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Albot81
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November 18, 2024
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Smart meters + direct unvented water cylinders + THTC. Any advice welcome!

  • November 18, 2024
  • 32 replies
  • 768 views

Thanks all for all the various valuable info about meters, the switch overs and installations etc pending. We currently are in the West of Scotland (mainland), have all electric and old economy 7 box etc. On the THTC. The folks at OVO I've spoke to already have offered economy 7 (overnight), economies 9 and 10, which is basically split over 3 periods in 24 hours. Questions are i suppose is as currently we're on overnight off peak, but aware that the off peak would be split over the day, which may be handy for being savvy with new style electric Quantum heaters we have (got rid of the old brick ones) and timings. The direct unvented water cylinder we have is 210 litres, any ideas whether then this would be heated 3 times a day for example with economy 9 or 10 rather than the current overnight? Plus am guessing it would be 2 smart meters for on and off peak? Many thanks, aware lots of this may have been already answered in various places, but just before i speak to them again about getting some engineers here.

    Best answer by Firedog

    Smart meter installed/replaced today, 
      

    Congratulations on your new toy! It may take some time for everything to settle down, but keep an eye on things and come back here if something’s not quite right.

    About hot water: a tank like yours probably has two 3kW immersion heaters, of which the upper one is for the boost. It takes about 11kWh to heat a full tank from cold (I used 10ºC to 55ºC), so that would take 3½ hours. You could do that three times every day if necessary at offpeak rates, but I doubt you could ever use that much hot water. I suggest you watch it carefully, e.g. switch it off one morning when you get up to see just how long the hot water lasts. You may be surprised. If you leave it on all the time, you may end up wasting electricity just to keep the temperature high in the tank when it’s not really necessary. (I’m afraid I’m not au fait with the differences between vented and unvented tanks, so I stuck to ‘O’ level physics.)

    32 replies

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    Albot81Author
    Rank 4
    February 21, 2025

    ​@Firedog that's really helpful thanks! I'll keep you posted. U was just a little baffled as the engineer says the IHD does not have a boost feature, and if was needed outwith at any point to use the old e7 boost box, which i thought was redundant. But will keep an eye on.

    Firedog
    Super User
    FiredogSuper User
    Super User
    February 21, 2025

    … the engineer says the IHD does not have a boost feature, and if was needed outwith at any point to use the old e7 boost box,
     

    Cross-purposes again, I’m afraid. Please have another look at what I wrote back in November. 

    Your boost box should still carry out its intended function: to energize the upper element in your hot water tank to heat up the top ⅓ of the tank (at peak rate, of course) should you run out. Only to be used in emergencies.

    The meter’s boost function (to override its load control switch) was available on some old IHDs, but I don’t think any modern one has it. You’re not likely to miss it.
      

    I found some statistics to show how much hot water British households use on average. The ones I found varied a lot, but something around 125 l/day seemed likely. Your 210 l tank would feed a whole household for up to two days after a 3½ hour charge. Something like 18% of a household’s heating cost is spent to heat water, much of it unnecessarily, I’m sure. Canny Scots will be less profligate ... 
      

    Noel | I have no official status; I'm just a volunteer who comes here to help other customers. My gear: Aclara SGM 1416-B Electricity-only E7 meter; Chameleon IHD3-PPMID-AAA | It may look as if I know what I’m talking about, but don’t let that fool you. |
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