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Heating and cooling: DHW - how?

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  1.  
    • CommentAuthormaxelaine
    • CommentTimeNov 26th 2015 edited
     
    We'll be having an ASHP for UFH, so outputting at about 40C. We want to avoid having to have this running at 55C for DHW, because of the overwork, the frosting up, etc.

    We'll be having solar PV, which could dump excess into the DHW on long & sunny days, and during daylight.

    We'll also have bottled gas for cooking (no mains gas, and just like cooking on gas).

    How do we lift the DHW from the 40c the ASHP would provide to the 55c (occasional 60c for legionnaires) on short days / in the dark?

    Can you get a propane-fuelled combi boiler that accepts variably-warm water input?

    Should we just have another immersion, powered by the grid?

    Going off the idea of an Ecocent, which was plan A.
    • CommentAuthorDarylP
    • CommentTimeNov 26th 2015
     
    maxelaine,
    Yes. Some clients had a similar set-up to yours, the answer was a Rinnai propane/LPG-fuelled DHW 'combi'.
    It takes water inputs up to 60 deg IIRC, can be controlled via a remote, and is very efficient.
    Note it is DHW only, no output for CH.

    Cheers...:smile:
  3.  
    jsharris, formerly of this parish, was in a similar position to you & he has fully documented his thought processes, designs and results on the ebuild website.

    http://www.ebuild.co.uk/blog/12/entry-347-part-forty-one-hot-water-at-last/

    His current solution uses an ASHP for UFH & DHW pre-heat, a Sunamp PV to store excess energy from the PV array and a 9.6 kW electric in line water heater to top up the DHW flow temperature as required.

    David
    • CommentAuthormaxelaine
    • CommentTimeNov 26th 2015
     
    Thanks, Daryl, have emailed them - sounds like exactly what we want.

    Thanks, David, I have followed JSH's blog, and last night downloaded his DHW diagram, but it's a bit complex! - And he's programmed his own controllers, etc - which is more than a little over my head.
    • CommentAuthorsnyggapa
    • CommentTimeNov 26th 2015
     
    I would go cheap, reliable, cheerful - electric immersion on economy 7 overnight electricity

    use the grid at the point of it's lowest carbon intensity so something like 3 - 5am overnight, cost is peanuts and harm is minimal.

    cost includes such variables as embodied energy, fixing it, finding tradesmen that understand it etc

    -Steve
    • CommentAuthormaxelaine
    • CommentTimeNov 26th 2015
     
    snyggapa, the problem with that is it's not topping up the heat during the evenings.
    •  
      CommentAuthorSteamyTea
    • CommentTimeNov 26th 2015
     
    Why would you normally need to top up the DHW during the evenings?
    • CommentAuthormaxelaine
    • CommentTimeNov 26th 2015
     
    To have a shower after the washing up? Just want hot water always available, and not limited to heating overnight, just before all the lovely free PV starts.
    •  
      CommentAuthorSteamyTea
    • CommentTimeNov 26th 2015
     
    That's just a case of sizing the system right and remembering that if you get it wrong you need to flick a switch for half an hour.

    I have lived with E7 for the last ten years, you soon get used to it. Even my old brain dead lodger (and not being nasty there, she had a stroke and only had half a brain), managed get to grips with it.

    I keep my 200 lt cylinder at 50°C, that is enough for a couple of baths and a shower or two.
    I could raise the temperature up a bit, to about 85°C and that would do 4 baths easy.

    What I did find recently, it that the cylinder looses 2 kWh/day when not used. This is down from about 3 to 4 kWh/day.
    But my cylinder is 28 years old. If/when I got/get a new one, I would go for more insulation, or depending on the results from Jeremy and his SunAmp, one of those.

    In that 28 year, as far as I know, it has not needed any work, not even a new element or thermostat.
    So it now costs about 30p/day or £110 a year.
    Really is a cheap and reliable system.
    • CommentAuthorgravelld
    • CommentTimeNov 27th 2015
     
    Posted By: maxelaine... not limited to heating overnight, just before all the lovely free PV starts
    Bear in mind for most of the year you probably won't get much PV for water heating until a while after the usual morning shower.

    Depends on a lot of variables of course... but we still have to top up with E7, even in summer.
    •  
      CommentAuthordjh
    • CommentTimeNov 27th 2015 edited
     
    We have a 250 L thermal store with PV powering the bottom immersion and E7 for the top immersion. Until around the start of October, PV was doing everything and I never used the top immersion. During October I started using it for one hour occasionally at night and now it is on a time switch to come on for one hour in the middle of each night. It's limited to 60 °C so the PV can heat the top of the tank by some more as well as heat the bottom of the tank. So whenever there's been enough sun, the E7 doesn't actually do anything when it comes on.

