Two immersion heaters.

[David Costelloe]

I am in the process of going unvented (as we speak...). I have to use immersion heaters, and the new cylinder has two. I have one feed to the existing immersion. It is on 2.5mm2 T&E and connects to a 32A MCM on the CU. We're adding a DP isolating switch in the cylinder cupboard. Will that be complaint for one of the immersion heaters?

To fully install the cylinder, I plan ask an electrician to either
(1) run an additional 2.5mm2 T&E cable back to the CU, connect it to the existing 32A immersion heater MCB at the CU. So the MCB protects two separate cables and the 2.5 T&E is within its capacities.
Or
(2) run a new 6mm2 T&E cable back to the CU, connect to the existing MCB and in the cylinder cupboard, split the feed to two DP isolators, one for each immersion and its timer.

I'd be grateful for advice - are both approaches compliant? Is one better than the other? The amount of work will be similar.

 

[Harry Bloomfield]

I am in the process of going unvented (as we speak...). I have to use immersion heaters, and the new cylinder has two. I have one feed to the existing immersion. It is on 2.5mm2 T&E and connects to a 32A MCM on the CU. We're adding a DP isolating switch in the cylinder cupboard. Will that be complaint for one of the immersion heaters?

Change the 32amp to a 20amp or a 16amp and that part will be fine.

Usually, when two immersion heater are fitted, one is at a high level in the tank and one towards the bottom. That allows use with cheaper off-peak supplies, the bottom one heating the entire tank. The upper one sometimes called 'boost', is used to give a smaller quantity of hot water when on peak cost.

1. You cannot run 2x 2.5mm from one 32amp MCB. It would need to be 2x separate 16amp.

2. Is possible but 1x 4mm is adequate, but you would need to add a 13amp FCU one for each heater.

2x 2.5mm each on a 16amp, would allow for remotely switching (kitchen) each heater or later using an off-peak supply on one heater.

 

[terryplumb]

Just ask an electrician to provide two radial circuits for the immersion heaters. You don't need to provide instructions on how to do it.

 

[EFLImpudence]

Yes, if you are "going unvented (as we speak...)" then presumably the qualified heating engineer will know how to do it and install whatever is required.

 

[AndyPRK]

Harry - Is probably not for an E7 setup, but rather to meet Part L (or whatever) heatup requirements.

What is normally done in this case I don't know!

If you could get a 25A MCB for your CU, that would be a great situation