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Thermostat only turns furnace on when batteries are pulled out and put back in


Our thermostat is only detecting that the furnace needs to be turned on when the batteries are pulled out and then put back in. Initially we were changing to fresh batteries, but when it was happening every day we started just putting the same batteries back in. The furnace runs for a while with no problems, shuts off at the appropriate time, and then never turns back on. The weirdest part is that the thermostat is reading that the temperature in the house is well below where it should be and still it does not tell the furnace to turn on. Once we pop the batteries out and back in it kicks on, and the process starts over again. What's up?
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Posted 2013-02-15T21:31:44+0000  by mollybethkern mollybethkern

How old is this thermostat?  If you adjust the heat up will it kick on? After you pull the batteries and go through a heat cycle, when it should be kicking on after that do you hear a click from a relay inside the thermostat at all?  Can you turn the fan from auto to on and does it turn on the fan?

Posted 2013-02-15T21:46:55+0000  by jwatkins82

Hi, thank you for your help.


The thermostat is old, but not ancient. It's prgrogramable and has a backlit LED display. I would guess that it is between 5-10 years old. 


It does not kick on if the heat is adjuted up.


I have not heard a click, but I haven't been listening for one ether. :)


Yes, if we have it on fan it will kick on. 


It seems that the function that doesn't work is the auto on when the temp gets too low. I've checked and double checked my temp settings.



Posted 2013-02-15T21:54:47+0000  by mollybethkern

You can try to remove the cover/thermostat from the wall to get to the wires. If its a honeywell you just pull toward you, if it a 3m then remove top cover. Which ever once you get to the wires jump the R and the W (this is for a gas furnace), if the heat comes on then just replace the thermostat. You'll spend more time trying to fix it. If the heat doesn't come on then its something in the furnace.  If it is the furnace check for fuses on the PCB if it has a board.  Check for pilot if it has one.  These things will depend on brand and age of furnace.

Posted 2013-02-15T21:55:36+0000  by jwatkins82

Thank you- we will try that!

Posted 2013-02-15T21:59:24+0000  by mollybethkern

It could be a micro trigger or a relay that isn’t working correctly inside the tstat.  Have any programming changes been made? I know on some brands you can change when the unit will turn on.  I.E. If you have room temp set at 70 degrees on 3m for example you can adjust when it kicks on, default is 1 degree below target but that can be adjusted.  If it was me I would replace it.  You don’t need a high dollar tstat that you can adjust from your phone, that’s neat but who really needs that, no sense in that. IMO

Posted 2013-02-15T22:00:30+0000  by jwatkins82


Capacitors are also a common issue, you can tell if you take the tstat apart and look at the capacitors on the board and see if any look like a can ready to burst.  They swell when they fail.  These things can be replaced, all the parts are available but it all depends on your ability to troubleshoot a circuit on a PCB and determine which component is bad, and replace the component.  If you go this route be sure to write down the capacitance rating, sometimes if they have been really hot you can not read the specs on the side of the cap.  My guess is a cap that is not holding a charge, they work much like batteries that store a charge.  Since it only seems to work after you pull the batteries that would indicate to me that when the batteries are put back in the cap charges real quick and is able to discharge quickly, but when left alone the cap isn’t holding enough energy to do the job.  I have spent a lot of time before fixing tstats usually caps, radio shack usually will have what you need but again its more time consuming than replacing with a new one.  Hope this helps.

Posted 2013-02-15T22:15:26+0000  by jwatkins82
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