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Old 01-21-2012, 06:10 PM   #1
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Thermostat 24V tripping off


Hello All,
Looking for a little help.
Im having an issue with my oil burner. It is shutting down before the tstat has ended its call for heat. The kicker is that there is NO TRIP in the lockout! I tested all safety features and they all worked properly (resulted in the lockout switch being tripped).

Being a warm air furnace I checked the plenum temp limit switch and that limit is not being reached and not killing power to the primary. (Working properly)

What I did was jump the tstat terminals so the furnace would stay on indefinitely. The result was the thermostat terminals would trip with some type of internal safety switch. I would get 24v before the furnace would start, I jumped the terminals and the furnace would start and waited for the system to stop then I checked again for 24v and it was not there! I waited about 5 mins until I heard the 'click' inside (not the lockout switch) and BANG, 24v again!!

What would cause the 24V transformer to trip like that?

Thanks for any help you can provide!!
Primary model number is Honeywell R8184G

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Old 01-21-2012, 06:17 PM   #2
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Thermostat 24V tripping off


when you say you checked for 24Vs and it wasn't there? or was are you using a meter and where are you reading it stat or burner. is the stat just for the oil fired furnace or AC wired into it also?

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Old 01-21-2012, 06:22 PM   #3
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Thermostat 24V tripping off


Hi, Thanks for the quick response.
I am checking for 24v right on the primary with a multimeter.
The tstat is only for the oil burner and the tstat is wired right into the primary, not through a fan control or anything. I think I have isolated the problem to the primary on the burner.

Thanks Again!
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Old 01-21-2012, 07:51 PM   #4
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Thermostat 24V tripping off


keep in mind when your reading any voltages 24V or 115V on a point that opens and closes from say the stat or limit control when the voltage shows the contacts are open...l l when you don't read voltage the contacts are closed..l/l ....when they are closed you need to go to a ground or neutral to read the voltage..can't read voltage in parallel
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Old 01-21-2012, 08:04 PM   #5
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Thermostat 24V tripping off


Haha, I'm almost insulted!! I did all that testing and you think I made a rookie mistake? Haha, thanks.

I thought of that and removed the leads from the stat terminals on the primary to take the reading when it shutoff. The reading was 0, with both contacts open, when I heard the click I checked again, with nothing connected to the terminals and the voltage was back. Is there anything that would trip the primary without affecting the reset button?

I wish I lived in the south!
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Old 01-21-2012, 08:26 PM   #6
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Thermostat 24V tripping off


sorry nothing meant...had to put that out there....
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Old 01-22-2012, 01:09 AM   #7
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Thermostat 24V tripping off


Now you have to figure out if the line voltage is going out.
To be honest it sounds like a limit is killing the power to the transformer.
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Old 01-22-2012, 08:14 AM   #8
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Thermostat 24V tripping off


Guys, thank you both very much for your help. Get ready to point your finger and laugh.
I am a total rookie. Retested the limit when the power killed and figured out, thanks to grounding the second multimeter lead, that the limit was killing power. Drilled a small hole in plenum and stuck a thermo in there, 230!!!!!!
Pulled the air filter out, turned on within 10 seconds and temp never broke 160. Put air filter back in and temp IMMEDIATELY started to climb.
Who's going for a ride to the store? Who's got two thumbs and is a dumb ass? THIS GUY!!!!

Thank you guys both very much!
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Old 01-22-2012, 09:21 AM   #9
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Thermostat 24V tripping off


LOL.... been there done that type of thing!!!! at least you now know to keep the filters clean!!! and you can check the temp anytime you want with the new hole you drilled, so in the grand scheme of things... not a too bad a job.....
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Old 01-22-2012, 10:57 AM   #10
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Thermostat 24V tripping off


Was the filter so thick with debris it looks like a piece of carpet ?
like this?

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Last edited by hvac5646; 01-22-2012 at 10:59 AM.
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