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Our thermostat died yesterday, but after replacing the dead batteries with fresh batteries, and confirming the LCD screen was operable, our AC no longer turns on.

I have attempted a factory reset on the thermostat, and I do hear a quiet but definitive "click" when I believe it is attempting to activate the HVAC unit, but that's as far as it gets.

One theory I have is that the batteries were long dead, and the thermostat was getting it's power from the wiring directly, which means it could be the HVAC unit that died. However, it's one of the first times I've had to deal with a HVAC unit personally, so I'm guessing. I reset the breaker a couple times to see if that would do it, but it's a no go (it is clearly marked on my panel with a aluminum bridge between both breakers).

Anything else I can try before I call a pro?

Totaline P474-1035 Thermostat

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do other components like the fan and heat turn on? also, did you label the wires before switching them out? you can't always go by color –  Steve Sep 3 '12 at 14:33
Fan/heat do not turn on. I have not modified this thermostat since I bought my house (aside from replacing the batteries this first time). –  Matt Beckman Sep 3 '12 at 16:06
Go check your furnace - often they will have LED's indicating what is happening (with a legend on the front panel). If no LED's, verify the furnace is getting power and check the fuse. Almost all furnaces have a 24VAC transformer - check the input is 120VAC and the output is 24VAC –  Steve Sep 3 '12 at 16:13
Did the heat/air conditioning/Fan ever work since you've moved in? –  Tester101 Sep 4 '12 at 12:02
Yeah. We've been running it fairly often these last couple months. –  Matt Beckman Sep 4 '12 at 20:28

2 Answers 2

up vote 1 down vote accepted

Sounds like you may not have power from the transformer. You can check this using a multimeter, by testing the voltage between the red R wire and the blue C wire. You should read somewhere around 24VAC, though depending on the system it could be anywhere from about 6VAC to 30VAC.

Most thermostats only use the batteries to power the thermostat itself, LED display, programming, date/time, etc. Thermostats with a C wire (like yours seems to have), sometimes only use the batteries to remember your settings during a power outage. The actual power for signalling heat/cool, is supplied by a transformer in the furnace itself (the red R wire). If there is no power on the R wire, the thermostat has no way to call for heat/cool. In this case you'll have to have an HVAC tech troubleshoot, and possibly replace the transformer.

You could use your multimeter to verify the furnace itself is getting power, though this is slightly more dangerous since you'll have to open the service panel on the furnace. This could put you in danger of electrocution, and should be avoided if you are not familiar with HVAC systems.

The only other thing I can think for you to check, is to look for a serviceman switch and make sure it's in the ON position. A serviceman switch is a switch on the feed line to the furnace, that allows servicemen to turn the power off to the furnace to service it. It should be located very close to the furnace, and within line of site of the furnaces service panel. If there is line of sight from the furnace service panel to the breaker panel that feed the furnace; or this is an old installation, you may not have a serviceman's switch.

Your best bet, is to simply call in an HVAC technician to service the furnace.

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I tested with my multimeter and there doesn't seem to be any power on the R wire. I made the call for a HVAC tech to replace the transformer. Thanks! –  Matt Beckman Sep 4 '12 at 22:56
After having the technician take a look, he said that a short in the unit caused a few relays to blow. I think he was being honest, so I'm glad I didn't dive deeper on this one... that would have taken me a LOT of diagnostic trial/error. –  Matt Beckman Sep 5 '12 at 21:29

My A/C stopped working because the batteries in my (Honeywell) thermostat needed to be replaced. I replaced them and plugged it back in the wall mount. The A/C still did not work, but the LCD lights were on. I had to remove the face plate and replug it back into the mount. This time I pressed the center of the LCD screen and got it fully connected which turned on the A/C unit.

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