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  • Plumberists & Heatingerists Q – Hot water cyls?
  • Premier Icon maxtorque
    Full Member

    We live in a small, 2 bedroomed terraced house, currently with small (iirc 7kW) old gas boiler, used for heating and hotwater with an vented, indirect, 170l tank. There is currently no electrical immersion heater fitted so the gas boiler does all our hot water heating. As the house is pretty well insulated, and in a terrace, we don’t actually do much space heating for a lot of the year, so we often run the boiler just to get hot water.

    Boiler is old, and probably due replacement, and the current plan is to move the hot water tank out of the bathroom situated airing cupboard (where it uses up about 1/3 of the space in the small bathroom).

    I’d love to at least prepare for the inevitable move away from gas energy, so i thought about installing a slightly larger (210L) unvented (to get decent shower hot water pressure) tank, and eventually a small solar array to drive an immersion heater to do most of the hot water heating. The question is, given any eventual move to say a heat pump for space heating (small house, so we don’t need a lot of heating power), is it foolish to move to purely electrical water heating straight away. ie fit a direct cylinder that doesn’t have the boiler circuit loop in it? Given that ‘lecy prices overnight have been rather low (our EV charges overnight and recently we’ve been paid to charge 😉 even without any solar assistance, i think the costs might not be that high.

    I guess we are talking about supporting a couple of quick showers a day, and a couple of decent baths a week, plus daily ot water for hand washing / washing up etc. How much hot water is that in terms of volume (M.Cp.DeltaT gives us the overall energy required if we know the mass of water and start / end temp)

    So, ST Plumberists, whats the best route forwards?

    (PS, i don’t think we will actually rip out the boiler and put in any heat pump or similar this year, it would be a next summer job at the soonist i think)

    Premier Icon sharkbait
    Free Member

    How many in your family?
    We have a 210L direct megaflo in our holiday place and from April to Oct all the hot water is produced by the PV (if we get an extended spell of crap weather then we will hit the boost button)
    Seems to deal with a family of 5 OK but a bit bigger would be great.
    In out main house we have an indirect 250L megaflo and it’s ample. Again, the PV at this house heats the water admirably.

    Here’s a link to a spreadsheet I did that shows the cost of heating various volumes of hot water which you may find informative

    https://docs.google.com/spreadsheets/d/1AIZjuQYUOV0HkUlLbJawAzY6UvYEn0wAs881CH3fk_Q/edit?usp=drivesdk

    Premier Icon maxtorque
    Full Member

    2 in family, no kids, and neither of us are really the HIPO type to spend hours in the shower 😉

    I’ll take a look at your data thanks, that’s really useful!

    It did strike me that i could get an indirect cylinder, initially connect it to the boiler loop, then when the boiler goes, simply disconnect it, and fit an inline thermosyphon immersion element right next to the tank, driven from either the solar array or from the mains for boost heating off-peak?

    As we have to move the cylinder (about 10M, across from the bathroom where it currently lives) not having to extend the boiler loop does make that job a little easier too.

    Premier Icon sharkbait
    Free Member

    I think (happy to be corrected) that an indirect cylinder will be slightly less efficient as the immersion element is a backup so will be higher up the tank – therefore a bit less hot water.

    But the one at our main house works great.
    I’ve turned the thermostat on the immersion element up to max to get the most hot water. Not necessarily a great idea if you have kids but easy to change.

    Premier Icon maxtorque
    Full Member

    An external immersion heater driven by energy from solar panels and replacing the boiler loop (ie plumbed into the normal indirect coils in the tank) solves the “heater too high in the tank issue” does it not?

    I’m a power electronics engineer, so i can actualy whip up an MPP controller to sit between the solar panels and that heater from the bits i’ve got laying around in my home lab 😉

    In fact, thinking about it, given that what we want is heat, then a simple variable DC load actually could connect the solar panels and simply be arranged to turn their DC electricity straight into heat. If we want lets say 65 degC water, then that load would want to sit at say 85degC, which is an ideal running temp for power silcon! Controller just adjusts the effective resistance to keep the input voltage at the optimum voltage for the panels.

