Replacing old UFH thermostats

[zakinho]

I have an electric underfloor heating system with thermostats that must be 50+ years old. I want to replace them with modern wifi versions that I can schedule/control. Based on the pictures shown here, what are my options and in terms of wiring how should any new stat be wired?

 

[ericmark]

I see the same pictures on the Screwfix forum, and you say electric UFH, so 1974 (50 years ago) I do not remember any homes with electric underfloor heating, clearly under floor heating was used in Roman times, so has been around a long time, but as to the way electric UFH was installed back then I don't know.

My first thing would be to test, insulation resistance tests are clearly important, seems likely to have lasted this long mineral insulated heating cables, which are well-known for drawing in water. To not add RCD protection could be dangerous, but adding RCD protection it may be impossible to stop it tripping.

Also, the maximum temperature could be a problem. It may need some floor temperature sensors, again, only testing will show what is required.

Personally, when hunting for a new home I rejected any with under floor heating, as nearly impossible to work out if insulated to stop heat going down into the earth, but if you already live in the home, you should know that already. I had the job to heat welds to 600ºC called stress relief, and so realise one can develop a program to allow fast recovery without over heating the floor, using programmable logic control, but this is expensive, so in real terms the recovery time tends to be slow, and one needs a thermostat which is either designed for UFH or has a program selection for UFH for example

The Google Nest Learning Thermostat 3rd Generation (3rd Gen) is compatible with many types of heating systems, including underfloor heating (UFH), but it's not compatible with electric underfloor systems.

 

[RandomGrinch]

It appears to be a Satchwell Type TM room thermostat...

Satchwell stat

And it seems to have a contact rating of 20A. This is quite high for some modern smart UFH stats - How are these circuits supplied with power? A photo of the fuses/breakers on the consumer unit may help us advise on suitable alternatives.

Also, as Eric suggests, it may be prudent to have the circuits checked out.

 

[zakinho]

Good spot…. The one I was looking at was painted over the identity plate but I have others which appear to be a model TM S15.

I now know they’re rated at 20A but the breaker in the board is rated at 16A and when test the load (clamp meter at the board) in the bedroom it’s about 11A. I am assuming that whoever upgraded the consumer unit tested and was aware that the current was below the breaker they’ve installed.

So my next question is, armed with that information does that open any other options for thermostats?

 

[RandomGrinch]

So my next question is, armed with that information does that open any other options for thermostats?

Something like the Meross for electric UFH may suit, it has a 16A max load...

https://shop.meross.com/products/meross-smart-thermostat-mts200hk?srsltid=AfmBOoo071ETgeevLzrQ1mNFHTsMU95TGcbtI1oQca7TniNJGGttkTva

 

[ericmark]

My worries have been answered as already on a 30 mA type A RCD.