timer and contactor in consumer unit

[LadySpark]

I am very embarrassed to have to ask this but final job on a VERY l o n g day involved wiring up a dual tank for a customer on economy 7 electricity tariff. The objective is to get the large night tank to heat only at night using cheap electricity and the smaller day tank to be switched on manually by the customer if they run out of hot water during the day. I just cannot think how to wire it all together.

I am using:
this timer -
https://www.tlc-direct.co.uk/Products/G ... gL7JvD_BwE

this contactor -
https://uk.rs-online.com/web/p/contacto ... lsrc=aw.ds

two of these mcbs -
https://www.screwfix.com/p/mk-sentry-20 ... lsrc=aw.ds

I have the wires from the tank to the back of the consumer unit on din rails 5 and 6. I could just wire the day tank straight to the mcb but I feel that it should go through the timer and contactor in case it is left on resulting in the night tank kicking in while the day tank is still heating.

I seem to be having a mental block with the wiring. The wiring diagrams provided show the black neutral wire going from the timer to the contactor and then on to the mcbs. How? Stick it in the din rail at the bottom of the MCB? Seriously? Maybe that might work but it doesn't feel right. Normally I can translate a wiring diagram into connections but I am having some kind of mental block about this.

PLEASE can somebody help. Either a really clear wiring diagram that actually shows the worse going into somewhere sensible or a translation of how on earth I am meant to wire the neutrals please.

[Someone-Else]

I am not there, so I am going to ask.

Are you sure you should be doing this? Its just that you are leading yourself down a very dodgy path.
1) What if the supply company change the "off peak" times, why not ask them to supply a contactor that energises when it is off peak.

2) Assuming for what ever reason 1 above is not done. What will you do when the clocks change, the timer will be an hour out.

3) Why are you suggesting a 12v contactor ? (I am only going by the link you provided)

4) Why are you making it so complicated? an immersion heater timer will be less hassle, and the customer can adjust the time by turning its dial.

5) If it helps, in my time I have seen a lot of people go "weak at the knees/sh*t their pants/forget" when relays/contactors are involved.

[Neelix]

You are over complicating matters.

You say the customer has economy 7 - but as I understand it no separate fuseboard

How many immersion heaters does the tank have?

[ericmark]

There are many methods used, standard is two immersions in the same tank one near top set to a lower temp to one near bottom, so even if left on it will not normally work unless excess water used that day. And there are a host of timers made for the job. Horsman seem popular.

There is also the Willis system where one external immersion heater is used, and the time switched on for dictates how much hot water.

The iboost+ is designed mainly for use with solar panels, but again is designed to have two immersion heaters.

There is nothing wrong with a bespoke system, however not sure why 12 volt, and the radio 4 signal is going to be turned off, so the time display needs to be where it can be checked as one walks past, as getting the time wrong can work out expensive.

There seems to be no fixed info as to what time is used, I have an EV supply which is for 5 hours, but not sure if the midnight to 5 am is BST or UTC so I start the charge at 1 am and stop at 5 am BST so it does not matter, the in home display shows BST and the smart meter shows BST but the phone app shows UTC.

I try to use systems every spark is likely to have seen, I could program a PLC and I have a couple spare ones, but if I was ill who could work out what I have done? So better to use timers designed for the job so any other electrician can fix it should it go wrong.

Also a consumer unit is a type tested device, so you can only fit items recommended by the manufacturer inside the CU so basic all items have to be same make.

[LadySpark]

Thank you for your reply. Please see answers below:

I am not there, so I am going to ask.

Are you sure you should be doing this? Its just that you are leading yourself down a very dodgy path.
1) What if the supply company change the "off peak" times, why not ask them to supply a contactor that energises when it is off peak.
I am not aware that this is a possibility. I have never heard of such a thing. My usual suppliers (TLC and Screwfix) sell these timers so that is where I have always got them from.

2) Assuming for what ever reason 1 above is not done. What will you do when the clocks change, the timer will be an hour out.
The timers are "magic" (radio controlled I assume). They have a little battery inside to keep the clock going and save the settings when not on the rail. They just do have the correct time and change when the clocks change. However, they can also be set manually. Changing something when the clocks change isn't a massive problem.

3) Why are you suggesting a 12v contactor ? (I am only going by the link you provided)
I think that is the battery but you've got me doubting myself now. The timer/contactor/MCBs ares 20 amp.

4) Why are you making it so complicated? an immersion heater timer will be less hassle, and the customer can adjust the time by turning its dial.
All too right it would! It is so annoying when customers have the least convenient setup. However, everyone in all the flats has this setup. The customer previously had this setup. I don't feel that I can expect the customer to change for my convenience. Unfortunately the plumber ripped everything out when changing the tank so a "simple" final connection job at the end of the day turned into a nightmare.

