Electrics provision for system boiler and megaflow with immersion heater

[flameport]

On the 35KW boiler and large house. My house isn't small but given the variance in sizing, I am inclined to over size rather than under. Worecester also advised me that the 35 CDI boiler is fully modulated which means it can be throttled back.

I would welcome your thoughts on the above.

A boiler should be sized on the heat losses of the building at the lowest likely outside temperature. The same information is used to size the radiators.

Modulating burners are there so that when the exterior temperature is not at the lowest expected and the heat losses of the building are less, the heat input to the system is also reduced.
They are not there so that people can bung in any boiler they happen to find and hope the boiler output can be adjusted to something appropriate.

As with the electrical side, there is a lot more to heating systems than randomly selecting items from a catalogue and hoping they will work as intended.
You should find someone who can design the complete system.

 

[Jupiter01]

as per advice, I will now be splitting my ring to become 2 radials based on the 2.5mm t&e cable. One will be for Immersion Heater and the other for boiler.

final question - I don't think anyone has mentioned heat resistant immersion heater cable. Is this necessary or will my 2.5mm t&e suffice?

 

[Taylortwocities]

The cable that goes from the isolator to the immersion should be heat resistant.
you would never run twin & earth to the immersion itself.
PS When will your electrician be released from prison, or wherever he is supposed to be. You really need him.

 

[Jupiter01]

OK. To be clear, the cable from Consumer Unit to DP switch can be 2.5mm t&e but the cable from here to the Immersion Heater needs to be heat resistant. If yes, this was my original understanding.

 

[Taylortwocities]

It depends on the value of the MCB on the radial. Do the results of the cable calculations say that 2.5mm t&e is the correct size, given the installation method & other factors?

 

[securespark]

FWIW, here's my input.

I agree wholeheartedly with TTC. Came to the conclusion very early on that it would make perfect sense to run the immersion on a 20A breaker using one of your ring final legs and a 3A CH spur using the other.

If you fitted a 20A MCB for the CH spur at the supply, you could use a switched spur at the other end and if you needed to fit a socket or whatever for anything else, you could tap off that.

Alternatively, if you don't need extra outlets off the CH radial and if you can get 3A MCB's for your CU you could fit one of those and a DP switch locally for isolation.

As far as the cyl is concerned, you need to check whether it is single or dual element.

For dual element systems, you may well need larger than 2.5. This is perhaps what the plumber was thinking of.

 

[Jupiter01]

Thanks guys. I can check on the plumber on whether we have a dual or single element system. The fact that I have a Heatrae Sadia 300l megaflo, does that give anything away?

On the wiring, I understood your consensual opinion to be as follows (not withstandiing the above):

- for boiler (and spare sockets) have a 20A MCB breaker with a 2.5mm twin and earth, going into a 3 amp switched fused connection unit and then on to a set of sockets too. This would be a radial and not a ring. There is hardly any insulation that is interfering with the path of this cable.

- for cylinder have a 20A MCB with a 2.5mm twin and earth, going into a 13 amp switched fused connection unit. This would be a radial and not a ring. There is hardly any insulation that is interfering with the path of this cable.

 

[securespark]

Looked up the installation instructions: it says there can be 1, 2, 3 or 4 elements fitted.

Each is 3kW.

 

[ban-all-sheds]

OK. To be clear, the cable from Consumer Unit to DP switch can be 2.5mm t&e but the cable from here to the Immersion Heater needs to be heat resistant. If yes, this was my original understanding.

When will your electrician be released from prison, or wherever he is supposed to be. You really need him.

 

[ban-all-sheds]

Thanks guys. I can check on the plumber on whether we have a dual or single element system. The fact that I have a Heatrae Sadia 300l megaflo, does that give anything away?

On the wiring, I understood your consensual opinion to be as follows (not withstandiing the above):

- for boiler (and spare sockets) have a 20A MCB breaker with a 2.5mm twin and earth, going into a 3 amp switched fused connection unit and then on to a set of sockets too. This would be a radial and not a ring. There is hardly any insulation that is interfering with the path of this cable.

- for cylinder have a 20A MCB with a 2.5mm twin and earth, going into a 13 amp switched fused connection unit. This would be a radial and not a ring. There is hardly any insulation that is interfering with the path of this cable.

When will your electrician be released from prison, or wherever he is supposed to be. You really need him.

 

[Jupiter01]

thanks for checking that @securespark.
in this case, I assume that my conclusions based on all the help you have provided are valid?

separate radial for boiler and immersion heater using 2.5 t&e with Switched Fused Connection Units on the end of each line.

 

[EFLImpudence]

You do not want a fuse for the immersion - just a switch.

 

[Taylortwocities]

 

[securespark]

But you need to contact the plumber immediately to check how many heating elements the Megaflo has.

If it has 2+, you will need to upgrade the supply.

 

[ban-all-sheds]

separate radial for boiler and immersion heater using 2.5 t&e with Switched Fused Connection Units on the end of each line.

When will your electrician be released from prison, or wherever he is supposed to be. You really need him.