Re: Installing wall thermostat for baseboard heater
Is the circuit breaker for the heater twice as thick as breakers for your lights & outlets? If so, it's most likely a 240V heater. Double-check the nomenclature plate on the heater (may need to pop the cover off) to make sure it's not 120V. Some will list both 208V and 240V.
If that's the case, then the previous installer simply put in a single-pole thermostat on a 240V heater; this is a common thing to do but not a very good idea as the other "hot" wire (in this case the white) is not switched. That means that the heater remains energized even when off.
If you can confirm that it is 240V, based on your description of what you had with the single pole t-stat, wire up one "half" of the t-stat just like it was before ("line" side to the two blacks; "load" side to the heater). Wire up the other "half" to the white wires exactly like you did with the black wires.
NOTE: do not put two wires under a single screw head; you will not get a good connection. Instead, use a wire nut to create a "pigtail" and wrap the single pigtail wire under the screw.
Can you give us the brand & model of your new thermostats? We can look up the installation instructions and that will allow us to give better advice.
The "Senior Member" designation under my name doesn't mean I know a lot, it just means I talk a lot.I've been a DIYer since I was 12 (thanks, Dad!). I have read several books on various home improvement topics. I do not have any current code books I can refer to. I was an apprentice plumber for two years.