Immersion Heater Wiring.

The traditional  horstman units used to be a double pole change over on the switch.

 If not any more than a DPCO relay will do it for you. (this sort of  size, but with a mains coil probably )

Mike

Several possibilities;

Connect the water heater to the 24 hour supply, via a local time switch such that it is energised during the off peak hours every night, but with manual operation at other times when required.

Fit a 2 pole changeover switch such that the immersion heater is normally connected to the off peak supply but can be manually switched to the 24 hour supply when needed.

Fit a 2 pole changeover relay but operated by a “run back” timer, this gives the same functionality as the 2 pole changeover switch, but prevents forgetting about it and leaving the water heater on the peak supply for an extended period.

How about changing the element for a dual element?

Sparkymark: 
 

How about changing the element for a dual element?

+1. That was the traditional arrangement - the off-peak supply had an element low down (or a long element if top mounted) so to heat the entire tank, the peak supply had a top element (or short element if top mounted) so could only heats the top portion of the tank. Thus the ‘boost’ function could produce a small amount of hot water quickly using the minimum of peak rate electricity. Trying to boost using the bottom/long element wouldn't produce properly hot water until most of the tank had been heated - requiring a lot more expensive peak electricity.

  - Andy.

Sparkymark: 
 

How about changing the element for a dual element?

The copper cylinder is very distant from the consumer unit and only has an off peak supply cable.

Z.

The copper cylinder is very distant from the consumer unit and only has an off peak supply cable.

The French approach would be to have a single power cable plus a “pilot” wire to signal when off-peak was available. Is it an E7 kind of tariff where the ‘peak’ supply is charged at off-peak rates during the night? If so that might simplify things - connect the immersion to the peak supply (no DP C/O switching required then) and use a signal from the off-peak supply to switch it on a night - possibly using one of those wireless units to get the signal from the off-peak board to the airing cupboard without needing an extra cable.

   - Andy.

does the change-over/ boost switch have to be near the cylinder, or would it be OK on the wall nearer the meter?

Mike.

There’s only one way of doing it, E7 controller from the peak supply.

Anything else is too involved.

mapj1: 
 

does the change-over/ boost switch have to be near the cylinder, or would it be OK on the wall nearer the meter?

Mike.

If the boost controller was downstairs near to the kitchen sink downstairs it would be very convenient. Then the lady of the house would not have to climb the stairs and walk along a long passageway to operate a switch in the airing cupboard.

Z.