Mad heater: tales of underfloor heating thermostat woes - Page 1
Forum section: electronics / general

author: PerranOak
author_rank: Frequent Contributor
timestamp: January 26, 2021, 02:06:00 pm
content: 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 87.6and the probe to be 13.5kwhen the floor was really at 18.6degC (measured with a thermal camera).I have been told conflicting things: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.

author: The Soulman
author_rank: Super Contributor
timestamp: January 26, 2021, 04:36:26 pm
content: 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.

author: themadhippy
author_rank: Super Contributor
timestamp: January 26, 2021, 04:51:14 pm
content: 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

author: Ian.M
author_rank: Super Contributor
timestamp: January 26, 2021, 05:13:31 pm
content: 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.

author: PerranOak
author_rank: Frequent Contributor
timestamp: January 27, 2021, 10:56:47 am
content: Great, thanks all (Hi Ian.M) I'll try out the ideas that I can get away with. As noted, SWIMBO must be appeased!

author: richnormand
author_rank: Unknown
timestamp: January 28, 2021, 08:56:41 pm
content: Probe:If you have an approximate idea of its location put a pot of boiling/warm water to warm the tiles while you read the resistance. Plot the resistance vs time.That should tell you if it is working and if it deviates too much at the higher temperature. Some can look OK at room temp and go open at higher temperatures.Also does the thermostat have not only a set temperature but a real-time display of the room or floor temperature. If so you should see it change during the warmup period.This assume it is not a thermocouple, as you said it is supposed to be a NTC 10k.Thermostat:Is there an actual relay to activate the heating coil in there? Can you hear it click as you adjust the temperature from low to high?Coil resistance seems OK assuming you operate with 230VAC. Do you have a way to monitor the voltage across it (clip a lightbulb in parallel for example).Do you have access to the thermostat insides. The heater controls are usually fairly simple (relay, TRIAC or the like).Final though (a recent video from Dave with his thermostat issues turned out to be a bad contact in a control button.Thermostat units are fairly standard and swapping a new one might not be that expensive before contemplating any tile work....Good luck with it.