5) If it helps, in my time I have seen a lot of people go "weak at the knees/sh*t their pants/forget" when relays/contactors are involved.
That does make me feel better. I am so embarrassed that I could not get it right first time. However, not one of the answers here actually tells me how to wire the timer/contactor/mcb arrangement so I am still stuck. Fortunately for me (obviously only fortunately for me), the customer's mother is seriously unwell and he is rushing to her side so I have time to get my head around this before I have to go back. Help to wire the customer's chosen configuration instead of alternatives such as a normal wall timer would be greatly appreciated.

[LadySpark]

To try and make the situation clearer, on these instructions, where should the black neutral wire go? Can't go into the bottom of the MCB surely?
https://www.tlc-direct.co.uk/Technical/ ... MTSD1T.pdf

water heaters.jpg (59.44 KiB) Viewed 687 times

[Neelix]

I note you haven’t answered my questions

[OnlyMe]

That wiring diagram is most unhelpful. L1 and L2 must come from the same MCB to avoid a borrowed neutral. So L1 powers the timer and is a permanent live. L2 must come from the same MCB but may be powered via switches between the MCB and the timer

[ericmark]

I must admit I had to look at that timer many times before I could work out what it was doing, it would seem top terminal L1 and N are the power the unit, and 1, 2, 3 are the contacts.

However I have failed to work out how it needs to be wired, as I don't know how the off peak is supplied.

In my own house the off peak and peak all come from the same supply, it is down to me to time when used, so I would use a 16 amp MCB/RCBO which would feed both the L1 (2) and the L2 (top) , the off peak connected to where it says load (1), and the peak immersion to terminal 3 vie a user 20 amp switch marked boost. The MCB/RCBO should be no more than 16 amp so if contacts stick the MCB/RCBO will trip.

If the peak and off peak are not the same supply, then can't see why it needs a timer.

As to neutral I would use a DIN rail terminal block if using a separate enclosure for the MCB/RCBO if direct to CU then should be a terminal block already for neutral.

[Someone-Else]

Ladyspark, I do not know you, so this is nothing personal.


Both screwfix and TLC supply an immersion heater timer

timer.jpg (32.21 KiB) Viewed 661 times

I think that is the battery but you've got me doubting myself now. The timer/contactor/MCBs ares 20 amp.

coil.jpg (22.59 KiB) Viewed 661 times

where should the black neutral wire go? Can't go into the bottom of the MCB surely?

I will pretend you did not say that.


As I said, contactors/relays often make people worried as they do not understand them. Learning about them is not something you should be doing on the job "unsupervised" I really have seen a lot of people get them/the wiring so wrong. I would suggest that in this case you, take a few days off, buy a contactor and a timer, and wire them up on a test board to see what they can do. (There are lots of things you can do with just a contactor.) If you really want, I will offer you advice via PM

[ericmark]

Timers like these have been used for years for off peak supplies and I can't see why one would want to go away from the norm, I am very aware of the problems when doing some thing however clever which is not the norm, as tradesman/woman arrive shake their heads and leave as no idea what is going on, to work it out takes time which they may not be able to charge for.

I do have a unit myself which can be used with two immersion heaters, although mainly designed to work with solar, I have the iboost+ fitted, which is designed as a expensive replacement for the timer shown and has a wireless CT coil to detect when there is excess solar.

There are other units also designed to work with off peak electric and the internal wiring does vary, I am sure you already know, when off peak or economy 7 came out we had white meters for the off peak and completely independent fuse boxes, before the consumer unit was invented, then we started to get single distribution units and trigger supply from radio 4 LW signals, which are about to be turned off, as the valves are no longer made so when it fails BBC is closing the LW transmission down, so there has been a move to smart meters.

However although the smart meter it seems can supply power at varying tariffs through the day, at least mine has not option for using any trigger signal, so Octopus may offer a very good variable rate, but what I want is fixed times so I can set and forget, however there still seems to be some uncertainty over if using UTC (unified time constant was called GMT) or DST(BST) in the summer months, so although my tariff gives low cost electric 00:00 to 05:00 am I have the batteries set to charge 01:00 to 05:00 am so it does not matter which time they go by, only takes 1.5 hours to recharge them so 4 hours is ample, however heating water may take longer.

My smart meter and in house display (IHD) show BST but the phone app shows UTC, and they all only show kWh at the moment the £ option is not working, I have an EV charging tariff, and my solar batteries are topped up over night, so from 1 am to around 5 pm to all day depending on amount of sun and time of year I am using power at 8.95 p per kWh only when the battery runs out some time during the evening if it does run out, do I need to pay premium rate of 31.31 p per kWh, it is a lottery as if you use same supplier to sell to as one buys from you get a better rate for exported power, and there is a leaving fee so swapping and changing is not really an option. I use British Gas as they took over the account by default when last supplier went to wall, but to calculate best options I need a years info as to what power used and exported, it seems rather a tangled web.

So it is not simple to recommend what to do, and one would need to know system being used.