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Furnace electrical transformer fried??

6K views 30 replies 7 participants last post by  raylo32 
#1 ·
Yesterday I moved my thermostat to a more convenient location. I got everything moved and hooked up and it awash all working great. A few hours later I noticed that the furnace stopped working. The pilot is on but the electrical system seems to be messed up. I tried turning the furnace switch off and on to no avail.

I think I could have blown some electrical components because I accidentally touched the wires from the thermostat before I remembered to turn off the electrical switch to the furnace.

Can anyone help me diagnose the problem?

I have a comfortmaker guf100a012an gas furnace
 
#6 · (Edited)
raylo32 said:
Never seen one withour a fuse. Did you remove both panels? Do you not see a circuit board? It would help if you take a couple of pics and post them.
This model has no circuit board it is an old style standing pilot and almost never had a fuse for low voltage. It would appear that the transformer is blown but I'd double check by putting a meter across the terminals on the secondary side and look for 24 volts AC.
If you haven't seen this type of unit before you haven't been to many older houses with original furnaces. They stopped making standing pilot furnaces in 1993
 
#14 ·
post some closeup and well lit pics of it with both doors off and the wiring. you need a voltmeter to test for voltages and if you don't know how to use it etc then better to call a Pro. Those old furnaces usually have cracked heat exchangers so it should be inspected by a Pro for safety. Google cracked heat exchanger>Images for more info.
 
#26 ·
do you get 24 volts across R and C on the terminal strip with the door switch pressed in. looks like somebody altered some of the factory wiring. R is fed by that pink wire and you may need to trace where it comes from, I suspect the limit control.
 
#2 ·
You probably just blew the transformer fuse on the furnace circuit board. It is ususally a 3 or 5 amp that looks like a plug in car fuse under one of the furnace access panels. Open the circuit breaker to the furnace, make sure your wires are connected properly, replace the fuse, and switch the breaker back on.
 
#8 ·
DNA said:
I was hoping this would not be the case. Looks like its the transformer. Where would be the best place to find a replacement part?
As a contractor ive always bought from any hvac supply places but they may or may not sell to you so trainers would be my suggestion. As long as the transformer it 120 VAC primary and 24VAC secondary. You can even get a multi tap one that will work with different supply voltages and is more of a universal one.
 
#10 ·
Doorbell transformers are too weak and will not work. Transformers are rated in VA=volt amps like horsepower for the layman. Doorbell ones are 10 VA or close and you need a 35-40 VA if you have AC. 20 VA minimum for heat only. try americanhvacparts.com
 
#11 ·
I think the ones I get are in the 40-50 range. Maybe they are really multi-purpose transformers sold in the doorbell aisle? I've got one at home I can check later but hopefully the OP will have his issue resolved by then. Hopefully he lives somewhere that the winter has eased up a bit. It would have been tough going w/o heat here last week, but not so bad this week.
 
#17 · (Edited)
the primary side/120 volts side is the black and white wires. the secondary 24 volt side is the blue and yellow. You should get 24 volts from them, check that. after that you should get 24 volts from the W wire from the tstat when it is calling for heat to the C terminal or yellow wire, if not then it is shot or you got a bigger wiring problem or broken wire. you can also take the W wire and R wire off at your box and using an ohmmeter with alligator clips on your meter ends put them on those wires and have it call for heat. should get 0 ohms or very low resistance. if it is infinity or OL open line then you got an open circuit for some reason. don't use the ohmmeter while holding the probes with your fingers as it will read continuity thru your body and give a false positive result. I am pretty sure yellow is common but really need to see the wiring diagram to be sure. post a pic of that box with the other side facing out so we can read what the terminals are or you list them from top to bottom starting at the transformer and going down to where that blue wire attaches.
 
#19 ·
Its usally pretty hard to smoke a transformer by bumping the wires together on the thermostat end of circuit,if you bump the wires against something metal inside the furnace on the other hand Id say all bets are off though!:laughing:
 
#20 ·
I get voltage across the blue and yellow wires off of the transformer, seems to be doing its job. The furnace won't switch on still. I tested using a spare thermostat and that didn't change anything so it is not the thermostat. What is the best way to find where the electrical fails?

Here are so pictures for you to look at.

This is the electrical box with the new transformer installed.



Here is the front of the electrical box.



Here is a larger view of the furnace



Is there any sort a wiring diagram that I could use for a reference? Its hard for me to follow what the different electronics do.
 
#21 ·
Disconnect the thermostat wires at the furnace.
Then put a jumper wire from the red/r to the white/w terminals,furnace should come on(you might have to hold in the door switch,couldnt tell from picture)
If that brings the furnace on its time to hook low voltage back up at furnace and check the wires at the stat,can you post a picture of the wires at the stat?
Did you splice anything when you moved stat?:)
 
#24 ·
Disconnect the thermostat wires at the furnace.
Then put a jumper wire from the red/r to the white/w terminals,furnace should come on(you might have to hold in the door switch,couldnt tell from picture)
This doesn't kick the furnace on, where is the next place to check?

If that brings the furnace on its time to hook low voltage back up at furnace and check the wires at the stat,can you post a picture of the wires at the stat?


Did you splice anything when you moved stat?:)
I did not I just got a new wire and hooked everything up where it was before.
 
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