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I’ve been wrestling with an electric radiant heat system that I installed roughly 6 years ago in my upstairs bathroom. I found one morning 2 weeks ago that the floor was cold and saw that the GFCI had tripped on my Honeywell TH115 AF-GA thermostat. For reference only, I have 2 other floors in my house that have this same type of system. This is the only problem that I’ve had with these 3 systems. The system in question has a 1’ x 15’ heating mat and it’s setup to run on 120 volts. It is on a dedicated 15 A line.
I would like to list all of the things that I’ve tried to find the reason for this situation.
1. Trouble shooting began with measuring resistance between the heating leads. Result was 64.2 ohms. This is suppose to be ok. Resistance between each lead to ground showed no change in value on my multimeter. Meter read “1”. (I measured another floor setup and got similar results from lead wires to ground).
2. I took thermostat from upstairs system and installed it on downstairs system. Thermostat worked ok.
3. I took downstairs thermostat and installed it into upstairs system and the GFCI tripped. At this point, one might think that the floor system might be failing and the thermostats are ok except step 1 showed mat was ok.
4. I borrowed an Insulation Tester and measured resistance between heater leads to ground again. I set tester to deliver 100 volts. Both leads read roughly the same. It would start at .2 mohms and climbed to .92 mohms within 15 seconds. I measured the system downstairs and both leads to ground read 110 mohms. These values are good so I’m told. The .92 mohms values are suspicious to me.
5. I then decided to call tech support and after listening to my attempts (the insulation tester results went over their heads which is ok - it went over mine too), I was told to hookup line voltage to heater leads and see if floor heats up. If floor gets warms, the thermostat is bad and floor mat is ok. Bathroom floor got warm. I let it warm up for about 30 minutes.
6. I ordered new thermostat and received it in 3 days.
7. Installed new thermostat into upstairs system and GFCI tripped. Installed new thermostat in downstairs bathroom. Thermostat worked correctly.
Ok???? Now what.
8. I inspected the heater and sensor wires from where it comes out of floor to thermostat looking for broken insulation etc and found one suspicious nick in heating wire. Cut that part off still leaving me barely enough to work with. Tested thermostat. Still trips.
9. Went to my breaker box and performed a few actions. Made sure all wiring was securely attached and also gently blew out dust and any other debris that was present. No change.
10. I ran a brand new length of 14-2 wire from upstairs bathroom to basement making it impossible to walk safely around my house. My wife loved this! Attached new wire to new 15 amp breaker. Thermostat still tripped.
I’m not sure how this idea got into my head but I pulled my generator out. I then installed a male plug on new wire and plugged it into generator. Tested thermostat and it didn’t trip! I left if for roughly 30 minutes and bathroom floor warmed up nicely. I am now completely baffled.
I can’t help but think that it’s something so simple and it’s staring me in the face. Can anyone shed some light on this problem? Could the .92 mohms reading in the heater leads be the fault shown by the insulation tester?