DIY Doctor

Easy to Swap Digital Programmable Thermostat for a Basic Dial?

Postby nickpan » Sat Nov 02, 2019 3:27 pm

The digital multi mode thermostat for my bathroom under floor heating turns itself off every minute, so the floor is cold. I hate that thermostat, it won't go below it's 5degree setting, so floor is either cold, or very warm, I just want a nice background warmth to the floor, even in summer, not unpleasant sweaty warm. I'm glad it's broke I tells ya.
If I replace it with a dial, is it an easy swap?
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Postby Mr White » Sun Nov 03, 2019 12:18 am

There are a variety of mechanical room stats, (Some require Live and Neutral to operate, some don't) It also depends on what your UFH requires. There is also the possible danger of having mains in a mechanical room stat in a room full of steam.
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Postby ericmark » Sun Nov 03, 2019 9:30 pm

You do not say if wet or electric under floor heating, with wet the thermostat opens the zone valve and the temperature of the floor is controlled by pumping water from return back to feed so water not over around 35 deg C, so the on/off is a separate thermostat to the floor temperature.

With electric the floor is limited to around 27 deg C, there are two completely different methods, Raychem is a special chemical impregnated core which auto controls floor temperature, the warmed the floor gets the higher the resistance of the cable. But the other method uses a simple resistance cable and to ensure it does not go over temperature there is a pocket between the elements and a sensor measures the floor temperature, so the thermostat is a special, one it works with a higher current to most central heating thermostats, and two it has two sensors one for room temperature and one for floor temperature.

I have not seen an electric underfloor heating thermostat that does not need a neutral, however there may be one I am unaware of, and unless using Raychem then Line in, Line out, Neutral, and two sensor wires at least.

With a bathroom often a more expensive type is used with two sensors, one under floor and one air so the thermostat can be mounted outside of the room, there are also problems with earthing, some systems need an earth mat over the heating wires when used in a bathroom.

I no longer have a house with under floor heating, and to be frank when I did it was rather useless, to install the first job was dig up floor and lay a rather thick layer of polystyrene, so that the heat would not be lost, net result even with floor switched off it stayed reasonable warm.

Idea was to dry the floor, mother was an amputee, only one leg, so did not want her to slip on the wet room floor, as soon as shower turned on it cooled the floor, so from turning shower off to having dry floor was 1/2 hour minimum with extractor running as well, she was not going to wait that long before leaving the shower.

Using a simple mop did a better job, but also floor very slow reheating, before fully up to temperature it took an hour or more, and without towel rail the wet room was rather cold with the extractor running which was drawing in cold air from the hall.

The under floor sensor failed, it should have be able to be replaced, pulling it out of pocket and fitting new, but it stuck, I used the thermostat without the sensor, and to be frank the floor never got that warm even without it, could still walk on floor even if I forgot to turn it off.

Son has under floor heating, but that is more to cool the rayburn cooker than to heat bedrooms, needs a heat sink to keep boiler in cooker cool.

The wet type does work, it can't over heat, and it holds the heat for a long time, so with oil boilers it reduces the hysteresis, not been in a house with Raychem that is rather expensive, but the simple wires is not very good even if everything is A1.
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Postby nickpan » Mon Nov 04, 2019 2:27 pm

Electric underfloor heating.
I found a manual dial which connects to a temp probe, but have been told: "The issue you have is that the Devireg has a floor probe of 15000 ohms and the Aube has a probe of 10000 ohms so it will not read the correct temperature. So if you were to change to the Devireg then you will also need to change the floor probe."
I'm not fussed about the correct temp, just can I turn it on low
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Postby ericmark » Mon Nov 04, 2019 9:47 pm

As you have found there are different types of sensor, all seem to have two wires, K type works on voltage PT100 is 100 Ω at zero degrees. Devimat says 10-20 kΩ.

I tried to find which thermo couple had that resistance, however there are so many types, and often the resistance is not what is measured to work out temperature, but it is a good method to find when they have gone faulty.

Theory is a sensor goes into a pocket, in other words a tube which allows the sensor to be replaced without lifting the floor, however when I tried it was stuck.

I would guess a Thermoplastic NTC Sensor is used, 27 kΩ at zero and 10 kΩ at 25°C but can only guess. However since it does not say at what temperature the sensor is at 10 kΩ it is really not helpful.
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