Central Heating control - wire size

[Andrew]

Hi all,

I want to add an additional wire from CH controller (HW Off terminal) to the junction box by the boiler so that I can have the heating on without the hot water. At present there is nothing connected to this terminal.

Is 0.75mm wire too thin? I wouldn't imagine that there is very much current required, even if it's still 240V

thanks a lot, Andrew

[Dave Plowman (News)]

There's usually a 3 amp fuse in the FCU feeding the entire CH electrics so

0.75mm is fine.

However, I hope you've got suitable controls to allow what you want. Basically it has to have a pumped HW primary with either a three port valve or two motorised ones. If the hot water primary relies on thermo circulation it won't work.

[Andy Wade]

Err... do you know what you're doing? What sort of heating controls have you got? Why is the system likely to be incorrectly wired at present? If you have a old system with gravity circulation for hot water it will be impossible to have HW off while CH is on, although a thermostatic cylinder valve could be fitted to limit the DHW temperature.

Wire? You cannot use a single unsheathed equipment wire, unless it's enclosed in a conduit or trunking system. Otherwise the wire should be a core in a sheathed cable, either a multi-core flexible, or flat twin-and-earth or 3-core-and-earth wiring cable. (Single and sheathed cable (6181Y) is also OK, but that's not very commonly available in the smaller sizes now.)

0.75 mm^2 is OK if it's a core in a flexible cable, and assuming 3 A or 5 A fusing. Cores in non-flexible permanent wiring cables are subject to a minimum size of 1.0 mm^2.

[Graham]

I did something similar. The Randall 102 electro-mechanical timer had off / dhw / dhw&ch but no 'ch only' setting, and the system is fully pumped with 2 x 2 port moterised valves. So I wired a separate switch box with two three position switches so DHW and/or CH can be either timed off or cont. Also wired a "booster" switch which shorts out the cylinder stat prior to filling the bath. All wired in 1mm T&E

That that 25 years ago, its still going strong.

[Andrew]

Thanks for the info all. Yes, I do have a 3 way motorised valve and fully pumped system. The system is currently wired with 4 core and has no connection between the HW OFF on the controller (Honeywell ST6400C

7 Day Programmer) and the 10-way junction box . I guess it is not connected because it's not essential - not everyone would care about having CH but no DHW. Rather than replace the complete run (~10m) with 5 core, it will be cheaper & easier for me to leave the existing and add a new cable alongside. The cable I have/was proposing is: 2182Y 0.75mm 2 Core Round Flex. 50m. HO3VVF CENELEC / HAR approved. 2182Y flexible cord for light-duty appliances. PVC insulated and sheathed.

The existing run is cable clipped around the wall of my garage and I was going to do the same with the new cable, alongside. I didn't think I would have to use additional conduit but I suppose regulations have changed since the existing run was installed, and I shouldn't use round flex for permanent installation anyway...

thanks, Andrew

[Lurch]

On Tue, 17 Jul 2007 08:51:10 -0000, Andrew mused:

Correct.

[Roger Mills]

Presumably the cylinder stat already has change-over contacts in order to provide a HW-satisfied signal? If not, you'll need to change that, too.

[Andrew Gabriel]

There is no such restriction in the regs. There are situations where flex is required in a permanent installation, but none where it is forbidden.

[Lurch]

On 17 Jul 2007 18:26:55 GMT, [e-mail] (Andrew Gabriel) mused:

Er, yeah. I read that wrong, what I meant was flex isn't really for fixed wiring.