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Old 11-30-2011, 05:48 PM   #1
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When blower comes on, gas shuts off


It is a mid 1980s Carrier(label is so worn cant read it), natural gas furnace.

Original problem...furnace blower was running continuously, while maintaining temp. It also would do false starts on shut down, 4 or 5 quick starts on shutdown after a cycle. I swapped the thermostat.

With new thermostat installed, pretty much same problem. Furnace would work in On, but in Auto would...would light burners...they would burn for several minutes, then blower would come on and shut off the gas. Pilot light stays lit.

Then I changed the circuit board, cuz thought maybe a bad relay. Nope that did not help either.

Lastly, I read in another post about cleaning the flame sensor and I thought I gave it a good cleaning and also cleaned out the inside of the furnace and removed burners and cleaned them. Amazing how much dust is in there beyond the filter...although it probably has not been cleaned in long time.

Now, I kind of feel like the temp sensor maybe causing the problem. The burners come on to prime the furnace with warm air, but the blower never kicks in...then the furnace must get to hot, shuts off the gas and blower comes on to cool things down...that's what it seems like to me anyway.

any help troubleshooting would be greatly appreciated.

Thanks all,
bill

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Old 11-30-2011, 05:56 PM   #2
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When blower comes on, gas shuts off


post pics of it with both doors off and maybe we can recognize it and narrow it down.

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Old 11-30-2011, 06:09 PM   #3
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When blower comes on, gas shuts off


here is a pic of the gas controller, circuit board, and the flame sensor.

Thanks for the quick response.
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Old 11-30-2011, 06:16 PM   #4
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When blower comes on, gas shuts off


you don't have a flame sensor with a pilot light but a thermocouple. I have worked on some of those units and the ones I have seen have the circuit board "time ON" the fan after 45-90 seconds. there is no temp sensor to turn on the fan. where did you get the board from. not Zee Bay I hope. lots of those are used or junk or bounced around in some guys service van etc. sounds like a bad board to me. get a new OEM one if need be. americanhvacparts.com
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Last edited by yuri; 11-30-2011 at 06:22 PM.
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Old 11-30-2011, 06:22 PM   #5
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When blower comes on, gas shuts off


yes agree with yuri looks like bad board......you said you changed board why does it look so bad with water marks around stat connectors....get oem board should solve problem.....
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Old 11-30-2011, 06:38 PM   #6
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When blower comes on, gas shuts off


Of course I got it from ebay, is that where all diyers shop?

I think it was $50 bucks or so.

the thermocouple was replaced a year or two ago. Any ideas what on the board would cause the problem? The false starting has always been a problem, even when the thermocouple was changed. I did not think it had much affect. Just wondering as I like to know how things work. Also, am afraid that I could easily have a broken wire or something that may cause a problem..the wires seem quite brittle.

One other note, on the cooling side, I never get the false starting...I realize that may sound dumb, but they share the same board, or so I thought, that is telling it when to turn on and off? my apologies for being a non HVAC guy.

I'll check out that site, Thanks Yuri,
Bill
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Old 11-30-2011, 06:39 PM   #7
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When blower comes on, gas shuts off


Ben, Good eye. that is a pic of the old board, but the new one is virtually identical less the water marks.
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Old 11-30-2011, 06:44 PM   #8
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When blower comes on, gas shuts off


different relay for heating vs cooling and different circuit on the board. stay away from ZeeBay unless it is a VERY reputable seller with LOTS of references. that board probably costs me more than $50 wholesale/tradesman price so it sounds suspect.
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Old 11-30-2011, 07:03 PM   #9
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When blower comes on, gas shuts off


I think you fixed it guys! I don't want to be to excited yet but we might have done it.

Mostly ignorance on my part, but while looking at the old board for a part number/model number to use to find the correct oem, I found another jumper. Jumper from Gas 1 to Gas 3. When I was switching the old to new board, I remember thinking that was the only difference between the two. I thought it was riveted or soldered on there, but I got it off and put it on the new board. Gas came on for about 60-90 sec and then the blower came on and did not shut the gas off.

So it has been one cycle, and it did not false start on shut down after the cycle.

Thanks for making me go check the board, I would have never remembered the jumper difference.
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Old 11-30-2011, 07:07 PM   #10
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When blower comes on, gas shuts off


I suspect the jumper is to set the blower off time. some guys like it longer to try cool the heat exchanger down more and get more heat out, others find it drafty. but we will have to wait and see what you find. The ON start timing is always set by the manufacturer so as to not overheat the exchanger and cause them warranty failures/returns and extra costs etc.
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Old 11-30-2011, 08:50 PM   #11
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When blower comes on, gas shuts off


if you had a ton of dust in the fan section past the filter check the fins on the fan section on that squirrel make sure there clean....run your index finger along them minmal dust is OK major will keep the fan from throwing the designed air... up thru the unit
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Old 12-02-2011, 10:31 AM   #12
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When blower comes on, gas shuts off


Thanks Biggles, I'll check that next time i open it up. Seems to be working well right now. The flames are much more blue than prior to the cleaning. The only thing left is to change is the wires to the thermostat. They are brittle and keep breaking and they are too short. I'll get to that, hopefully before Christmas.

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