furnace cycles twice

[ponch37300]

Hi,

I believe I have an issue with my furnace. Everyone is sleeping right now so I don't want to make noise opening up the furnace door to see what brand it is so I will check tomorrow. The mobile home is a 2000 so the furnace is roughly 9 years old and it just has onr of the cheap thermostats that has the slider to set the temp and the switch to go from off to heat to cool. It will come on and do it's cycle and then shut off. Then sometimes but not all the time it will turn right back on again. I am thinking it might be an issue with the thermostat, telling it to shut off and then back on? Not really sure. I was an electrician for 3 years so I have some trouble shooting skills but don't have much experience working on furnaces so don't really know what to check first. Is there some things I can check out to try and trouble shoot this problem? I think I am going to get a new programable thermostat and see if that solves anything.

Thanks for any advice and help with this, Scott

[ponch37300]

Well I checked today and it is a coleman furnace so maybe I will start a new post in that section?
Thanks for any advice with this

[Robert]

Hi,

Is it just the blower coming back on or the blower and burner ?


What is the furnace model number ?


Thanks,
Robert

[ponch37300]

Hi Robert,

When it cycles twice the burner and blower come back on.

The model number is DGAT070BDF

Thanks, Scott

[Robert]

Hi Scott,

When it does it, test across control board W and C for 24VAC.


Then trace gas valve wires back to molex plug on the board. Unplug and find board side of plug pins/holes corresponding to the two wires from gas valve.

Test and see if you have 24VAC there and then plug back in and test at gas valve for 24VAC.



Thanks,
Robert

[ponch37300]

Thanks Robert. I don't have a garage so only keep the basic tools here. I will pick up my fluke and test those. For the W to C test it should be running in the second cycle when I test right? That will be fun to try and get in there when it cycles twice since it doesn't do it everytime!

Thanks again and I'll post after I do the tests.

Scott

[Robert]

Hi Scott,

Yes, only test when/if it goes for a second cycle. Go ahead and remove control box cover and find the places for the tests at W and C and also the molex connector plug where gas valve wires come to.


Those will pretty much do it, so don't worry if you don't get gas valve tested.


You may can get control box cover to stay on w/o screws for a cycle or 2 to be quicker for testing.


Thanks,
Robert

[ponch37300]

Not sure if it's a good thing or not but the furnace hasn't turned off and come back on while I'm at home since I picked my fluke up. I did do the test while it was running in it's normal cycle and I got 24.xx volts on them all. I'm going to replace the thermostat with a programable one, more so because I've wanted one than for trying to fix this.

Hopefully it either started working right on it's own(not sure if it was something to do with the first uses of the winter) or it will do it again when I'm here and will test it out and figure the problem out!

Thanks for the help and I"ll post more after I know more!

Scott

[Robert]

Hi Scott,

Them furnaces are skeered of the ole Flukemeister !! Be sure you get programmable that runs off batteries or you will need to run a common wire from transformer to t-stat C terminal.


Intermittant problems, my biggest pain for last 30+ years of hvac service. Drives homeowners bazookas as well.


Those were correct readings when running correctly.



Thanks,
Robert

[ponch37300]

I got lucky this morning. Was relaxing on the couch and the furnace cycled twice so I jumped up and grabbed the fluke.

W and C was 24 volts. I looked threw the window and the burner was on. I stuck the fluke in the molex plug(while it was still plugged in) and the two browns got 24 volts. I unplugged the molex plug and tested the board where the brown wires plug in and didn't get 24 volts, think it was 2.xx or 0, don't remember exactly but it wasn't close to 24. When I unplugged that molex connector the burner shut off but the fan stayed on. I plugged it back in and after about 20-30 seconds the burner kicked back on and when I tested the browns at the plug it was back to 24 volts. I didn't test at the burner, I tried but couldn't get any metal with the 2 tabs plugged in and didn't unplug them to test. I figured if I had 24 volts at the board end of the wires and the burner was working that the wires must be good and would have 24 volts at the burner end of the brown wires.

Hope this helps and can come up with something! Thanks, Scott

[Robert]

Hi Scott,

IF you have 24vac across W and C, then you are getting a call for heat from t-stat.


IF temps show t-stat should be satisfied and off, then t-stat may be bad.


Then, with 24vac at W and C, you should have 24vac at the molex connector on board going to gas valve wires also.



Thanks,
Robert

[ponch37300]

Thanks Robert. I had 24 at W and C and also at the two brown wires when the plug was plugged in. When I un plugged the plug I lost 24 V at the brown wires and the green light began flashing. I figured this had something to do with the fact that I unplugged the molex plug and it turned the burner off. I will get a new T stat and see what happens. Thanks for the heads up with the battery power! Is there anything else I could check in the mean time or did this pretty much mean the T stat is the culpret? Thanks again, Scott

[Robert]

Hi,

If you get 24vac at W and C when t-stat calls for heat immediately and it remains for entire heat cycle, t-stat is ok.


Then, after about 30 seconds, you should have 24vac at connector holes corresponding to gas valve wires and it should remain for whole heat cycle.



Thanks,
Robert

[ponch37300]

Well I bought a new thermostat and installed it 2 weeks ago. Don't want to jinx anything but so far it seems to be working right. Maybe the cheap thermostat that came with the trailer was shot, I'm hoping that is all the problem was.

Thanks again for all your help trouble shooting everything, really appreciate your advice Robert.

Thanks, Scott

[Robert]

Hi Scott,

You're very welcome, happy to help.



Take care and best wishes,
Robert