Why does my digital thermostat not keep temperature?

[sickwarne]

I bought a digital thermostat this past summer, I'd say about June. I worked great, I can't believe how much my bill went down. Up until about 3 weeks ago, it worked fine with the furnace as well. But now it suddenly stopped keeping the temperature. The furnace just runs constantly, and even if I switch the thermostat to "OFF" the furnace just keeps on running. I have to turn it off by using the breaker switch to cut off the power.Does anyone have any idea why it would do this ? A loose wire or connection maybe? Is it possible the thermostat went bad so soon ? It's getting really annoying having to turn the breaker on and off. If I leave it on while I'm at work, I'm likely to come home and the house will be 86 degrees even when it's 35 outside. If I leave it off too long, then the house is freezing cold.Any suggestions at all before I spend a fortune on a repair guy would be appreciated. The furnace itself is only about 5 years old.

[Antman]

Don't know which stat you actually have, but in general....

1) Check the batteries - even if the display works, it is possible that the relay won't operate. Alkaline batts usually last 5 years+ but cheaper zinc cells often supplied when new may not. To maximise battery life, battery operated digital stats use memory relays - you pulse them one way to turn on, then pulse them the opostite polarity to turn them off. Theya are not like standard relays that are energised all the time they are 'on'.

2) It sounds as though the relay is stuck in the on position. It is therefore possible the relay contact has welded closed - usually only happens if the load is higher than the design spec.
This can be checked by turning off all power, then measuring across the relay contacts with a multimeter. If it reads a short circuit, then the relay is stuck. Relays may be replaceable but only by a electronics engineer that can desolder it from the pcb and replace with an identical spec relay.

Regards

Antman

[johnrae]

If it's a radio stat, have you changed the batteries, as previously suggested.

If not then ask yourself a couple of questions:-
a) How much did the stat cost. 
b) How much did the extra fuel burn for the last three weeks cost

If (b) was greater than (a) then replace the stat immediately and then take up the matter of the defective thermostat with the manufacturer

If you want to do a simple diagnostic check then disconnect the signal wire from the stat control into the boiler control.  Does the boiler still run.  If yes then it's not your stat that's the problem.

jack