Water over-heating in immersion heater - new thermostat hasn't fixed the problem

Discussion in 'Plumbing and Central Heating' started by JukeBoxUK, 5 Jun 2021.

  1. JukeBoxUK

    JukeBoxUK

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    I have a Fabric Excelsior 130 litre unvented water heater in my loft. I have it timed to come on for 1.5 hours each morning. During these recent hot days, I'm finding that the water coming from the hot water taps is far too hot, even steaming.

    A few days ago, I drained the tank of hot water by leaving the bathroom hot water tap on for a while. Then I turned the thermostat on the side of the heater down to 30C, turned the heater on for a few hours, then tried the hot water tap again. The water from the tap was again steaming hot.

    Suspecting a malfunctioning thermostat, I replaced it like-for-like, rewiring it exactly like the old one. Again, I drained the tank of hot water by leaving the bathroom hot water tap on for a while, then set the new thermostat to 30C just to test it, turned the heater on for a few hours. Expecting to now have just warm water from the tap, I again had steaming hot water.

    I'm pretty sure the heater only has this one thermostat.

    Wondering if anyone can suggest what might be going on here and what is causing the water to overheat? I've uploaded some photos which might help. Any advice greatly appreciated.
     

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  3. Gasguru

    Gasguru

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    First of all those thermostat temperature settings are very inaccurate...I have the identical model and it's set at 45 with the water actually getting to the required 60 degrees (measured with proper test gear)...so it's guesswork getting the correct hot water temp.
    Secondly the overheat stat and user thermostat have not been earthed. The earth terminals should be linked and an earth wire installed...whoever fitted the cylinder bodged it due to not having 4 core cable.

    Since the earth wire has been pinched to be used on the hot water off terminal I'm guessing you have a mid position zone valve and an additional 2 port zone valve for the cylinder.
    Looks like a Sunvic 2 port feeding the cylinder and these are notorious for jamming open and therefore no longer controlled by the clock and thermostat.
    So have a look at the zone valves...if stuck open the cylinder will heat to the temperature of the boiler thermostat and that could be 80 degrees plus.
     
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  4. flameport

    flameport

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    To clarify - what exactly are you turning on to heat the water? A gas or oil boiler located elsewhere, or a switch to heat the cylinder using electricity?
     
  5. JukeBoxUK

    JukeBoxUK

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    @Gasguru thanks for the reply, I'll investigate and post again later
    @flameport: it's a timer switch to heat the cylinder using electricity
     
  6. flameport

    flameport

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    The thermostat you have replaced is what controls the temperature when it's heated from a separate boiler, usually gas or oil.
    The electric element has it's own separate thermostat which is located under the round grey cover below the white thermostat box.

    Unless you have electric solar panels installed and are heating the water during daylight hours, the electric option will be significantly more expensive than using the gas boiler.
     
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  7. CBW

    CBW

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    Immersion left on?
     
  8. JukeBoxUK

    JukeBoxUK

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    Ah ok. I have a slightly uncommon system which perhaps I should have mentioned - my apologies, I didn't mention this since I thought the immersion heater was a completely separate system. My mistake. Let me explain further....

    There is no gas supply to the house (it's a very rural location), only electricity. The central heating system is run via a Maxton electric boiler (I've uploaded three more files to show this, all starting with the word "Maxton"). Although this electric boiler can be used for both the central heating and to supply the hot water, it has never been used for the hot water in the seven years I've lived here, the hot water being supplied by the immersion heater instead (I'm not sure why, it's simply the way the previous owner used the system and was what he recommended when we moved in - perhaps it was he who is responsible for the bodged wiring? He was a 'handyman' electrician, after all!).

    OK, so from your post I think I now follow that the thermostat I replaced is the one which controls the water temperature if it were to be supplied by the Maxton electric boiler.

    So I guess I now have three questions:
    1. is it easy enough to replace the immersion heater electric element's own separate thermostat (the one located under the round grey cover)?
    2. would it be more efficient/better to change the system so that the hot water is sourced from the Maxton boiler instead of the immersion heater? (guessing this would be a job for a professional plumber?)
    3. would having the hot water supplied by the Maxton boiler mean that hot water would simply be heated on demand, whenever a hot water tap is turned on?

    (it might help to know that, whenever I've turned the hot water switch on for the Maxton boiler, it makes a very unpleasant screeching sound, so I've always turned it off again immediately).

    Thanks so much for the helpful replies and sorry again for not mentioning the whole system in more detail - I'm on a bit of a learning curve with this.
     

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  9. polesapart

    polesapart

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    @flameport

    I'd like to thank you for video and wiring diagram elsewhere on t'web on unvented cylinder wiring (Y-Plan+ safety 2port on HW side) .

    Case of virtual beer due!
     
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  11. polesapart

    polesapart

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    Yes

    But try turning it down first?
     
  12. JukeBoxUK

    JukeBoxUK

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    Ha, yes of course!
     
  13. JukeBoxUK

    JukeBoxUK

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    Yup, that thermostat was set to max, so I've dialled it down and we'll see how that goes. Thanks for all the help guys. It seems I've changed the other thermostat for no reason, but at least I've learned a bit.

    But going back to one of my last questions - do you think it would be more efficient/better to change the system so that the hot water is sourced from the Maxton boiler instead of the immersion heater?
     
    Last edited: 5 Jun 2021
  14. Gasguru

    Gasguru

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    There is no advantage to use the electric boiler to heat the cylinder...both options use very expensive electricity. Perhaps you have a lower rate night tariff.
    Electric boilers are generally nothing but a con, they are sold on the idea they are 99.9% efficient which indeed they are at turning electrical energy into heat energy but it's at huge expense.
    If your heat the water with the boiler you should consider the extra cost of wear and tear on the boiler (the elements and switching system) and pump rather than just the cylinder immersion heater.
    Far simpler would be a direct cylinder (with immersion) and electric panel heaters of some sort.
    I'd be seeing if that Myson boiler is even repairable anymore...
     
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  15. JukeBoxUK

    JukeBoxUK

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    Since there is no gas supplied to my area, I have to use electricity (unless I want to install a new oil fuelled boiler, which I definitely don't want, nor use expensive bottled gas). My average electricity bill is less than £100 per month, that's for central heating with water radiators, hot water, lighting and all appliances - and there is no gas bill. So it doesn't work out to be too expensive in the end, when you think that the average monthly gas and electricity bills are £56 and £58 respectively (2018 figures). Servicing costs are much less too. But more importantly, I have little choice due to location.

    I guess the one remaining question I have is whether the Maxton boiler can supply hot water on demand (like a combi boiler) instead of using the immersion heater at all?
     
  16. Gasguru

    Gasguru

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    No you can't have hot water on demand....gas combi boilers are in the order of 25kW power (for low hot flowrates) and upwards.
    Your immersion heater is only 3kW (takes hours to heat the water) and the boiler brochure states what kW you have.
     
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