Warm Tiles In-floor heating GFCI problem

[sb11]

I have a 240V dedicated circuit installed in my bathroom under tile. It is the GFT 2118 cable kit from warmtiles. It was installed in 2009 and ran perfectly until 2015 when the thermostat had an end of life issue. The original thermostat was a GT-2 and I had no issue with it or GFCI trips prior to the end of life issue.

I installed a new FG thermostat and it has been intermittently tripping the GFCI since it was installed.

I checked the resistance in the heating cable:
Black to white - 38 ohms (exactly what it should be according to manufacturer)
Black to ground - infinite resistance
White to ground - infinite resistance

I have also checked the resistance in the sensor cable:
12,000 ohms (exactly what it should be according to manufacturer)

Initially I thought it must be a thermostat issue so I replaced the original FG thermostat with a new one and the same issue has continued.
It appears to trip the GFCI every few days at the current temperature setting, but if I increase the temperature setting it will trip sooner.

any ideas? I am not sure why it would work perfectly well for 6 years and suddenly I would have a cable issue at the exact same time I change my thermostat.

 

[George Stolz]

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