I have two A/C units - one upstairs and one downstairs. Today I tried replacing my downstairs thermostat (Honeywell) with a new Nest thermostat. Full disclosure, I did not shut off the power to the A/C unit at the circuit breaker before doing this. Once I got the Nest hooked up, however, it did not automatically power on like it was supposed to. I took the Nest off and hooked the Honeywell back up, but the A/C unit would not respond to changes made on the thermostat. I tried setting it to heat, cool, fan on, etc, and the outside unit would not respond at all, even after waiting the obligatory 5 minutes after changing from heating to cooling. There was nothing wrong with the Honeywell before this.

To make sure the thermostats themselves were not faulty or didn't get fried since I did not kill the power at the circuit breaker, I installed both the Nest and the Honeywell on the upstairs circuitry, and sure enough both thermostats are working fine. The Nest powered up immediately like it should, and the unit responded to all the adjustments I made on the Honeywell.

The problem appears to be that no power is coming to the downstairs thermostat. If it were a problem with the actual A/C unit outside, I would expect the Nest to still power up but for it to just be unable to successfully adjust the temperature.

Any suggestions?

 

Check in your air handler to see if there is a fuse. Some systems have a fuse in the 24VAC line coming from the transformer to protect the transformer should the output get shorted (that's probably what happened when you were changing thermostats with the power turned on). If you don't have a fuse (not all systems do), you may have burned out the transformer. Check the output of the transformer and see if you have 24VAC. If you don't, check the input to see if you have 120VAC present (you should). If you have 120VAC at the primary but nothing at the output (secondary), you'll need to replace the transformer.

 

Can you tell me where I should start looking first just based on this photo? If I remove the panel with the orange sticker would this be the most likely place to find it?

 

It appears that the picture is rotated 90 degrees. I assume that the left side of the picture as shown is actually the top. The fuse (if you have one) will most likely be on a circuit board, which will probably be located between the burners (the four things behind the black pipe) and the blower assembly at the bottom of the furnace (right hand side as shown in your picture). Unfortunately, the area where it's most likely to be isn't in the picture (should be to right of your picture as shown).

 

Thanks, Bob. Here are a few more shots showing the entire length of the equipment. And actually, the air handler does sit as shown - the photo is not rotated.

The only other panel I can see that looks like it would be at all easily removed is the one to the right of the burners, as you mentioned. It's the one with the clear circular opening that is displaying the LED status light.

 

Well, I found one 3 amp fuse that was blown. It's the purple one plugged into the board. I guess I should assume this is the culprit? I don't see any others to replace.

 

Blake, you found it! Any auto parts store should have the fuse. Chances are pretty good that when you replace the fuse you'll be back in business.

 

Buy more than one fuse. Chances are something caused it to blow.

 

Fuse replaced and A/C is humming along beautifully now! Thanks for all the help, Bob!