No voltage at thermostat

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Lancerdad34

Lancerdad34

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I have a 19 year old Trane XE 80. Yesterday I turned the air on and I did hear it kick on. About an hour later I heard nothing running and my Honeywell programmable thermostat said "system on". However the furnace blower and compressor were not running. I checked for 24 volts in the wires going into the thermostat and got nothing. About 6 years ago I had to have the furnace circuit board replaced. Does this sound like the same issue? I take it is not a repair for an amatuer?

Thanks,

Brian in MI

Read more: http://www.doityourself.com/forum/g...ltage-thermostat.html#ixzz36tcfMSrK:confused:
 
Lancerdad34 said:
I have a 19 year old Trane XE 80.

nothing running and my Honeywell programmable thermostat said "system on".

I checked for 24 volts in the wires going into the thermostat and got nothing.
Click to expand...

I got banned, twice, from that forum. . .:D

You're due about four more years out of this unit with the oldest I've heard of running for 80 years. Mine was born in '82 and I have replaced the inducer motor & its circuit board, air handler motor, gas valve, flame sense switch and a control board relay. And, had to sand the control board edge connector contacts that were coated with calcium from a water leak (probably from the AC drain).
I don't want a new furnace because I will not be able to repair these computer-controlled, touchy & complex gadgets.

If the display works but no 24vac then the thermostat is running on its battery.

Check if the 24vac transformer in the furnace
http://www.google.com/search?q="hva...Mount-Transformer-PF42440%2F203565058;300;300
is putting out. If no 24vac there, check that the transformer's primary winding is getting its 120vac.
Depending on what you find, you will be
checking upstream or
downstream of the transformer
or replacing the transformer (but they are pretty hardy).

You could post a schematic found on the inside of a furnace panel to aid us in efficient troubleshooting of this system but most DIYers lose their nerve when they see the schematic. :(
 
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remove the front panel and check the 5 amp blade style fuse mounted on the circuit board. If it fried then you have a short. If you have to go buy one, get about 3. One to see if it holds, one if it doesn't hold and you have to eliminate a short in the wiring and a spare.
Tranes will not run on just any thermostat without a conversion board.
If the thermostat is not new to the scene then that is most likely not your problem but, if you find a bad fuse, replace it and power up without the thermostat attached to eliminate the thermostat as a source of frustration.
 
Since my last post I was able to get the blower motor running from the thermostat set on cool. The compressor would not start unless I pushed in the contactor. I measured the voltage on the circuit board Y C was 14.5 and G C was 11.5. The voltage at the wires outside was about the same.
After all that testing I smelled something burning at the furnace and sure enough the transformer burned up. Not sure if I should try another unless I put an inline fuse in the 24 line coming out to protect it. I sent a couple of pics. Thanks!

Brian

contactor1.jpg

circuit board.jpg
 
Lancerdad34 said:
The compressor would not start unless I pushed in the contactor

I measured the voltage on the circuit board Y C was 14.5 and G C was 11.5.

After all that testing I smelled something burning at the furnace and sure enough the transformer burned up.
Click to expand...
Implying that the voltage is too low to pull in the contactor.

The low voltage and the smell may confirm an overload on your 24vac bus. Finding where this excess current is going will likely not be easy unless you want to start replacing circuit boards.

There is a way to use lengths of wire between circuits as shunts, and therefore as ammeters, but you will need a DVM (on the AC mV scale), a calculator, an AWG table and you need to measure the wire lengths.

Most HVAC transformers are rated at 40 VA, so (40 VA)/(24 V) = 1.7 Amps. Steady currents of more than this are almost certainly a problem.
 
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I had a setback with this thing. I ended up burning up the transformer and not sure why. I went and got another one and mounted it with an inline fuse on the hot 24 volt wire. Now I am not getting any power to the transformer. I used the same wires from the old one! Checked with volt meter and nothing. Checked breaker at main panel and that was good. Checked fuse on circuit board and that was good as well. Yes the safety switch was not engaged. I called a friend who work in the HVAC industry and he is coming tomorrow. What do you guys think happened?

Thanks,

Brian
 
Your problems are increasing.
First you just had a mild overload and now you've also got an open circuit. Opens are easier to trace than shorts or overloads but this is still not a good omen.

Post a schematic and get a clamp-on ammeter.

On this furnace,
http://www.google.com/search?q=furn...Write%2F396GAWguide%2F396GAWguide.htm;600;600
the interlock switch and connections are the only things that can prevent 120v from reaching the transformer primary winding.
 
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I had a friend of the family come by today and we are back up and running! He was able to get 120 to the transformer right away so I must have done something wrong there! He found that the wire from the air handler to the compressor was bad so we replaced that. Then that the contactor was bad as well. I guess that is why I smelled something burnt out there. Anyway thanks for all the help here I learned a lot!

Brian
 
snatch victory from the jaws of defeat

Cliché to win at the last moment. At the last moment, the team snatched victory from the jaws of defeat with a last-second full-court basket.

:D

Now that you have a working system, carefully measure the resistance of, the current drawn by, and the voltage across, many components. You'll need the numbers for next time.
Since it's AC, for coils, the voltage divided by the current is not the same as the resistance measured by a meter.
 
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I learned more then I thought I could ever learn about HVAC here. Now I feel confident tackling any issue if one comes up again.

Thanks!

Brian
 
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Lancerdad34 said:
I learned more then I thought I could ever learn about HVAC here. Now I feel confident tackling any issue if one comes up again.

Thanks!

Brian
Click to expand...
That's the best compliment I've ever seen paid to a forum.:trophy:
 
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