Complicated dual tariff arrangement with four main switches.

[Selfmade]

Hi Folks

I'm used to (and understand reasonably well) simple dual tariff arrangements with two main switches (either two separate consumer units or a duplex). But I occasionally come across, and am now quoting for changing consumer units for a more complex arrangement that I don't full understand. There are four main switches, one for regular 24 hour loads (light, sockets, etc.), one for 24 hour heating, one for off-peak space heating, and one for off-peak water heating. I've seen this type of arrangement in a single duplex consumer unit, four separate consumer units, or two consumer units (one with all heating loads, one with just the 24 hour non-heating loads). So my questions are:

1. Why are the 24 hour heating and non-heating loads separate?

2. Why is the off-peak water heating load separate from the off-peak space-heating loads?

3. Is there any reason not to combine the loads onto two main switches, one with all 24 hour loads and one with all off-peak loads?

P.S. I can't remember the exact setup with the meters, but as far as I can remember, there were separate live tails to each main switch, each fed from different meter terminals, i.e. none of the main switches were fed from the same live meter terminals via a Henley block.

 

[Rpa07]

Photos always help out here @Selfmade. It does sound like they’ve over complicated things and just two consumer units would suffice. It may have been how the installer could understand to do it in the first place. It all works, right?

 

[Selfmade]

Photos always help out here @Selfmade. It does sound like they’ve over complicated things and just two consumer units would suffice. It may have been how the installer could understand to do it in the first place. It all works, right?

Hi Rpa07, I didn't think to take any I'm afraid, and I don't like to request a second visit before I've even provided a quote. I've seen similar arrangements in at least three other dwellings, so I don't think it's a one-off crazy design. Though I guess it's possible they might have all be done by the same spark...

 

[Selfmade]

I'm kind of wondering whether at some time it might have been (or might still be) the case that the 24 hour heaters would operate on the low-rate at off-peak times, while socket, lights etc. would always be on the high rate. Just a guess really and still wouldn't explain the water heater.

 

[Selfmade]

OK looks like they are called multi-tariff CUs, or in Scotland 'total control boards'. I've found some reading material on these, but if anyone knows anything about them I'd still be interested to hear from you. Looks like Hager at least still make and sell them.

 

[SJD]

It is possible to have a supply with more than 2 tarrifs. I have a customer with 3 tarrifs, 1x 24 hr, and 2x off peak with different off-peak unit rates. I was told it was historical, don't know that you can sign up for one now, if you don't already have it.

 

[Toneyz]

It is possible to have a supply with more than 2 tarrifs. I have a customer with 3 tarrifs, 1x 24 hr, and 2x off peak with different off-peak unit rates. I was told it was historical, don't know that you can sign up for one now, if you don't already have it.

I was thinking along those lines the 24hr lights sockets has always been there. A upgrade for heating and water heating has been added so another C.U for 24hr heaters and another two for water heating peak and off-peak it could be that the off peak rate may be the same but the time switching different for W/H and heating.

 

[Selfmade]

I was thinking along those lines the 24hr lights sockets has always been there. A upgrade for heating and water heating has been added so another C.U for 24hr heaters and another two for water heating peak and off-peak it could be that the off peak rate may be the same but the time switching different for W/H and heating.

Or maybe that the water heating switches between high and low rates, while the storage heaters just switch off at high rate times.

 

[Selfmade]

I just read on another forum the suggestion that it's a good idea to put the water heater on its own RCD as it's a higher tripping risk, so this avoids loosing the storage heaters if the water heater develops an earth fault. So I'm thinking of fitting a new multi-tariff CU, but replacing the main switches with RCDs. I also have to work out how I'd integrate AFDDs in this arrangement, as I said I'd quote for these as an option, but I guess that's another story...

 

[Selfmade]

Astonished! Just read the following on the SSE website: "The off-peak rate is available 24 hours a day for panel heaters, focal point fires, bathroom heaters, towel rails, the hot water boost and even electric showers." Wow!

 

[Toneyz]

I would try to find out the tariff rates and switching on times?

 

[Selfmade]

I would try to find out the tariff rates and switching on times?

Thanks for the suggestion. I will probably do this if I get the job. I think I have enough information now to know that each of the four sets of tails are indeed different and the four groups of circuits need to be kept separate - whether that's rates or times.

 

[Selfmade]

This thread: Scottish Hydro Total Heating Total control - MoneySavingExpert.com Forums - https://forums.moneysavingexpert.com/showthread.php?t=1131259 and in particular this post:

"Meter 1 - a standard single rate meter meter (MPAN#1, first 3 digits on MPAN normally 171) This meter measures the Domestic supply (lights, sockets, cooker etc) This is a 24 hour supply....no point in putting washers etc on at night as it does not have a cheap rate.

Meter 2 - is a radio Telemeter (MPAN#2, first 3 digits on MPAN normally 172) This meter measures the heating supply and can have 2 or 3 seperate outputs as follows- a 24 hour supply for any panel/convector heaters in the house. An 80A switched supply, for the storage heaters and hot water (overnight charging). A 35A supply for 'boosting' the hot water through the day...if connected this is done by the meter, but can be done manually...if not connected then the boost is supplied through the 24 hour supply."

seems to describe what I have seen perfectly. Apparently it is a north of Scotland thing only, which explains why some on here haven't come across it.

 

[sunray]

I had a job with 3 tariffs in a rental property where they insisted on a single point of isolation, my solution was a large CU:
12+14 Way High Integrity Fuseboard 2x100A RCD without Knockouts | Mastertrade - https://mastertrade.co.uk/hg-vml91214cu-hager-vml91214cu-con-unit-1214-way-100a.html?gclid=Cj0KCQiAtf_tBRDtARIsAIbAKe2yJdUvT7qbuduOOWO8JAkr0azsgmreiFG-VFU0prBFIbsKAbqqS9AaAlp0EALw_wcB
and used a 4 pole isolator, 2 pole RCD and MCB for the immersion, RCD and 5 MCB's for storage heaters in one row, 2xRCD and MCB's for peak rate on the other row.

 

[richy3333]

It’s a commonly found set up in north Scotland for THTC setup. If you don’t know or understand it I’d suggest leave alone.
If you’re pulling the bullet to do the work, to isolate, presumably your either an NICEIC or SELECT registered contractor.