My sister had a similar problem a few months ago. There are a lot of different things that could be the problem! Your best bet would be to call a technician so you don't waste your time or money messing with the wrong thing! <a href='http://www.blakesplumbingandheating.net/heating' >http://www.blakesplumbingandheating.net/heating</a>
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1. Here I assume a forced air system, since that's what I'm familiar with. If your thermostat is a two-wire unit, it's basically just a switch, so the problem would likely be at the furnace. IFF (if and only if) it is a 2-wire unit, then you can test the furnace by shorting the two wires together. If the furnace powers up normally and heat is generated and the distribution fan kicks in, then it's time to replace the thermostat. If not, look for power failure to the furnace, to the thermostat power supply, a pilot that has gone out, a bum fan, or a bad control valve. The last time I had a problem with that symptom, it turned out that the valve control was broken and it cost me ~$700 to have it replaced.
While you've been rather vague about the device - sometimes they fail and need to be replaced. L
Check the Battery is #1, I think. Same thing happened to me on a foreclosure house I bought and after 2 hrs of checking stuff like Re-design mentioned, I called the service man. Fortunately he asked me on the phone if I had checked the battery...saved me a $75 service call. When I checked, there was NO battery!...former owners took with them! I put in a AA and everything worked fine.
How do you know that the thermostat is not working? Find out if there is voltage to the thermostat. Most I've seen run on 24 volts dc. IF there is power to the thermostat, then there is 1. broken therm., 2. broken line between therm. and unit. 3. broken unit. If the unit is broken you'll need a service man. If the therm is broken you must be sure that you get the correct replacement or you can destroy you unit. If there is no power to the therm. then find the transformer that powers the therm. Usually it is on the hvac unit. Be careful that you are testing the right thing and not fixin to get into something that is going to hurt(kill)you. If there is power at the transformer then 1. there is a break in the wire between the therm and the transformer, 2. you weren't testing the right connections at the therm., 3. broken therm. If the transformer is bad you might be able to get a replacement but it must be the right voltage. With out being there I can't tell you which connections to test or if that transformer is the right one etc. It would just be easier to call a service man out.
Check the battery in the thermostat?
I didn't even think about that one. Mine at the office has a battery so #4 is check the battery.
Agree with Re-design, call a service man. Is it possible the problem is with the furnace?