Help with a home thermostat that went blank

[LA1]

i had one of those free PEPCO ( our power company) thermostats. One day in the summer i touched the thermostat and got a static shock. The thermostat was blank, no signs of life. I called Pepco and they came a day or so later and replaced the thermostat with a like model worked fine. Everything was good for about 3 years i even got a new furnace last year. I still have the same thermostat, About 2 day ago , very cold out, I touched the thermostat again to turn down the heat. i got a mild static shock. Again it happened nothing on the display and the heat went into cool down mode then shut off like i turned the heat off. I ordered a new thermostat on Amazon and in the meantime i jumped the white and red wire to get heat. Itt was a Honeywell Xp2 or something close to that. I turned off the power at the furnace, luckily all the 5 wires are the correct color to the thermostat. I disconnected the wires and then connected up to the new base plate, blue to C, white to W, red To R, green to G, yellow to Y. I turned the power back on at the furnace, measured the voltage from R to C and i have 28VAC. I turned the power off at the furnace then went up and plugged in the thermostat, then turned the power on at the furnace. the new thermostat is blank. I did multiple checks, i even got out a Variac I used for TV repair, dialed in 28vac and connected up to the C and R pins on the back of the thermostat.....no display. did i get a bad thermostat or did my furnace somehow kill my new thermostat?

 

[mepstein]

Does it take batteries. If so, remove and replace. Might not be the reason but might as well start with low hanging fruit.

 

[BurtEggley]

read the manual on how to connect it would be my suggestion. Verify what the wire colors connect to on the HVAC system also. Don't assume they are connected correctly at the system, and be aware there are many different configurations.

 

[Ohmthis]

My bet is batteries too. Some don’t use the 24v to power the Tstat. The batteries do and the Tstat just passes the control voltage on the the correct terminal. Check for a secret trap door. What Tstat did you buy?

 

[Ohmthis]

If you were referring to the Honeywell XP2 above, it does require batteries.

 

[fitter30]

28 vac will not kill a stat. Look at the transformer is it in the air handler or furnace? Furnace it will only have 120 vac tap, air handler might have 208 or 240 vac tap with push on terminals. What line voltage?

 

[bonneyman]

I'm with the other guys here. Change the battery first. I can't count how many times I've had electronic devices stop working and I do all sorts of troubleshooting then find out the damn alkaline cells have burst and leaked.

 

[BurtEggley]

one possibility here is that the supply voltage is not there, either miswiring or a broken wire etc., and the thermostat is running on batteries until it dies each time.

 

[HoosierBuddy]

Batteries. Surprised they lasted 3 years. I have mine on a 2 year replacement rotation to avoid the "heat quits in the middle of the night" scenario.

 

[finn]

one possibility here is that the supply voltage is not there, either miswiring or a broken wire etc., and the thermostat is running on batteries until it dies each time.

I would look to see if there is power to the “green” wire. From memory, some thermostats don’t “need” it to function, but won’t work long term if power is disconnected for an extended time period.

I had a dead Nest thermostat that was replaced a week before the warranty was up.

The technician I spoke with over the phone while diagnosing the dead thermostat blamed it on the microprocessor in the thermostat overheating and failing when the internet was disconnected for an extended time period before we moved in. (I didn’t actually believe him, as I didn’t see the logic, but the new thermostat he sent under warranty has been fine for six years now, so why argue).

 

[bonneyman]

Batteries. Surprised they lasted 3 years. I have mine on a 2 year replacement rotation to avoid the "heat quits in the middle of the night" scenario.

I agree. In the old days batteries seemed to last longer. Now the dang things burst and leak and ruin devices.

I used to tell my customers to change all their batteries (thermostat, smoke alarm, TV remotes, etc.) once a year. To the men I told them their birthday - the women I told them their anniversary. Figured nobody could forget those!

 

[CoogarXR]

I have my old "basic" digital thermostat sitting on my desk as a thermometer, since the batteries are still powering it, lol. (hint-hint: the 24vac doesn't power it).

However, my newer thermostat does not have batteries, and does run off the 24vac thermostat power.

So yeah, it can be either way.

 

[bonneyman]

I have my old "basic" digital thermostat sitting on my desk as a thermometer, since the batteries are still powering it, lol. (hint-hint: the 24vac doesn't power it).

However, my newer thermostat does not have batteries, and does run off the 24vac thermostat power.

So yeah, it can be either way.

Ha, me too! There's a second thermostat that I used to run my evap cooler in years past. I don't use the cooler anymore, but the digital stat is in my office area and I keep batteries in it for the thermometer.
My new A/C and heater stat is dual powered (either 24v or bats), but don't have a common wire so I just run it off the batteries.