# No power to my thermostat



## rolltrain

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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## guyod

Every digital thermostat i seen has batteries.  I would look harder. Did you take it off the wall?


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## Daryl in Nanoose

What kind of thermostat is this and what kind of heat ?


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## glennjanie

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.
Glenn


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## kok328

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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## triple D

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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## jack3140

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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## wolfenpr

Maybe you just have to replace the thermostat.


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## falconbrother

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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## brywilliams

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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## keepinitcool

Those darn condensate pumps and their safety features! Usually don't see this issue unless furnace/air handler is installed in an attic or upstairs area.  But good post about what could be a possibly overlooked cause of the problem.


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## mcluvin

brywilliams,

Thank you!  Thank you!  Thank you!  My Labor day weekend hell is finally over.  I just did not expect that a float switch would also kill power to the thermostat, but not kill power to the wiring that controls the fan.  I don't even want to admit everything I've done to troubleshoot and how many internet posts I've read.  After reading your post I went out and looked at the float line.  Sure enough it wasn't level.  Got it level and now reading 28 volts between R & G at the thermostat.  Yeah!!!!!!!

Chris

Also for anyone else troubleshooting a similar problem, voltage would fluctuate between R & C at the thermostat.  The float would never totally kill the voltage but would keep it low enough to not power the thermostat.  A couple times voltage was high enough to power everything, but it would soon fail again.  I'm sure due to water again tripping the switch.


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## sandmyth

This post saved me having to call for HVAC service.  I checked the air handler in the attic, and the float switch had been tripped due to standing water in the pan below the air handler.  I drained the water, and AC was back on in 10 mins.  I'll have to check the outside drain line to find out why it was draining into the pan instead of the external drain, i'm guessing i have a clog somewhere.  I guess any pvc safe drain un clogger would be fine to run down the drain line?

Glad this thread was the second google result for "themostat no power".


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## Wuzzat?

sandmyth said:


> This post saved me having to call for HVAC service.  I checked the air handler in the attic, and the float switch had been tripped due to standing water in the pan below the air handler.  I drained the water, and AC was back on in 10 mins.  I'll have to check the outside drain line to find out why it was draining into the pan instead of the external drain, i'm guessing i have a clog somewhere.  I guess any pvc safe drain un clogger would be fine to run down the drain line?
> 
> Glad this thread was the second google result for "themostat no power".


I guess that's why my control board edge connector had corrosion; no float switch to turn off the AC.

I usually blast out clogs with a garden hose.


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## 168zhong

My Honeywell Thermostats power indicator is off but still shows time and actual temperature.  Battery is optional for this model. Is safe to touch all the wires of the thermostat without turning off power?
By the way, I checked the furnace and found power indicator on the control panel is off? Do I still have reason to check the thermostat?


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## nealtw

Welcome to the site. Did you check the breaker?


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## wrightcomputing

Mine was the float problem also.  Just drained a full bucket of water from the drip tray and I am back in business.  Glad that fixed the *No Power to the Thermostat* issue.


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