Installing a wireless thermostat

10 Nov 2008
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United Kingdom
I have a Worcester 24CDi RSF combi boiler and would like to fit a wireless thermostat to it. There is no thermostat or heating timer unit currently installed.

Here are the installation instructions for the boiler:

I have the Thermsave DRF 3 Wireless Thermostat that I would like to install ( The installation instructions for the thermostat are here

Looking at the instructions for both the wiring (page 12 boiler and page 3 thermostat) seems not too complicated but I just want to make sure that I'm 100%.

On the boiler ST8 part we have connections Ns (Neutral), Ls (Live Switched), Lr (Live Remote??), Spare. On the receiver I have N (1), L (2), COM (3), NO (4) and NC (5). Can anyone tell me what NO and NC are for?

From reading articles on this and other forums I expect the wiring to be:

Ns -> N
Ls -> L
Lr -> COM

Presently there is a connection on the boiler between Ls and Lr. I believe that this should be removed and the connection should now be on the receiver between L and COM. Can anyone confirm this?

Any help greatly appreciated.

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On the Rx:

NO = normally open
NC = normally closed
COM = the relay contact which will toggle between the two above contacts depending on whether the relay is energised or not.

All the above 3 refer to the relay internal to the wireless receiver.

On the ST8 connector:

Ns = Neutral send
Ls = Live send
Lr = live return (ie the switched live which is energized by the thermostat)

In your sys I would imagine the connections would be thus:

On your ST8 part:

Terminal 1 (Ns) connected to 1 (N) on the wireless stat receiver
Terminal 2 (Ls) connected to 2 (L) "" "" "" ""
Terminal 3 (Lr) connected to 4 (NO) "" "" "" ""

Finally you will need to put a short link wire between 2 (L) and 3 (COM) on the stat receiver.

In this manner, you are supplying 230V mains to the stat receiver via the L,N connections, you are looping the L connection across to the COM (moving contact) on the stat receiver internal relay, and then returning the switched 230v live connection (from the NO terminal on the internal relay) back to the boiler into the ST8 Lr pin.

As you've mentioned you will find a short link wire already fitted to the boiler ST8 connector block between pins 2&3 which effectively mimic the effect of the relay contacts in the star receiver. You'll need to remove this and you can probably re-use it for the link across the stat receiver pins L and COM.

This is all based on your description of the system parts provided, I can't vouch for the accuracy of the information provided above but someone else should be able to confirm or refute what I've suggested..

(Based on your information your combi expects to see 230v (mains) on the Lr pin to enable to boiler, whereas most current models of combis use 'volt free' contacts ie they don't expect to see mains on the 'stat' terminals, they simply expect a closure across 2 contacts. Connecting 230v mains to the 'volt free' stat contacts on a 'volt free' combi boiler can cause irreparable damage to the controller pcb so double-check you're happy with what you're doing before you start or get someone else to assist)

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