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? 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.ac unit has ice on it Most thermostats only use the batteries to power the thermostat itself, LED display, programming, date/time, etc. cost of installing a central ac unitThermostats with a C wire (like yours seems to have), sometimes only use the batteries to remember your settings during a power outage. ac unit measurementsThe 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. So I had the same issue and called my buddy who is a certified HVAC technician and he helped me trouble shoot the problem. First he had me check to make sure all the wires were secure behind the thermostat, but they were secure. Then he had me take off the panel on the heater/AC Unit and push in the door trigger to see if there were lights.

There were lights, which meant that power was going to the unit, just not making it to the thermostat. Then he had me check a fuse on the main circuit board that had the number 3 on it. The fuse was not blown. Next he asked me to check the AC condensation overflow pipe to make sure that there wasn't a clog because if there is a clog the safety valve will cut off power to the thermostat. Sure enough, this was the issue. There was a 1 inch PVC pipe running up along the left out-side of my AC/heater that went down into a water pump that was clogged. I simply unclogged it and the AC cut on immediately. I just had this exact issue. I called the HVAC emergency service number and the technician called me back. He advised to unscrew the bottom of the kill switch or float. There is a yellow wire that leads to it. Once I drained all the water ,the system turned on instantly. 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. Make sure you put the batteries in right. Don't let the springs fool you like they did me. I put both batteries in with the - (minus) sides on the springs. Put one battery with the + side on the spring. It should show you the + signs on the battery holder. Before anything, try reset button at the furnace itself hidden behind the cover. It's one of those that activates when the cover is on and deactivates when the cover is off. My problem was exactly that. I put brand new Panasonic batteries in my thermostat and it did nothing. However, it worked with the old batteries, one of which was Energizer, and the other was Duracell. Clearly it takes a stronger battery.I didn't read all the responses, but thought this would help. If your thermostat has a R and Rc, then there has to be a physical wire (jumper) connecting the R and Rc, along with the red wire going into the R slot.

I didn't know this, as I inherited the thermostat from a friend. Heat turned on, AC did not. Once I read up, installed the jumper, everything worked fine. Hope this helps someone! Simple trick that fixed my problem. Replace the batteries but put them backwards per direction. It will shut of the lo batt warning then put the batteries correctly. Browse other questions tagged hvac thermostat or ask your own question.At T. N. Bowes Heating & Air Conditioning, Inc., our goal is to exceed your expectations in every possible way. By hiring a team of highly trained and dedicated technicians and installing only top quality products, we uphold a higher standard of excellence. When you trust your comfort to T. N. Bowes Heating & Air Conditioning, Inc., we provide the personalized attention and conscientious job performance to achieve your goals for temperature control, air quality, budget, safety, and system longevity. Results that save you time and money, while protecting your investment, comfort, and home.

We offer prompt scheduling, punctual arrival, affordable service & proven results! We deliver superior results that will save you time and money and ensure your winter comfort! Quick and accurate installation, we ensure a clean, healthy indoor environment for your family Established in 1988, T. N. Bowes Heating & Air Conditioning, Inc. provides a trusted source of new installation, replacement, seasonal maintenance, and prompt repair of heating, cooling, and air quality equipment in Waldorf, MD & surrounding areas. As a Carrier Factory Authorized Dealer, our team is fully licensed, extensively trained, and consistently updated in a wide range of industry-leading options to meet any challenge. All projects are handled to your specifications and completed within a strict timetable. For top quality heating and cooling repairs, installations, and maintenance, call T. N. Bowes Heating & Air Conditioning, Inc. today! We’ve lived our entire lives, and continue to work in Waldorf, MD & surrounding areas, and are committed to the local community.