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Old 02-11-2008, 08:04 PM  
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Default No power to my thermostat

Checked for batteries, it doesn't have batteries. Checked the breaker, no tripped. Where would be another place to start looking?

Thanks for any ideas.

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Old 02-11-2008, 08:19 PM  
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Every digital thermostat i seen has batteries. I would look harder. Did you take it off the wall?

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Old 02-11-2008, 09:21 PM  
Daryl in Nanoose
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What kind of thermostat is this and what kind of heat ?

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Old 02-12-2008, 12:08 PM  
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If it is not battery powered then there must be a 120/24v transformer to power the controls. Transformers fail occasionally and have to be replaced. You can completely disconnect the transformer and use an ohmmeter to see if you have continuity on the high side and the low voltage side.
It that is not the problem I would look for a wire that has lost connection.
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Old 02-12-2008, 06:23 PM  
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Check the primary side of the transformer for 120VAC.
Check the secondary side of the transformer for 24VAC.
If you have 24VAC then your T-Stat could be bad.
Check to see if your transformer has a "reset" button, reset if tripped.
All else fails, I usually turn off main power the unit, power it back on and kick it.
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Old 02-12-2008, 10:30 PM  
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Default One thing to consider

I dont know what kind of furnace you have, but if it is electric, watch out!Your transformer is probably 240volt. Either way it should be located inside furnace. And there usually is a safety switch on the cover like dome light in car, you might have to depress this to test things. Let us know what you find, and how many wires are in t-stat cable, we might need to use these later.. Good luck KOK328 is dead on on test methods, but make sure your not a electric furn. or you'll want to have meter on 240v...
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Old 02-17-2008, 06:23 PM  
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how are you testing for power? if you try across the thermostat wires you will get nothing because it is only a switch connected in series go to the source ie the transfo god luck
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Old 10-14-2009, 10:44 AM  
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Maybe you just have to replace the thermostat.
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Old 10-22-2009, 08:22 AM  
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If you have a burned out transformer there is a good chance that the contactor shorted and burned up the transformer. There is no good way to test the contactor. You can get one cheap off of ebay. I always keep a 30amp double pole laying around. Last spring my transformer went and nothing on the system would work. I have AC and gas pack. Check the furnace and find the transformer. Make sure you have 120 coming in and 24 going out. This 24 volts probably powers your thermostat. The 24 also powers the electronic control unit and the call for AC (contactor) on the coil. Once I understood my system it became fairly simple to diagnose issues.
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Old 05-28-2013, 02:32 PM  
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No power to thermostat - allow me to share my $84 lesson. Symptom: no power to my thermostat. checked ckt brkr, checked power across connections for thermostat (have basic Honeywell model - pull control module from base plate to expose wire connections. red is hot.). going into heat pump or air handler for checking is a little more than I'm comfortable with but issue wasn't a transformer or other 'significant' electrical component. issue was my float switch that checks for condensation excess/flooding. in my case, the line to the float switch is not rigid and it had gotten depressed thereby letting gravity help feed the line and tripping the float switch. I would have expected the float to prevent the a/c from running but not shut off all the power to the thermostat. re-leveled the float line/drained water (power on to thermostat) and after 5 min system reset the a/c was back on. take comfort that I didn't need a new compressor or something much more expensive but...

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