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Old 10-27-2011, 07:59 PM   #1
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Luxaire Gas Furnace - Short Cycling


Hello,
I am having some problems with my furnace, i think its from overheating??

Got back in town today and turned the furnace on this evening, and it sounded like things were working, but it wasnt getting warmer. Here is what happens-
Turn up the thermostat to activate furnace, burners come on and work like they should, but blower doesnt come on when the burners are doing their thing. Then after the burners stop( I am assuming for safety/overheating), the blower runs for a minute or so then stops. Then the cycle repeats, it did this for about 15-20 minutes then I turned it off.
What would be my starting point to see if this is something I can fix myself?

Thanks in advance for your help,

Dan

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Old 10-27-2011, 08:21 PM   #2
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Luxaire Gas Furnace - Short Cycling


We know the blower works when the high temperature limit switch (safety) trips. This is supposed to happen, the gas shuts off and the blower comes on to push out excess heat.

We also know that in call for heat the board is not sending the signal to the blower to come on.

Try cooling mode and see if it comes on. Try fan only, then try heat again.

Either way, my guess is the control board.

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Old 10-27-2011, 08:32 PM   #3
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Luxaire Gas Furnace - Short Cycling


Something you can do to check this out is with a meter and on call for heat, remove the blower wire that's attached to the "heat" terminal on the control board. Obviously the service doors of the furnace will be removed so tape down the door switch to get power to the board. Check the voltage on the heat terminal while on call for heat and with door switch taped down. If there is no voltage on that "heat" terminal than the board is bad.

You should have 120-ish volts at that terminal.
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Old 10-27-2011, 08:44 PM   #4
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Thanks Doc,
BTW - Val Kilmer as Doc, one of the best roles ever, lol

I tried the AC, it works fine, as well as the "fan on", both turn on the blower. I have had an issue before with the high temp limit switch, and I had to reset the safety button/breaker. But that switch isnt tripping this time. It happened once before when it was really humid out, think it has something to do with air desity/pressure, but I am no scientist.
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Old 10-27-2011, 08:53 PM   #5
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Luxaire Gas Furnace - Short Cycling


It was after Tombstone came out that I began using this screen name. Favorite gunslinger of all time, Doc Holliday is.

We need to know if the control board is sending voltage to the heat terminal. If it's not the high temperature limit switch tripping it's the flame roll out switch cutting the gas off and turning the blower on.

Either way, while on call for heat is where the problem lays. We know at least one of the safety switches is working so that's not the problem.
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Old 10-27-2011, 08:54 PM   #6
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Quote:
Originally Posted by dvan76 View Post
think it has something to do with air desity/pressure, but I am no scientist.

I'd put my money on air flow problems.
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Old 10-28-2011, 12:12 PM   #7
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I wasnt able to locate which terminal it was, none of them were labeled. I tried to find a wiring diagram, but its an old furnace.
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Old 10-28-2011, 03:35 PM   #8
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Model and serial number of furnace, please.
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Old 10-28-2011, 03:37 PM   #9
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In general, this is what a board would look like. Note the heat and cool on the right.

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Old 10-28-2011, 05:59 PM   #10
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Hello,
It doesnt have anything that look like that. I took some pictures of all the componenets, but I cant upload them to the site.

In the bottom cabinet, where the blower is there is only a small brown block, about 2" by 3" or so. Thats where the wires for the from the blower are.

The model number is PDCU-LD08N080A
serial number is ECYM038362

I checked all the wires today and everything was making good contact. I also did a little bit of internet research and was wondering about the heat anticipator on the thermostat. My kids are always knocking the cover to that thing off, could that need to be adjusted?

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Old 10-28-2011, 08:10 PM   #11
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Sent e-mail.
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Old 10-28-2011, 08:34 PM   #12
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if you don't have a solid stae relay board as shown then the fan has to be thru a thermal disc that closes the fan circuit during the heating mode and LO or Med LO speed look for an access slide plate in front above the controls or on the side... http://www.bestbuyheatingandaircondi...-adjustablefan like these
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Old 10-31-2011, 11:06 AM   #13
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It does have a control limit switch, how would I test that.
Also, when i turned it on for the first time this morning, it worked fine. It started up, burners came on and then the blower came on. Eveything worked as it should but only for a nimute or two, then it went back to the same issue.........burner on....burner off, blower on...blower off, burner on...etc.

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Old 10-31-2011, 11:21 AM   #14
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check the rounds in the squirrel cage for dirt or heavy dust....your fan limit isn't closing on the burner heating up and goes into hi limit cutofff.remove the cover on the bottom of the furnace so you get return there not from the ducted return...see if the fan kicks in after the burner comes up and stays on.if your cold just keep the fan in ONat the stat and set for heating as usual but the end result is you going to draft when the stat satisfies...
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Old 10-31-2011, 01:47 PM   #15
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it happens with the bottom cabinet panel removed as well

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