Please help with this central heating wiring question!

I’m hoping someone with more experience of central heating systems can help with this problem, as I’m sure it must be a common situation.

A plumber has replaced a system boiler, which heats the central heating and also a hot water cylinder. The system comprises a programmer, pump, valve actuator, hot water cylinder (with cylinder stat) and room stat which are all located in the same place – an airing cupboard. The boiler itself is in a different place – the kitchen.

There is twin and earth (i.e. no switched live) between the wiring centre in the airing cupboard and the boiler. This means that when the programmer/stats stops calling for heat the power to the boiler is killed completely, so the pump overrun in the boiler doesn’t work. Likewise, when the programmer/stats start calling for heat the boiler effectively gets powered up from scratch and has to warm up before it starts producing heat.

It’s not practical to run 3-core between the airing cupboard and the boiler, so I think I need some kind of wireless solution so the boiler can be left permanently powered up and just receive a signal when it’s time to produce heat.

Ideas I have had are:

1. Taking a spur off a plug in the kitchen for a permanent L+N, and using one core from the existing twin and earth as a switched live. This doesn’t seem ideal as the boiler would then be powered from two different circuits.

2. A wireless programmer. Seems unnecessarily expensive.

3. Using the existing twin and earth as a permanent L+N and installing an RF stat, left on its highest setting, to tell the boiler when heat is required. This seems like a bodge.

4. Some other kind of wireless contactor?

This must be a common situation with a proper fix. Please can someone help?

Thanks in advance.

How about mounting a relay in a box next to the boiler. Run a supply from a 3A fused FCU as you've suggested to supply the boiler. Take a line into the relay box and connect to one side of the relay contacts. Then take the output of the relay as your switched live. Run the relay coil using the existing T&E from the airing cupboard.

This then provides a single point of isolation for the boiler. The only thing with two supplies is the relay box, which should be appropriately labelled.

Or as above and fit a 12v psu/tranny in the cupboard so as to send 12v down to a 12v relay next to the boiler.

Boydy

replace the boiler with one that doesn't need 3 core!!

Practical or not, the only way to go is run a new t+e to the boiler, use one cable as perm L+N and the other as switched live and pump live. If no other route is available go up the airing cupboard into the loft and down external wall in tube, drill back in by boiler and terminate into triple pole switch(fan iso), 5 core h/r into boiler, easy life!

Practical or not the only way to go is run a second T+E from the wiring centre to the boiler, use one cable as perm L &N and the other as switched live and pump live

Murdoch said:

replace the boiler with one that doesn't need 3 core!!

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That'll be my one, that was brand new in 1975 then.

Most of the modern boilers with low water content heat exchangers will be designed for pump run-on to remove the residual heat when the burner is turned off.

The new potterton only needs a s/l,n,earth.

Sharpend said:

The new potterton only needs a s/l,n,earth.

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So it does. Interesting. Thanks.

Thanks very much for all the helpful replies.

I think BoredSparky has the right idea and I should just bite the bullet and run a new cable if possible. I'll go and have a closer look to try and find a suitable route.

As for the relay idea, I've not played with relays much. Any chance HandySparks or some other kind person could send me details of a suitable product I could use?

What's anyone's view on just using the existing cable as the permanent supply and installing an inexpensive (£30) RF stat, left on it's highest setting, to give a switched live?

Thanks again

Or, for a 230V one:

230v AC 8 Pin Relay 2 Pole

Stick it on a proper base in a large adaptable box.

Like this:

One thing I didn't think of - the FCU for the heating controls is on its own circuit, so I could just put it into the breaker for the downstairs ring main as a spur. Then I can take a fused spur from a socket near the boiler, use one core from the the existing boiler T+E as a switched live, and everything in the boiler will be on its own circuit. Huzzah!

If I've understood you correctly, it's a good idea to feed both boiler and controls from one final circuit, then at least the heating only loses supply if one circuit fails rather than two. However, I'd be very careful how I labelled the FCUs. Plus I think a prominent label on the boiler, warning of the two supplies would be advisable. I still think the relay would be worth doing, given that anyone servicing the boiler would expect to be able to isolate it completely at one place only. If you don't use a relay, maybe fit a 3 pole isolator for the boiler only?

sparkol said:

One thing I didn't think of - the FCU for the heating controls is on its own circuit, so I could just put it into the breaker for the downstairs ring main as a spur. Then I can take a fused spur from a socket near the boiler, use one core from the the existing boiler T+E as a switched live, and everything in the boiler will be on its own circuit. Huzzah!

Click to expand...

FFS! wtf, etc. (oh and to quote MJ, GASP!!!!)

Boydy