I have installed a 240v flextherm heating cable under a floor hoping to use existing 240v. Heating circuit to power it. There are 2 other baseboards on this circuit
GFCI thermostat for floor is rated for 15 amp and heating circuit is on 30 amp breaker.
I am not getting any power through the GFCI when I connect it up so I think that I am getting an automatic trip on the GFCI because of the overload.

Any suggestions?

What is the total wattage of the heating system?

Why a GFI breaker? Does the manufacture instructions recommend or require this?

What is the total wattage of the heating system?

Why a GFI breaker? Does the manufacture instructions recommend or require this?

Wattage of existing baseboards is about 2250
Wattage of underfloor heating is about 60

I am on north american 120 volt system. Manufacturer recommends( and supplies) the GFCI thermostat

OK, are you trying to get 120 volt GFI protected power from the 240 volt baseboard heat circuit?

What type GFI breaker do you have?

Where does the neutral connect from the floor heat?

Exactly which Flextherm product do you have?

This would have been simpler if you ran a 120 volt GFI circuit just for the heat.