Replacing a 2-wire, low voltage room stat

[philos4424]

I need to replace a failed thermostat in my living room. I have a simple electric boiler supplying hot water to underfloor heating - this is switched by an OJ Electronics WLM Master Underfloor Heating Controller (now discontinued). As well as the boiler, this controller also switches the pumps and opens the actuators on the UFH manifold and has a 2-wire low-voltage bus to which 4 seperate zone (room) thermostats are connected. The thermostats are basic OJ Electronics WLTA-19 dial adjusted thermostats (not mechanical), also now discontinued. One is no longer working and I need to replace it. There are 2 thin wires running from the thermostat to the low voltage bus (+ to + and - to -), they obviously don't carry any great current. I seem to remember reading somewhere that the thermostats were rated as 5 volt but I can no longer find the spec. I need advice on a suitable replacement thermostat as I am confused as to the different types of thermostats (2-wire, 3-wire, 24V, 240V, etc). I'm happy to consider one of the newer smart thermostats if it would work with the system I have. Thanks in advance.

 

[RandomGrinch]

Hi,

I don't know for sure, but I believe this would be compatible?

https://www.radiatorshop.co.uk/radi...mostat-Underfloor-Heating-5v-Rehau-p173750944

You could also try the following brands:

"Although manufactured by OJ Electronics, these WLM controls are often branded as Rehau or Giacomini"

I may be wrong, but I think the bus system limits you to using certain 'stats from the above brands (for a direct replacement). If you wanted a smart thermostat, it would have to be wired in to your system elsewhere.

I hope that helps

 

[Madrab]

Most 2 wire thermostat should work. The fact some may be rated at 240V just means they can carry 0>240v to handle switched live boilers. They are also used on volt free combi's etc and are basically just on/off switches.

 

[RandomGrinch]

Most 2 wire thermostat should work. The fact some may be rated at 240V just means they can carry 0>240v to handle switched live boilers. They are also used on volt free combi's etc and are basically just on/off switches.

Thanks - I was only going by the manual:



But we all know how wrong they can be sometimes!

...I think it might depend on how you emphasise 'only'!

 

[Madrab]

That's a fair enough consideration and I guess it comes down to why they specify that only OJ stats can be used. If it's a volt free bus that is only looking for an on and off connection then any stat should be compatible, that being said there may be internal component considerations that the bus requires that are part of the OJ stats internals to allow them to be compatible.

@philos4424 I guess a call to their tech would be advised to understand if only their components can be used and why.

Looking further into the manual it may be down to correct communication and zone control, where the OJ stat must be configured to control it's own specific zone and be able to talk to the bus properly, looks like I was being too generlised and didn't read the the original post properly!!

 

[philos4424]

Okay, thanks for the advice, one and all. It would seem that the OJ stat has a selector that must be set to a specific output and that in turn operates a specific actuator/zone. So a "bog standard" 2-wire stat won't do as it hasn't got that ability. - unless anyone knows different??? The Raditor Shop sells an updated OJ version that is backwards compatible - for £90!!!! I've searched high and wide for a cheaper alternative, without success, so it seems like if I don't want to freeze this winter, I'm going to have to fork out for it.