Power Supply to CH Boiler

[DavidM]

I'm having a replacement CH boiler (Worcester Bosch Greenstar 24Ri) fitted in a couple of weeks in the airing cupboard in the first floor bathroom. The existing boiler is immediately below on the ground floor, so all of the existing pipework is quite convenient.

I've agreed with the plumber that I will provide an FCU in the airing cupboard, and put in a new cable from the room thermostat on the ground floor. I think I have 3 options for providing the power to the FCU.

1. A short spur from the first floor ring main. This is the most obvious and probably easiest, however I'm not sure about the integrity of that ring main (it's been extended in various ways over the years, including to a second floor loft conversion) plus the cable is buried in the walls in many places (17th regs, RCDs and new CU - shudder!).

1. A short spur from the supply to the immersion heater in the HW cylinder (also in the airing cupboard). Simple, plus the cable is not buried at any point (runs down via a ground floor cupboard into the underfloor void and from there to a cupboard containing the CU), so no RCD needed. The heater is on a dedicated radial via a 15a MCB. However the heater is rated at 3Kw, plus say 3a for the boiler, so pushing the MCB a bit (maybe). Pretty sure it's on 2.5 T&E but need to check.

2. Either extend the existing spur/FCU from the old boiler up to the airing cupboard directly above or put a new spur from the ground floor ring. Fairly easy to do. However the existing FCU is a spur on the ground floor ring main (separate MCBs for ground and first floor rings) and this ring main is also hidden behind some wall paneling in the kitchen (elsewhere is just in the under floor void). So maybe another 17th regs and RCD issue again.

Obviously if the boiler is anywhere other than on the first floor ring the FCU will be labeled accordingly.

The main thing I'm trying to avoid (without being unsafe - as distinct from obeying all the "rules") is the plumber's sparky trying to insist on a CU upgrade to 17th standard, plus all the other bits associated with this.

What's the "best" option for the boiler supply, or maybe a variation on the above?

Any constructive advice gratefully received, and any other points to watch out for that the sparky might be tempted to wring extra business out of, and where I can do something to head this off.

Thanks, David

[David Hansen]

On Thu, 04 Sep 2008 10:21:54 +0100 someone who may be DavidM wrote this:-

The essential thing is that all the central heating bits are fed from the same circuit, so there is only one place to switch the lot off. Boiler, pumps, thermostats, valves and so on should all be connected to one circuit.

Not actually a spur, just part of the radial circuit. Fit a switched fused connection unit beside the switch for the immersion heater and that's that. If there is not a local switch for the immersion heater fit one, it can go alongside the connection unit in a dual box

Immersion heaters are generally used as backup devices. If they are in use then the boiler is not working and thus the pump(s)/valve(s) and boiler itself are not drawing much current. I doubt if many heating systems draw more than 1A at any time anyway.

In theory the flaw with this arrangement is that it is a common point of failure for all water heating. This is a slight risk, but only a slight one. If there is a fault with the heating system or the immersion heater the faulty one can be isolated and the other used. That just leaves a fault on the common cable. How often are these damaged? Extremely rarely in most houses.

[DavidM]

Yes they will be. All the bits will be adjacent to the boiler, except the room stat, and I'm putting the cable in for that such that it will pick up it's connections local to the boiler.

OK, thanks. I'll label the FCU and the MCB to make it clear what they connect to. Can you or anyone else think of any regs that the sparky might throw at me to say this is not allowed/bad practice? David