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Mad heater: tales of underfloor heating thermostat woes
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PerranOak:
I have a Warmup 3iE thermostat using an NTC 10k "probe" (which is what they call the thermocouple embedded in the floor, under tiles) in a very small bathroom. It's the only heat in there and so is pretty important.

It's been working well for nine years but now has had a Mad Hatter hissy fit! The floor is cold but the unit is reading 50degC!

So, I checked the resistance of the mat (the actual heating element in the floor) to be 87.6 \$\Omega\$ and the probe to be 13.5k \$\Omega\$ when the floor was really at 18.6degC (measured with a thermal camera).

I have been told conflicting things: |O

1. Since the probe is reading about right (data tables for the NTC 10K show that this resistance is spot on for 18.6degC) it must be the actual controller that has gone mad and I need a new one at around £100.

2. Regardless of the probe resistance, if you set the controller to measure air rather than floor temp and it shows about right (which it does) then it's the probe that is mad not the controller and I need a new one at around £2,000 (smash up the old tiles and heating element, lay new element and probe, lay new expensive tiles chosen by the boss).  :'(


Can anyone please help with guidance/advice - I will beg if necessary.
The Soulman:
It looks like your probe is fine, no reason to tear-up the floor.
The controller can be partially faulty.
A new controller would be the easy fix, repairing the old one is another option but swmbo may not approve.  :-DD
themadhippy:
how about bodging in a cheapo controller,an stc 1000 uses 10k ntc probes,are good for 10A and can be found for around a tenner
Ian.M:
Get a new NTC 10K thermistor, duct tape it to the tile somewhere out of the way but over the element, with a smear of heatsink compound for good thermal contact and a small square of expanded polystyrene over it so it reads surface temperature rather than air temperature, and patch it in to the controller.  That will give you positive proof of whether or not the controller is bad.  CAUTION: the thermistor terminals may not be mains isolated.  If not, insulate the added thermistor very carefully, don't allow anyone else access during testing, and totally remove it as soon as you have finished testing.

If (and its unlikely) the probe is bad, you may be able to get away without ripping up the floor by drilling a shallow hole where the corners of four tiles meet, and permanently epoxying a bead thermistor into the hole.  Run new wiring by grinding out the grout between tiles to form a channel, tack-gluing the wire into the channel at intervals, then re-grouting over the wire.
PerranOak:
Great, thanks all (Hi Ian.M) I'll try out the ideas that I can get away with. As noted, SWIMBO must be appeased!
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