UK Plumber Forum 

A place of resource for Plumbers and Gas Engineers in the UK.

Ask your general plumbing questions here. Please have a quick check to make sure it wouldn't fit in any other category.
100 %
By Bobbyspit
Hi guys

I am changing my room thermostat on my underfloor heating system!

I have removed a very old Horseman three wire HRT3 replacing with a modern digital two wire unit from Sallus

My issue is the underfloor hearing control board (24v) has three wires and the new thermostat is the two wire with the options of normally open or normally closed circuits.

The wiring into the control panel which is a standard gmfour zone 24v generic unit has the options for up to 4 wires
The old set up utilised the negative as well as the positive and the switches positive.

I am looking for the correct way to wire up the new thermostat to the control board , I initially blanked off the neutral wire behind the thermostat and used the feed into the COM and the switches live into the NO but getting no activation to the system when when increasing the stat which I know is working correctly

Any assistance appreciated

By joni os
The two wire salus requires just a positive feed in and sends a positive feed back when heat required. Blanking the neutral is correct because it is not required. If not working check:-
When programme calling for heat, wire to common terminal is live. When stat clicks on, return wire becomes live. Then check where these wires terminate in junction box/ wiring centre.
Be sceptical about wiring, it is not unusual to discover a blue neutral or green earth has been used as a live conductor.

Have a look at the Vaillant ecotec 938 - just to t[…]


Hello everyone! I am Athena Faris, happy to be her[…]

Thanks guys, let me try an allen key in it and see[…]


I like the old systems the greeks and romans i[…]