Electrically heated hot water - options?

Discussion in 'Plumbers' Talk' started by rogerk101, Oct 26, 2020.

  1. rogerk101

    rogerk101 Screwfix Select

    Hi Plumbing gurus

    I'm at the decision stage for providing hot water to my garage-to-grannyflat conversion, and am finding that each option has considerable drawbacks.

    12 kW electrical instant water heater:
    only heats water when needed
    easy to install with no requirement for G3 certification
    no annual maintenance requirement
    can't be used to fill a bath (flow very restricted so bath will be cold by the time it's full)
    can't use with thermostatically controlled shower
    requires special shower controller with hot on RIGHT side (so opposite from normal)

    50 litre or 80 litre unvented water heater
    can use with normal thermostatically controlled shower (with hot on LEFT)
    can be used to fill a bath (although 50 litres of 70 degree water might be quite small)
    must be installed by G3 certified plumber and notified to local authorities
    must be serviced annually by G3 certified plumber (so add £100 to annual running costs)
    quite big, so takes up a lot of space in a small granny flat

    I'm sure these issues are very familiar to the pro plumbers, so what other factors am I missing?

    Many thanks in advance for sharing your experiences.
  2. The first decision Roger is whether a bath is essential, desirable or whimsical.
    Life will be a whole lot easier without the requirement to fill a bath.
    We don't have any information about your set up but, presumably, it totally precludes tapping into the DHW of the main dwelling, does it?
    Gas (if you have it) is sooo much cheaper than lecky.
  3. rogerk101

    rogerk101 Screwfix Select

    Thanks for responding.
    It's about 30m from the main house to the garage, so much too far to take my oil boiler heated water down there.
    I use 19kg gas bottles to cook with in the main house, because we're not on mains gas.
    I have considered using gas bottles down there to fire an instant gas boiler, but I'd consider that a last resort option, unless I can be convinced otherwise (for which I'm open to convincing!)
    A bath isn't essential ... it was simply a case of if we ever have grandchildren and the kids that come to stay with us want to bath them in a bath, at least it'll be there. By then I'll probably have found another solution, so my thinking is that in the meanwhile I'll probably just go with the instant electrical heater. I have some experience with the 9.6kW version from Galaxy, and you can get a reasonable shower from that, so a 12kW from the likes of Redring should be fine.
    Do you know if there are any safety issues with trying to fill a bath from a 12kW instant electric heater or is it simply the fact that the flow is so slow that the bulk of the bath is cool by the time it's full?
  4. masterdiy

    masterdiy Screwfix Select

    1. I would go Instant hot water heater under sink & basin. Either 5Lt or 10Lt depending on projected usage.
    2. Electric shower that way only cold feeds needed.
    If both sink & basin are not far away from each other, one would more than likely be ok.
    Heating different matter. Maybe bottle gas.
  5. rogerk101

    rogerk101 Screwfix Select

    The instant 12kW electric water heater I'm looking at has no storage capacity at all ... it turns the cold water into hot water on the fly, with a flow switch and a monstrous 12kW (almost 50A) heating element. The 5 or 10 litre ones have much less poserful heating elements and can only really be used to wash a few dishes in a kitchen sink or a few faces and hands in a bathroom basin. You'd need more than 10 litres for a decent shower.
    The building is so well insulated with 200mm external insulation on the outside of all walls and the same amount in the roof, so we'll heat with a couple of electric blow heaters plugged into the wall sockets. Last winter I heated it with one 1.5kW blow heater even on the coldest nights.
  6. kiaora

    kiaora Screwfix Select


    maybe not possible, but...

    i have and apartment on the first floor of my house, it's not used all the time, so have a 200lt unvented hot cylinder in the roof void, its connected to cheap rate electric on the bottom immersion and normal rate to the top half.

    It feeds a bath, shower, kitchen sink and basin. Its never run out on cheap rate, yet..

    If it did I have day rate electric available

    I think its cost effective, and a quality hot supply, on demand (well 24 hrs before guests arrive !)

    good luck
  7. rogerk101

    rogerk101 Screwfix Select

    Thanks for that suggestion.
    In my case it's a 1 bedroom granny flat that will only get very occasional use (unless I decide to AirBnB it some time down the line).
    I don't have the space for a 200 litre tank and if I go down the tank route, I think 80 litres at about 65 degrees should be enough for one bath or two showers. I could easily fit a horizontal 80 litre tank in the space under the sloping ceiling.
    Funnily enough I discovered that I can get my very own G3 certification for the princely sum of £85 for a one day course, so I think I may just do that and do the installation and annual service myself. I already have a pressure tester, so have all the kit I need to check that the pressure relief valves actually work when they're expected to.
    kiaora likes this.
  8. You won't have any trouble getting your G3 but you still need to be a member of a competent person scam in order to notify Building Control.
    If you're doing your garage project under a building notice you can include the UV cylinder in that. Otherwise . . .

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