Came back from a vacation and found the downstair thermostat's display blank. I tried all - reset breaker, reseat the unit. The upstair system is OK. The downstair system is not operational.
Took the upstair thermostat downstair, and it also went blank. Replaced it back to the upstair wall mount and it is OK.
There is a heater/fan in the attic that I have not checked.
TIA for some pointers...
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Re: Honeywell T8000C Thermostat Problem
If you have a volt meter, remove the thermostat off from the mounting plate and check for 24 volts AC across red (R) and White (W). Then check across Red and Yellow (Y). If you do not find any voltage, follow wire as much as you can all the way back to the air handler that is connected to it. Look inside and see if there is a small Auto (car) type of fuse in the wiring harness. Test and replace if necessary. Jump out the Red and Green terminals on the air handler and replace the door. See if the blower turns on. If so, you have narrowed it to the wiring to the thermostat. If not, you have a power problem and will need a technician.
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When did it last work? The first thing I would do is turn off the breaker that powers the A/C system. If it's a central system there are probably two 2-pole breakers (outside unit and air handler) turn them off, wait a few minutes and turn them back on. Let the system re-set (compressor time-out is at least 5 minutes after turning it back on. After the system is reset, see if your thermostat is working correctly and if not, try this link for an overview of Honeywell troubleshooting. Honeywell Thermostat Troubleshooting Six Step Guide
page 13 . Look at home depot Lowe's for similar models as this is not made anymore (you can buy used from eBay.
Model # RTH221B is $28.00. According to your model number it is a standard 24 VAC thermostat. You can pick from several models and prices, but the I showed is the closest to yours
I would say that you have one of two issues. Either your thermostat is bad or you do not have 24v coming from the heating unit. In this case the problem would be in the low voltage circuit of the heater. At the heater you should have 24v AC when you place an electric meter across the R terminal and the C or W terminal. If you have no vaoltage across the R to either the C or W then you have a bad transformer or a blown fuse in the furnace. If you do have voltage ther and also at the t-stat then the t-stat has gone bad, replace it.
This unit doesn't require a battery. It runs off the 24v from the unit. Check that your transformer is still working. Check the control board for 24v pwr. and main pwr. Check your breaker to the unit. It could be as simple as a bad stat too. You can check by using a jumper wire and jumping across from r term. to y term.
Sounds like it may have lost power. Check the breaker and the fuse at the furnace. If they are all in the on position and working then it sounds like you may need a technician. If the display is blank on the thermostat you may also want to replace the batteries.