Wet underfloor heating with electric thermostat

[Marritza]

I'm hoping for someone to help explain the below - I'm not an experienced DIYer but always willing to learn...

We have wet underfloor heating on the ground floor and one of the old underfloor heating thermostats (from about 2012) blanked out after a power cut. Basically, the screen never came back to life (it had blanked out previously when being programmed but then recovered). While looking for a replacement we were advised by Heatmiser that the direct replacement would be their Slimline V4 model (photo attached). Thinking it is a straight swap we opened the box and found out that we seem to have an electric underfloor heating thermostat - a PRT-E (photos attached).

Called Heatmiser back to ask what to do - they told us that they have no idea why an electric thermostat should have been installed on a wet underfloor system and that we should still use the Slimline V4 rather than the Slimline-E V3, their electric variant (in that case I have no idea where the black wire would go that is currently going into the Load terminal...).

Would anyone have an idea about why the installers might have used an electric thermostat on a wet underfloor system and what the issue would be with replacing it with the Slimline-E V3 version again? Taking into account that:

- There are 4 additional thermostats on the same floor, which I suspect would be the same electric version
- We definitely have wet underfloor heating with a system boiler (hot water tank) - photos attached
- The house was built at the same time the thermostats were installed (2012) and although we've only been here for a couple of years, we haven't noticed any issues (the last check of the electrics was in 2022).

Also, I'm not sure why there is no probe - the new E-V3 version seems to come with a probe. TBH, all the thermostats seem to be showing an ambient temperature that is too high but we have got used to it.

Thank you!

 

[Murdochcat]

As long as the thermostat has the option to sense the air temperature, then it should work ok.

 

[ericmark]

New thermostat will need a link as volt free, the sensor will need to read instructions, with an electric system the cheap version not chemical type it needs a sensor to stop floor over heating, but with water the water temperature is controlled to stop over heating, and with chemical electric the heating strip varies resistance so also self regulating, so no need for sensor in the floor.

However the control and point of sensing do not need to be together, with a standard radiator the air moves with thermals so best place to sense is under the radiator so it senses the return air, but to work the thermostat it needs to be higher, and to see the thermostat also, so eye level in a wheel chair is 1100 to 1200 mm, those not in wheel chairs can bend down but those is wheel chairs can't jump up, so thermostats normally placed at 1200 mm above floor level, which may not be best place to record temperature.

I have no idea how thermals work with UFH, I did look at the Hypocaust (from the Greek word meaning 'Fire Beneath') in Chester, but it does not show any way to control how hot the floor would get, I am imagining the Romans invented dancing as the floor was too hot? But joking apart where one would measure the temperature with under floor heating not a clue.

Can't understand the concept, to my mind I want to spend as little money as possible on heating, to do that reaction time is rather important, i.e. how long before I want to use a room, do I need to heat the room? So I would set the TRV heads with my radiator systems so first kitchen, then dinning room, then living room, and after a good gap the bedrooms were heated. In other words in the same order as I use the rooms when returning home.

It does present a problem with office and craft room in that we have to set heating to come on as we enter the room, and accept it will take time to warm up. This house is a lot slower than the last house, with last house 20 minutes and living room warm, this house more like 2 hours, but the electric UFH in mothers wet room would take nearly 4 hours to reheat the room unless towel rail also used.

So unless the heating is never turned off, can't see how UFH works? The whole idea today is analogue control. So not turned on/off but up/down, so not a solenoid but a motorised valve so it can adjust how much it is open. This is how the TRV heads work. So my TRV is both motored more open or more closed, and I fail to see how any on/off thermostat can work with today's modulating boilers. At least in an efficient manor?

 

[Marritza]

Thank you both. My main question is why there might be a UFH thermostat for electric floors installed at my house even though I have wet/water UFH heating. Heatmiser told me that wouldn't work but clearly it has worked for 12 years. I will need to replace the thermostat ASAP, so I'd like to understand whether there is any reason NOT to install a thermostat for electric floors again (which would be easier for me as an inexperienced DIYer).

 

[DetlefSchmitz]

The PRT-E was being used simply as a room (air) temperature thermostat. That was one of its functions. If the user had wanted they could use it with electric underfloor heating so that it could ensure that the floor itself didn't get too hot.
Feature 08 – Sensor Selection: On this thermostat, you can select which sensor
should be used. You can select between air temperature only, floor temperature,
or both.
It was perfectly OK not to have a floor sensor, and just use it as a room temperature controller.

So that is what the replacement thermostat should be set up to do.

The one you bought was really intended for electric UFH with a probe. The instructions seem to suggest, though, that you can use it as just an air temperature controller using the built-in sensor. See the settings instruction to achieve this.

 

[ericmark]

I look at the cheapest programmable thermostat as a start point battery powered and cheap no frills, seem to remember 4 programmable slots or even non programmable and as long as electronic rather than a bi-metal strip type then the slewing range is likely 0.5ºC and likely all you need. Only reason for more expensive is remote control.

I used the programmable one in my old house for years, batteries last around 2 years, so I changed every year so did not run flat. The house was open plan, and boiler non modulating, so one thermostat did whole of down stairs. Upstairs TRV's stopped over heating.

What you have is fine, only way to improve is go for analogue controls, and I don't know how you do that with UFH.

 

[Marritza]

Thank you for the advice. I will then replace the thermostat with the electric version with air temperature option.