On Thu, 04 Sep 2008 13:31:02 +0100, David Hansen
wrote:

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

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.

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.

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.


2. 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.

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
http://www.screwfix.com/prods/48234/Electrical/Switches-Sockets/Mounting-Boxes/Appleby-Galvanised-Steel-Dual-35mm

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.

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