[guest]

What is the current draw on that 8' heater? Is it running at 240 or 120 volts?

One problem I can forsee with a true electronic thermostat is that the triac that actually switches the current WILL generate it's own heat internally, which may cause the problem the OP is having.

Come to think of it, since it is only a single heater, is it possible that the current drawn by the heater is not enough? (Kinda like how some LED dimmers have a minimum load requirement?) From the manual:

Supply: 120/240 VAC, 50/60 Hz
- Minimum load: 2 A (resistive only)
500 W @ 240 VAC
250 W @ 120 VAC
- Maximum load: 12.5 A (resistive only)
3000 W @ 240 VAC
1500 W @ 120 VAC
- Display range: 0 to 50°C (32 to 122°F)
- Setpoint range: 5 to 30°C (40 to 85°F)
- Storage: -20 to 50°C (-4 to 120°F)

So is the heater load within range?

Hold the phone, just noticed this:

The thermostat registeres the correct temperature in the room until heat is called for. Then the temperature registered by the thermostat actually goes down instead of up. And it just continues to draw heat (indefinitley I guess). As I said I've tried 2 of them and they do the same thing. Once I turn down the setting on the thermostat and it stops drawing current, the thermostat again registers the correct temperature.

Possibly the thermostat display is playing the game of showing the low temp in the room and will gradually raise to the set point?

The manual is infuriatingly short on details of what the display does.

I have baseboard heat here and may pick up one of those 'stats and see what happens.