Our thermostat (which has been serving solely as a thermometer since the last time we used the heat in the spring) blanked out yesterday afternoon. I checked the inside temp. during the day, then last evening the face was completely blank. It is completely unresponsive to any button pushing. I checked for batteries, but it appears to be wired to the house electrical system. The tech at BatteriesPlus saw no battery in the diagram and digital photo I made. What might be the problem? It's only 3 years old. Thank you for any help you might offer.

First place to start is the batteries...

Most programmable thermostats have some form of battery - either replaceable or rechargeable - otherwise all the programmed information could be erased anytime there was even a momentary loss of household electrical power. 3 years would likely be the life limit of replaceable batteries. Do you still have the owner's manual? If not, try looking your t-stat up online. Many manufacturers have online manuals...

How many wires are going to your thermostat?

Only four of the ten wires coming out of the wall are connected to the thermostat. 3 are connected at one box at the Heat Stage terminal, the Fan terminal and the 24 VAC Hot terminal. The fourth wire is connected to a separate box at the 24 VAC Comm terminal. It is a Carrier Battery-free thermostat, I have learned.

I spoke with someone at Carrier and he felt that there could be some problem with the system, not just the thermostat. I'm interested in any input, but will probably end up calling for service. I appreciate the help all have offered. Thanks!

Sorry we couldn't be of more help. A battery-free T-stat has a memory chip (PROM) that stores info that can be recalled - even after a power failure. But otherwise, it does need power to function. Start with simple first - make sure the breaker isn't tripped / fuse blown in your breaker / fuse box. Next, check the t-stat itself for a fuse. Lastly, you could test the 24 volt transformer using an AC volt meter across the 24 VAC comm and 24 VAC hot terminals.It should read around 24 volts.

Thank you! I had checked for a tripped breaker already, but looking at the circuit board, I feel I'm entirely out of my league. All the responses are much appreciated and very helpful. I feel more confident now that I really DO need to have a service person come to the house and that makes the cost a bit easier to take! I will definitely use this service again. All of you who are volunteering your time and knowledge are to be commended! Sincere thanks.:)

Janalan,

Please be sure and let us know the final outcome in as much detail as you can. It could possibly help someone in the future and will only add to the knowledge base on this site. Thanks... ;)

I phoned to request a service call yesterday afternoon and in talking with the scheduler, was reminded of an on/off switch that is mounted to the side of the furnace in the laundry room. It had gotten switched to off. Upon turning it back on, the thermostat began working just fine. I felt fortunate that the scheduler had just had a similar thing happen to her and that she was service-oriented enough to mention it to me. I feel a little silly, but I'm so grateful to have found this forum where problem-solving is offered so kindly.

I have never signed up for a forum, but HAD to share my excitement! I, too, did EXACTLY like the initiator of this discussion. Saw the house temp in the early part of the day, had a totally blank readout later in the day. I looked online for help, ended up here. Many thanks and much gratitude to the individual who asked him to post the outcome. THAT is exactly what my issue was. We have a switch indoors and another in the furnace room/utility room, where I store gardening equipment, etc. and this being spring and all... The switch was accidentally moved to the "off" position. One flick and I was back on track. I cannot thank you enough for having this information available to the homeowner!

Wow. I was about to go buy a new thermostat and replace it myself because the quote to have someone come out and check it was $100 plus around $350 for the unit which seemed extremely high. I took the advice from above about checking the switch to the furnace (mine was located in my attic as a normal wall switch which said "fan" written on it and was 5 feet from the furnace). Just switching it on made the thermostat downstairs come back on. To be honest, I had switched it off several weeks back and did not know it was connected to my thermostat (I just thought it was an attic fan or something useless). Just wanted to say thank you for all the posts above. You saved me some Cash and much time!!

Hey I had that same problem and it seemed to be that the unit have a "safety switch" (floater) which is locate aside the unit (in a cabin inside the home)that switched off whenever it get overload with water from too much cooling.

Just pull out that safety switch, drain water, put the switch back into it's place and the unit should now start working normally. You should now be able control the unit via the thermostat.

Hope this will help.

You guys rock!! The same thing happened in my Casita! Had no clue what was going on! Discovered an on/off switch in the Casita bathroom!! Now I have heat in there! Thanks a million! Especially to the person that asked for an update!!