Two electrical supplies and a single immersion element

[onefff]

Good afternoon, new joiner here with a DIY (ish) question. I have a hot water cylinder with a single immersion element connected to Economy 7.
My economy seven has two separate consumer units and the hot water is connected to the off-peak unit.
Problem is, I'm running out of hot water before the day it out.
Updating the cylinder to a two-element system and running a second power supply from the 24x7 consumer unit is not an option because it would mean ripping up £100s of new flooring and possibly trashing a load of tiling too.
In my head, my ideal would be some sort of device by the fuse board that powers the single element from off-peak normally but has some sort of boos button that would give a 1 or 2 hour boost before reverting back to the off-peak supply ready to pick up the night-time main heating. Know ye of such a device?
I guess the alternative is to replace the element with a dual element heater and try and pull a second cable from the 24x7 consumer unit to supply a boost. That could be a really tricky cable pull and removing the element could trash the cylinder if I'm unlucky.
Any advice good people?
(I appreciate that new circuits would need to be done under part P and so I would need a sparky involved.)

 

[snowhead]

Depending on the existing meter / off peak switching arrangement you may already be very close to achieving your aim without ripping up floors, possibly only requiring a simple timer with boost control.

What else is on the Off Peak consumer unit?

Is the meter Dual rate / Dual output, i.e 5 wire, L and N In then N out and L1 to Main CU and L2 to Off peak CU?

Or Dual rate 4 wire with only 1 Live out which then goes to a time switch / RTS switching unit (where RTS is the BBC time signal being switched off next year, was this year but delayed)?

If you are on E7 then all usage in the off peak period is lower rate (or should be, but may not be if the meter is dual output) so you only need a timeswitch with boost, connected to Main C.U set within Off peak times and a boost control that would then work during normal rate.

 

[onefff]

Thanks for getting back to me.
The off peak unit has storage heaters and the immersion heater. It's a dual rate, dual output meter. There are 2 live outs, one goes to CU1 for 24x7 power, the other gores to CU2 for the storage heaters and the immersion heater. That's why I don't think the simple time switch solution works for me.

 

[Chuck]

I'm assuming the E7 CU supplies power rather than a switching signal to control contactors on each consumer:

Have you considered swapping the feed to the cylinder from the off-peak CU to the standard rate CU and then controlling the tank with its own time-switch? You should get a qualified electrician to do this for you as adding the 'new' load may require upgrading the CU and/or the earthing and bonding arrangements.

I'd certainly be considering getting a two-element tank and controller such as the Horstmann Electronic 7, which would give you quite a lot of options for programming and boosting. With this type of time-switch you need to check them every so often, at least annually, as they tend to gain/lose ten minutes a year and will otherwise gradually drift out of sync with the E7 times as a result.

 

[onefff]

I'm assuming the E7 CU supplies power rather than a switching signal to control contactors on each consumer:

Have you considered swapping the feed to the cylinder from the off-peak CU to the standard rate CU and then controlling the tank with its own time-switch? You should get a qualified electrician to do this for you as adding the 'new' load may require upgrading the CU and/or the earthing and bonding arrangements.

I'd certainly be considering getting a two-element tank and controller such as the Horstmann Electronic 7, which would give you quite a lot of options for programming and boosting. With this type of time-switch you need to check them every so often, at least annually, as they tend to gain/lose ten minutes a year and will otherwise gradually drift out of sync with the E7 times as a result.

Thanks Chuck. The problem with switching to the standard CU would be that the hot water would then always be heated at full rate electricity. I think the way my meters work is that my 24x7 CU is always charged at the full rate. That's certainly the way EDF always billed it.

I have realised there is a 13A spur (2.5 FTE) in the airing cupboard already to drive the power shower (pump only) so I could probably run a boost from there on a simple boost controller timer.

The issue with swapping the tank out for a two-element tank is how tight the airing cupboard woodwork is around it. It's quite possible that changing the tank over would trash the brand-new woodwork :-( That's why I was thinking about a twin element going into the existing element port.

 

[onefff]

Thanks Chuck. The problem with switching to the standard CU would be that the hot water would then always be heated at full rate electricity. I think the way my meters work is that my 24x7 CU is always charged at the full rate. That's certainly the way EDF always billed it.

I have realised there is a 13A spur (2.5 FTE) in the airing cupboard already to drive the power shower (pump only) so I could probably run a boost from there on a simple boost controller timer.

The issue with swapping the tank out for a two-element tank is how tight the airing cupboard woodwork is around it. It's quite possible that changing the tank over would trash the brand-new woodwork :-( That's why I was thinking about a twin element going into the existing element port.

Except your response got me thinking. Going back to when my parents owned the house, we had two separate meters, one for E7 and one for 24x7. The two never met and so, the 24x7 consumer unit was always billed at the higher rate. What I did not realise was that a 5 hole smart meter charges ALL electricity during the off-peak hours at the lower rate so it does not matter which consumer unit you're pulling power from. Thus, your solution is ideal, move the supply to the other consumer unit and install a timer with boost in the siring cupboard. Job done.
The 24x7 consumer unit is brand new with spare ways so a sparky can easily swap these over. I could do it as I have the knowledge and skill but I'm not certified to sign it off.