Electrics provision for system boiler and megaflow with immersion heater

[JohnD]

An immersion heater is about 3kW and uses about 13Amps.

The size of the cylinder, and the power of the boiler, are both immaterial.

In the same way that a 20W lightbulb always uses 20W, however big the room you put it in.

However, if the cable passes through insulation (e.g. in the loft) which surrounds it, it may need to be thicker. It would be easier and more sensible, though, to route it so it is not buried in insulation.

You will presumably only be using the immersion heater when your boiler is out of action.

 

[JohnD]

I am preparing a boiler room to accommodate the above and was wondering if someone can advise me on what provision I need to make by way of electrics. Currently I have a separate ring main which comprises of a double power socket and a FCU.
Thanks

A ring final is not normal for this.

It sounds like you are not using a qualified electrician.

Provision of a new circuit is notifiable work under building regulations. How will you do that?

 

[kai]

Single 20A radial circuit, 2.5mm cable would typically be suitable, at the end a 20A switch with neon indicator for the immersion and a 3A fused spur for the boiler & controls.

Should it be a B16 breaker feeding a 13amp immersion heater controller, plus a 3amp fused spur, which nicely makes it up to 16amps?? - Just asking.

 

[Jupiter01]

Thanks @JohnD.
The cable will be routed through the loft but hardly any insulation will be touching it. The power supply to the boiler room is on its own circuit too and not shared with anything else. Seems like I have ample here?

Yes, the immersion heater will only be used as a backup.

 

[kai]

You could use one of these purpose-made immersion controllers, they mean that the immersion can't be left on for extended periods by mistake, as the timer automatically shuts down the immersion after the set time has elapsed. It's rated at 13amps ideal for standard 3kW elements.

http://www.timeguard.com/products/t...fbt4-2-hour-electronic-boost-timer-fused-spur

 

[JohnD]

Ordinary Immersion switches are rated at 20A, they may be run continually for long periods. A Megaflo or similar often has an immersion heater near the top and one near the bottom, to be selected depending how much hot water you want, and how quickly. In which case you might use something like this
https://www.tlc-direct.co.uk/Products/MKK5208.html

It does not matter if the cable touches insulation (as long as it is not plastic foam or polystyrene) but it should not be surrounded,

 

[EFLImpudence]

Should it be a B16 breaker feeding a 13amp immersion heater controller,

It should be whatever breaker is suitable for the load and cable.
16A breaker and 2.5mm² cable is usual but that's a remnant from BS3036 15A fuses which required 21A cable.
Nowadays with 16A MCB only 16A cable is required.
16A timer/controller would be better.

plus a 3amp fused spur, which nicely makes it up to 16amps??

Not really, the boiler won't be more than 1A.

 

[Iggifer]

Single 20A radial circuit, 2.5mm cable would typically be suitable, at the end a 20A switch with neon indicator for the immersion and a 3A fused spur for the boiler & controls.

Should it be a B16 breaker feeding a 13amp immersion heater controller, plus a 3amp fused spur, which nicely makes it up to 16amps?? - Just asking.

You could use a 16A, a 20A or a 25A. It doesn't matter, each one is adequately protecting the fixed wiring.

 

[Jupiter01]

Thanks All.
Can I please clear up once again, that the following is sufficient (possibly OTT) for what I am installing here:

This is a separate ring main which is using 2.5mm twin and earth. The intention is to fit 2 FCU (Boiler and immersion heater) and have a double socket for anything else.

This is wired as a ring main e.g. cable comes from Consumer Unit, into first FCU, then to the second, then to the sockets and then back to CU

 

[kai]

It should be perfectly safe, if that's all you are supplying (3kw Immersion plus a 3amp fused spur for the Boiler and its Pump). A correctly installed Ring Final Circuit can have (from two to a large number of outlets in it!). One thing to watch out for though, if you have a B32 breaker at the origin of the circuit, the 13amp protection needs to be present for the immersion (immersion manufacturer's instructions often state that they need to be fed either with a B16 breaker, 15amp rewireable fuse, or a 13amp cartridge fuse or equivalents for a 3kw unit).

 

[Taylortwocities]

Who is undertaking the wiring for the boiler controls, especially the overheat safety trip in the Megaflo for the DHW circuit?
What provision has been made for that, or is somebody who actually knows what they are doing responsible for it?

 

[Taylortwocities]

Also

Personally, as the immersion is the standby in the event of boiler failure, it is nuts to have the boiler and the immersion on the same circuit…
Think about it: you wake up and the RCD has tripped, you trace the problem to the circuit with the boiler on it, you try everything but the darn RCD keeps on tripping.
You cannot use the immersion because the RCD keeps tripping.
You have no hot water, nobody can have a shower and the electrician/plumber is 2 days away.

Immersion on its own radial, boiler on a separate circuit. you know it makes sense.

 

[Jupiter01]

Immersion on its own radial, boiler on a separate circuit. you know it makes sense.

Yes it does make sense...

My job was to lay the ring main to first fix level and the electrician will be connecting in to CU and the services on the boiler end.

overheat safety trip in the Megaflo for the DHW circuit

Does this amount to the need for a further FCU/socket?

 

[Taylortwocities]

[
Does this amount to the need for a further FCU/socket?

No. You obviously need to provide a mains feed and FCU for the boiler itself.
There is other wiring to provide heating & hot water controls/programmer/thermostat and safety trip circuitry. Who is running the wiring etc for all of that?

 

[Jupiter01]

I need to lay two further cables and they are for the purpose of the thermostats/programmers (2 zones). Can you please advise on the cable I need to use for this?