I have Mears Thermostats in my home. They have (3) three wires on them
RED is load
BLACK is line
WHITE??
Thermostats tried a singel pole Thermostat with (2) wires RED & BLACK tieing off the White
I have tried a double pole using t1 for load (RED) and L1 for line (BLACK) and tie off l2 & t2 togather.

Are these line voltage thermostats for electric strip heat? Or are they low voltage thermostats?

This is for baseboard heat (220 volt) the wires are 12's

Since this is 240 volt, both wires (black & white) coming from the power source are hot. Each wire going to the heat will connect (thru the tstat) to each of the load wires. If you use a double pole thermostat, connect each incoming power wire to the line terminals of the tstat, and each wire that goes to the heat to the load terminals on the tstat. If the tstat is single pole, just wire nut one of the line wires to one of the load wires and hook the other wires to the tstat as I described above. I prefer double pole line tstats because when they are off, the heat unit has NO voltage hooked to it.

Since this is 240 volt, both wires (black & white) coming from the power source are hot. Each wire going to the heat will connect (thru the tstat) to each of the load wires. If you use a double pole thermostat, connect each incoming power wire to the line terminals of the tstat, and each wire that goes to the heat to the load terminals on the tstat. If the tstat is single pole, just wire nut one of the line wires to one of the load wires and hook the other wires to the tstat as I described above. I prefer double pole line tstats because when they are off, the heat unit has NO voltage hooked to it.

I have tried both of these ways and my heat iis still not working. What dose the WHITE wire do. I have been told that it is a common line but what function does it preform and how or where dose it fit in to the single or double pole thermostat. Thank you for your help so far it has been good to talk to someone that know what I am alking about .

If you hooked the thermostat up the way I said, it would sound to me like your heat strip is dead. If you check the resistance between the two LOAD (heater) wires, you should get a reading somewhere in the range of 12 - 25 ohms (for 16 ft of heat) up to 115 ohms or a bit above (for 2 ft of heat). That is a range of readings that covers most installations. You should also verify the voltage on the two LINE wires. If that doesn't show something close to 240 volts, you could have a problem there. You could also do a current check and verify the wattage used, but that's a bit more technical. If you don't have the equipment or feel comfortable doing those measurements, call an electrician.

The white wire is actually another hot conductor in this situation. It carries the power from the opposite leg (referenced to the black conductor) of your electrical system. In this situation, it is NOT a common or neutral conductor. Technically it should be marked with tape or something to show it is a hot conductor.

I have 22 ohms at this line. I know that the strip works because them old thermostat is bad it will not shut off or turn down. What will happen if I connect the white wires to a 2 polled thermostat

Connect both black and white LINE wires to the LINE TERMINALS on the new double pole thermostat. Connect both LOAD (heater) wires to the LOAD TERMINALS on the new thermostat. It will work correctly.

Connect both black and white LINE wires to the LINE TERMINALS on the new double pole thermostat. Connect both LOAD (heater) wires to the LOAD TERMINALS on the new thermostat. It will work correctly.

I have found a MEARS thermostat and found that the white wire is an optional antcipator and canbe connected to the load line thank for all the help