    Premier Icon TheBrick
    Free Member

    Not a plumber, but for some reason love plumbing and have done something similar to what you have planned.

    I have a 250l unvented indirect cylinder. The loop is plumbed up the the CH circuit that is duel power of wood burner and an electric system boiler as backup so irrelevant for water heating.

    I have never user the electric boiler to heat the tank. I do however have an immersion heater in the tank that actually does most of our hot water. The immersion heater is low in the tank so does the entire tank. The tanks that only have immersion heaters have two tappings. A low one for most of the heating and a top one for “boost”. Two adults one child. A big charge of heat overnight and a boost in the afternoon and it’s always hot.

    If I were you and planning on running the tank off of a solar array I would have that linked up to the lower immersion, plus couple of contactors to switch over to a off peak mains electricity overnight. Then the top tapping wired into the normal electric for top up boost.

    Premier Icon slowol
    Free Member

    Might be worth considering getting a solar thermal panel as the renewable heat incentive will pay for it over a few years at current rates. Check out the website.

    https://www.ofgem.gov.uk/environmental-programmes/domestic-rhi

    Premier Icon sharkbait
    Free Member

    Solar thermal is a one trick pony though.
    Hot water – nothing else.
    PV will do so sorts and is easier to install with no pipework in the roof space.

    Premier Icon maxtorque
    Full Member

    A quick estimate, and we used something like 75 litres of hot water a day, on average. Assuming water coming in to house at 15 degC and 65degC in the tank, (50 degC upheat) that’s 4.5 kWh of energy required per day.

    So how big a solar array do i need to gather 4.5 kWh of energy on average, per day, over a year? (obviously insolation is highly seasonal, and a 4.5 kWh system in summer is not much in winter)

    I guess, taking an approx solar panel efficiency of say 20%, that means we need 22.5 kWh of solar insolation, at the panels, we’ve got a south facing roof, completely unshaded so that’s an advantage.

    Solar insolation looks to average around 900 kWh/m^2 over a UK year (365 days) in the midlands (horizontal), So that’s 2.5 kWh/m^2 per day, so that looks like we need pretty much 9 m^2 of pannels just to heat our water over an average year? Sounds quite high?

    Premier Icon sharkbait
    Free Member

    Assuming water coming in to house at 15 degC and 65degC in the tank,

    Max you’ll get the tank up to is 61c …. And that’s really hot.

    If you have a look here it has all the PV data uploaded from our holiday place, gives you an idea of what you can expect from a 3kw system.
    https://pvoutput.org/intraday.jsp?id=6085&sid=5802

    Premier Icon maxtorque
    Full Member

    How many m^2 is your array, Says 12 panels in total?

    19 kWh today, but it’s been a perfect cloudless day 🙂

    Premier Icon sharkbait
    Free Member

    That’s just a 3kw array but South facing.
    This is the data from the East/West facing 4kw array at our main house for comparison.
    https://pvoutput.org/intraday.jsp?id=6085&sid=4867

    Premier Icon sharkbait
    Free Member

    So how big a solar array do i need to gather 4.5 kWh of energy on average, per day, over a year?

    You should also bare in mind that, unless the PV is only connected to the cylinder (and nothing else) then anything running in the house will get first dibs on the power generated with only surplus power being sent to the tank. So that needs to be 4.5Kw of spare generation.
    During 5 months a 4Kw system may not even generate that in a day.

    Not sure if this is of interest but this shows the temperature of the hot water (and other stuff) at our other house. As you can see the water is being heated from 7am at the moment but then the only other things running in the house are a fridge and the wifi.

    https://go.init.st/25ekolj

    because it’s in Wales we’venot been there since New Year and one temp sensor is playing up so no readings

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