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Instant Water Heater with Thermostatic Mixer Shower

Discussion in 'Plumbing and Central Heating' started by AWJS01, 30 Jan 2013.

  1. AWJS01

    AWJS01

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    We have an outbuilding with a 9.8kW instant water heater serving a basin and a shower. There's no gas supply and no space for a tank.

    The shower has a thermostatic mixer valve and really should be off a gravity fed system or combi as it doesn't do much better than a warm dribble at this time of year.

    Someone suggested it might be possible to raise the temperature of the shower, without reducing flow, by adding a second water heater in series with the first. Sounds logical to a non-plumber, but is there an obvious reason why this won't work? (assuming we can sort out the electrics).

    The other question - we understand that instant water heaters aren't recommended for use with thermostatic showers in the first place? I'd like to understand why this is the case (other than the performance being rubbish - I thought there might actually be safety reasons?)

    Many thanks,

    Alan.
     
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  3. petertheplumber12

    petertheplumber12

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    I cannot condone another one in series as it is not something a manufacturer would recommend, the reason a thermostatic mixer won't work properly with rising main cold and gravity or reduced pressure hot is that the thermostat has to be quite sensitive to work properly and a low pressure thermostatic mixer will not work for long on a high(3bar or greater) water pressure. So different pressures on hot and cold won't work well.
     
  4. stem

    stem

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    Some instantaneous electric heaters specify that they need to be used at full flow. They can be used with a manual shower mixer where the hot tap is opened fully, and then temperature control is provided by manually adjusting the flow of the cold water. With a thermostatic mixer, the hot supply may be automatically reduced.

    I assume the 'full flow' requirement is because, less water flow with the same electrical heat input, means the temperature of the hot water produced by the heater will increase, even to the point of the overheat stat operating, turning the element off, then back on, then off again. This will shorten the life of the unit and cause temperature stability problems.

    You don't say what make / model yours is, but the manufacturers installation instructions should tell you if it needs to operate at full flow or not.

    The Redring Powerstream is one that needs full flow the manufacturer supplies a special mixer for it to ensure those conditions are met.
     
  5. AWJS01

    AWJS01

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    Thanks for the replies so far...

    Some more information - first off, the water heater is actually 10.8kW and it's a Galaxy from screwfix. I've feeling it might be the a rebranded version of the Redring that stem mentioned, so I'm fairly sure it can't be used with a thermostatically controlled shower, for the reasons explained by stem and peter.

    The shower that goes with this, also available from screwfix, is seriously ugly. I think I just need a shower with a non-thermostatic mixer, but I don't think Galaxy/Redring are going to recommend anything other than their own.

    Any advice on a shower, make and model if possible, I can actually use with this? Or is it really just any non-thermostatic mixer?

    Or should I just call it quits and get an electric shower instead?!

    Cheers, Alan.
     
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  7. tomkut

    tomkut

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    AWJS01: as you pointed out the power of this heater is not sufficient for your needs, hence I don't see any reason thermostatic mixer would be reducing the hot water flow at any time.
    Running two in series may not be a good idea, but how about parallel (if half the pressure will actually turn it ON). If that is too much power, maybe two smaller ones, e.g. http://www.ebay.co.uk/itm/Dafi-in-l...kless-hot-water-electric-boiler-/271325125286

    stem: These can be used with taps too, which one may be running low, hence the heater will have to go on/off; That's what its temperature control is for. As I understand, http://www.cnmonline.co.uk/cex-electronic-instantaneous-water-heaters-39247.html benefits from doing it all the time.

    petertheplumber12: why would the pressure be so different on hot/cold inlet to the mixer?

    Warm 2014 to all DIYnots
     
  8. gailkm

    gailkm

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    can you tell me waht do did as I want to buy a similar heater and need a good non thermostatic mixer shower.
     
  9. tomkut

    tomkut

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    gailkm, I don't have any of these installed.
    Not much information on the net on these instant heaters or multipoint showers as some called them; you would just have to give it a try. I don't see any reason why two in series or parallel should not work.
     
  10. Norcon

    Norcon

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    Would be easier to fit a 50/L multipoint. :idea:
     
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