen to this guy, he knows his shyt. Hey Kyle! that time of year again, eh? lots of furnace problem posts to keep yer brain occupied, huh? =o)

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Old 10-03-2008, 08:26 PM   #5
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Furnace coming on, then going off


you have 2 limits on the furnace #1.....the fan on cycle and the burner #2 high limit if the fan doesn't work is considered a safty.you might have a silver Honeywell control dial above the burner with 2 settings on it one 140F-160F FAN ON ,and the high limit 200F-220FBURNER OFF.the burner is coming on the heat X is heating up, but the fan is taking the heat off it too fast.the setting is to low thats why your hearing the fan cycle on for a couple of minutes then off then back on.
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Old 10-04-2008, 06:14 AM   #6
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Oh great... here we go again with the Kat and mouse thing. Next will be crazy canadians...eh!
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Old 10-04-2008, 06:25 AM   #7
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now hold on there.... i resemble that!

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Old 10-04-2008, 09:34 AM   #8
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Furnace coming on, then going off


I would start at the limit switch.
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Old 10-04-2008, 10:35 AM   #9
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Quote:
Originally Posted by MdangermouseM View Post
hi boulder, listen to this guy, he knows his shyt. Hey Kyle! that time of year again, eh? lots of furnace problem posts to keep yer brain occupied, huh? =o)

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Yeah I suppose...sorry about that dude...I'll get to your PM here in a little bit....now where did I put my shyt???

Quote:
Originally Posted by biggles View Post
you have 2 limits on the furnace #1.....the fan on cycle and the burner #2 high limit if the fan doesn't work is considered a safty.you might have a silver Honeywell control dial above the burner with 2 settings on it one 140F-160F FAN ON ,and the high limit 200F-220FBURNER OFF.the burner is coming on the heat X is heating up, but the fan is taking the heat off it too fast.the setting is to low thats why your hearing the fan cycle on for a couple of minutes then off then back on.
I thought about this too Biggles, but the OP mentioned blinking diagnostic codes, which lead me to believe the furn probably doesn't have a fan limit switch of this nature...but it good that you mentioned it...always a possibility. Don't you love diagnosing furnaces without actually seeing them?

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Oh great... here we go again with the Kat and mouse thing. Next will be crazy canadians...eh!
...and the wheels on the bus go 'round and 'round...

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Originally Posted by Marvin Gardens View Post
I would start at the limit switch.
I agree the limit sw is 'active' in this problem...my question is more so of why.
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Old 10-04-2008, 10:40 AM   #10
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Furnace coming on, then going off


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I agree the limit sw is 'active' in this problem...my question is more so of why.
As we say in the medical profession when asked what is wrong with the 92 gentleman we just brought into the hospital by ambulance.

TMB. To Many Birthdays.

My guess is that the limit switch has has TMB.

Or in manufacturing terms it is two days past the warranty date.
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Old 10-04-2008, 10:43 AM   #11
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Furnace coming on, then going off


or MAYBE even three days? lol too funny Marvin. (but unfortunately true)

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Old 10-04-2008, 05:50 PM   #12
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Furnace coming on, then going off


ok....replaced air filters and cleaned out the vent covers. it is a coleman evcon model# DGAT056BDD about 15 years old. now the blower motor is not even coming on. the small motor comes on and the igniting element heats up, but just as the gas comes on, the furnace shuts down and starts all over again. i would really like to try to fix this myself, i am pretty handy and can do without heat for a little while. any suggestions on where to start?
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Old 10-04-2008, 06:08 PM   #13
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Furnace coming on, then going off


Ahh...

This sounds like a flame sensor problem, which maybe could be the entire problem. It might need replacing, but you can try to clean it first.

Looking at the burner assembly...opposite end of where the igniter is....you will find the flame sensor. It looks like a thin metal rod...either bent 90 degrees or straight. Usually held in with a 1/4" hex drive sheet metal screw, and has a wire coming off it. Carefully take it out, and you can try to gently clean the build-up on it with some fine sandpaper, or a wire brush, or similar. Re-install it and try again. Your computer board may also have an LED for the flame sensor, that lights up when the fire is burning and the sensor is 'telling' the board fire is present. If it's a steady light, things should be working well...if it's weak or flashing, the sensor isn't sending a strong enough signal, hence the board doesn't know that flame is actually present.
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Old 10-05-2008, 11:06 AM   #14
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Furnace coming on, then going off


i took the burner assembly out and cleaned off the sensor. the furnace came on and the blower motor came back on and seemed to work for a couple of minutes. then it started to do the same old thing. I am going to try to replace the sensor. does anyone have a suggestion on where i can pick one up in west michigan or possibly order it online from somewhere?
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Old 10-05-2008, 12:10 PM   #15
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Quote:
Originally Posted by Boulderv7 View Post
i took the burner assembly out and cleaned off the sensor. the furnace came on and the blower motor came back on and seemed to work for a couple of minutes. then it started to do the same old thing. I am going to try to replace the sensor. does anyone have a suggestion on where i can pick one up in west michigan or possibly order it online from somewhere?
Just about any hardware store has them. They are called thermocouples.

Measure the length of the one you have and get one that is close to that. They are all the same with a few rare exceptions.

If you have a multimeter you can actually check to see if it's functioning. But my money is that it is toast and for about $5 you can make sure that isn't it.

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