Spur from Boiler Circuit for Volt-Free Switching Thermostat

[Chris_J]

Hi everyone.

I'm about to install a Wireless RF Thermostat to my combi boiler. The receiver unit which does all the switching requires a permanent mains supply. It will not actually switch the live pole - the switching circuit is completely voltless, but the relay unit needs a 240v supply and I'd like to know the safest way of doing this.

My boiler is located in a cupboard right next to the CU. It is wired into the CU on its own circuit, via a DP fused switch. Initially, I wanted to take a spur from the switch, but since the switch is wall mounted, I would have to do some chasing which I want to avoid. So, I have three ideas:

1. Tap into the supply cable for the boiler, just before it enters into the boiler's PCB. I guess this would mean cutting the cable and wiring in a junction box to spur from. I can safely mount the box on the back side of the false wall the boiler is mounted on and this means the DP switch will control both the boiler and the receiver.

2. Spur off a local double plug socket next to the CU (presumably used in the past to power a freezer in the cupboard) as this is fixed to a wall mounted backbox, and the cables are not chased into the wall.

3. Last idea is to spur off the boiler's circuit directly at the CU, and run a cable straight to the thermostat receiver.

Option 2 I'm not keen on, as I think running the thermostat receiver on a separate circuit to the boiler is a bad idea. However, the receiver is a volt-free relay, so it would never make the boiler become live if the boiler circuit was isolated. On the other hand it would mean having to isolate the sockets in order to isolate the receiver.

Option 3 bypasses the DP switch and fuse, could this be a problem if the fuse blew, but the circuit didn't trip?

What do you think is the safest and most sensible way to do this?

Thanks in advance
Chris.

 

[sparkydude]

Hi From what your saying the feed should come from the boiler itself, can you not run another flex into the boiler and connect it into the same terminals as the feed is connected to?. You would need to be getting into the boiler anyways wouldnt you to connect up the switching termianls for the stat ?

Normally boilers have plenty of cable camps and grommets to allow wiring up of the external controls.

Nick

 

[Chris_J]

Yeah, that was my initial thought when I bought the thermostat, but then I was having a look at it this evening and the boiler PCB is housed in a control unit casing, where all the supply connector blocks would be, but there are no spare cut-outs or grommets to run another flex, unfortunately.

The boiler manufacturer has supplied a long cable coming out of the casing which is the two cores required for the volt-free switching (currently bridged so boiler runs normally.) The instructions for the boiler state this is how the thermostat should be connected, but mentions nothing about tapping a power supply from the board, which has lead me down the path of spurring from elsewhere.

Thanks for your reply

 

[sparkydude]

In that case then , can you not take the flex from the boiler to the spur and break into it, by taking the two ends into the transmittter and connecting them together in the transmitter terminals. Or use a single surface box with a blank plate and some connetor blocks to break into the feed cable .

Nick

 

[Chris_J]

Not sure what you mean by your first comment? Do you mean run the supply to the receiver box first, then spur from there to the boiler?

Is you second idea similar to my first option, except I wouldn't cut the supply to the boiler, just remove it and split inside the surface box?

Cheers.

 

[sparkydude]

yes, take the cable that goes from the spur to your boiler now and cut it in half, take the two cut ends into the reciever unit and connect them both to the terminals in there. Or as you said a surface box and split the cables in there.

Nick

 

[Chris_J]

Good stuff. I think I'll go with splitting the supply cable and linking them in the receiver unit. Means I don't have to buy anything then!

Thanks for all your help mate.

Cheers.