My thermostat wont come back on

If it's a new thermostat that you have recently installed, check your installer configuration. Make sure the tstat is set up for gas or electric heat, depending on what you have. Some t-stats also have a small switch on the back to switch from gas to elec. A gas furnace has components to control the fan, electric unit uses the tstat to control the fan. Hope this helps!

you need to change the differential pressure to 3 degrees.that means the unit will not kick on unit it is 3 degrees warmer then the setpoint

Have you tried pressing the 'Reset' button (with the tip of a pencil)? After doing this, you will have to re-enter the various times. It could be that your rechargeable battery is nearing the end of its useful life, by the way.
George

Remove main stat from subbase. On back of main stat there are some screws used for different settings. Screw "C" if for fahrenheit and celcius. Screw in for fahrenheit and out 1 turn for celcius.

I use ball valves.

Your honeywell should have battery back up but the primary power for this is a 24 volt transformer located in the air handler probably in your attic. All these controls run on 24 volts

Make sure your fan switch set to "auto". If there is a switch or jumper in the t-stat labeled "G" and "E" or "GAS" and "ELEC", it should be on "E".

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go to the main power box and find the either breaker or fuse that runs the system and turn it off

There are so many products, we cannot help without the model no. Try typing 'Honeywell programmable electronic thermostat' into google and searching for an image of your thermostat, then get the model no. Then you might be able to get some help.