central heating thermostat wiring

[Bill Wright]

I have a new Worcester-Bosch boiler and a Honeywell radio thermostat. Because the 'receive' part of the stat was moved some years ago to a place where the only reasonably possible connection to the boiler was via an existing two core cable, the 'receive' unit is powered from it's own mains supply, which is not via the boiler.

Because of our frequent power cuts I want to be able to power the heating from a generator. It appears to be a simple matter to replace the fused spur feeding the boiler with a 13A socket, thus allowing the boiler to be connected to the generator, but there is the problem of powering the stat. It is not an option to power the circuit the stat is on from the generator.

So I'm wondering about simply bypassing the stat during a power cut and running the boiler continuously at a low temperature. The stat connections in the boiler are to the LR and LS terminals. The ground and N terminals for a stat (on the boiler panel) are not used. The stat connections pass through a junction box adjacent to the boiler, and it would be easy to add a switch that would connect LR and LS together. Is this reasonable/sensible/safe?

Bill

[fred]

In article , Bill Wright writes

The Honeywell boiler control provides 'volt free' isolated contacts for switching boiler demand that are not referenced to the mains supply to the control.

If the wiring to those contacts has not been referenced (connected) to the mains supply (live or neutral) for the boiler control in any way then it is safe to add a switch in parallel with those contacts and use it as you suggest. Common sense would suggest marking it clearly something like 'C/H thermostat override'.

What is more worrying is the fact that the boiler control is not fed from the same supply as the boiler which is an absolute no-no. To protect anyone maintaining the heating system, all boiler controls are supposed to be isolated when the boiler isolator is operated so I'd recommend finding a way to change that some time soon.

[Graham.]

So why can't the receiver be sited near the boiler?

[Bill Wright]

It's out of RF range. It's in a different building.

Bill

[John Rumm]

It should be ok - the stat will look like a pair of volt free contacts (although depending on how its wired the boiler will also probably support a live mains in as a call for heat as well).

Note that if running the boiler form the genny you will probably need to make sure that earthing is correctly connected for the flame sensing to work properly. So basically set the genny up with an earth spike like a TT system, and tie one leg of its output to the earth.

[Bill Wright]

(snip)

Thanks for that. Most helpful. I think I'll get onto my favourite electrician and see what he can do.

Bill

[Bill Wright]

Invaluable info there. Cheers.

Bill

[Bill Wright]

Yes, this turned out to be necessary. Cheers.

Bill