    At the moment we're logging manually so I don't yet know exactly how much I'm using for what. Oh, there are normally two of us.
    • CommentAuthorGotanewlife
    • CommentTimeNov 27th 2015 edited
     
    This is exactly the sort of solution I think I need - being a hateful wood burner but with lots of solar and living in Italy I get up to 20 days a year in the shoulder seasons when I have 3-400 litres of 38 deg water - no immersion, and so I have to burn to heat 2000+ litres from 25 deg and 3-400 from 35+ deg up to 70+ deg. When all I really want is 150 litres from 35 to 45 deg!!!! Unnecessary humping of wood and cleaning of burner just adds to my frustrations. I had seen Jeremy's solution but I only have a 6Kw supply to my house (max possible) and with 2 families living here it would have to be a tanked immersion solution or say Max 1.5kW element. I too have loads of PV but of course it doesn't help in this situation.

    So, an instant bottled gas heater accepting 35 deg water in and supplying (say) 1 power shower plus one appliance (taps being short lived draws) and costing peanuts - mmmmm. So what type of Rinnai? Looks like I can get some models in Italy - the infinity 11e perhaps? (a friend will bring it over for me if I get it in the UK). Meanwhile I'll have to brace myself for a chat at my local plumbers merchant.
  13.  
    I have a 210l cylinder, heated to 50c, seems fine for a family of 2.5 . Heats straight off the ASHP at 3-5am

    It then occasionally tops up mid evening, but using economy 7 and heating to higher temps. may be more economical than buying additional boilers etc...
    • CommentAuthorsnyggapa
    • CommentTimeNov 27th 2015
     
    "is it's not topping up the heat during the evenings."

    Well, it can do if you program the timer to do so, but our experience is that an overnight top up (180 litre tank) lasts all day (2 adults and 1 baby) - we've only run out of hot water twice, both correlated to when a stray teenager took a loooong shower . Solved by pressing the boost button on the timer and waiting 30 minutes.

    Our timer is set to run for 3 hours overnight (so 9KWh of heat into the tank potentially.) Normal usage for us which would be a couple of showers , washing up and anciliary usage means that the water only needs heat from the immersion for a little over an hour until it shuts off, so just heating overnight works well enough for us and has roughly the same again headroom in the tank.

    Your usage of course may vary. We don't have a PV diverter but if we did then this would top up during the day when the sun was shining, meaning that the night time top up would be eliminated or reduced. The good part is the immersion has a thermostat so you would just time it to come on and it would only call for heat if the tank wasn't hot enough from the PV, no fancy controls.
    • CommentAuthorborpin
    • CommentTimeNov 27th 2015
     
    Posted By: maxelaineoccasional 60c for legionnaires
    Ok, been through this lots of times; the risk from legionella is extremely small (probably overstated) and eliminated completely if you do not directly use the water that is heated (i.e. water to be used is passed through a coil in the stored HW).
    • CommentAuthorborpin
    • CommentTimeNov 27th 2015
     
    Posted By: davidfreeboroughjsharris, formerly of this parish, was in a similar position to you & he has fully documented his thought processes, designs and results on the ebuild website.
    I think he is pretty much spot on.
    • CommentAuthormaxelaine
    • CommentTimeNov 29th 2015
     
    Thanks to all contributors, am thinking again - maybe mains immersion would do - certainly cheaper & simpler.
    • CommentAuthorgyrogear
    • CommentTimeDec 7th 2015
     
    Posted By: Gotanewlifeso I have to burn to heat 2000+ litres from 25 deg and 3-400 from 35+ deg up to 70+ deg. When all I really want is 150 litres from 35 to 45 deg!!!!


    Could you not have a little tank inside the big tank, to which you could divert stove heat ?

    gg
  19.  
    What I have is a WBS, the output from which goes into a coil at the bottom of my 2500 ltr TS. At the top of the TS inside the tank is a 340 ltr tank (ie counted as part of the 2500 litres); this is my potable DHW. In the top of the tank is another coil connected to my solar panels. For the 6 months when I don't need space heating my solar panels produce all the DHW I need, except for a few days during the shoulders seasons; it is then that I need a way of taking my large quantity of nearly hot enough water and making it slightly hotter. Seems certain I will go for an instant GPL solution idc.
    • CommentAuthorgyrogear
    • CommentTimeDec 8th 2015
     
    OK, you've got the little tank already, therefore.

    So could you not create a bypass, using appropriate shutoff valves, branched off the lower "real" WBS pipes, to send stove water directly to the DHW tank-in-tank, as "expeditive wood heat" ?

    gg
    •  
      CommentAuthordjh
    • CommentTimeDec 8th 2015
     
    Posted By: gyrogearsend stove water directly to the DHW tank-in-tank

    The water in the DHW tank is potable water, I believe. The water in the WBS is not.

    I like the efficiency of tank in tank designs, but they bring back the need for regular heating to 60 °C I think? And testing, or not?
    • CommentAuthorringi
    • CommentTimeDec 8th 2015
     
    How much would it cost to heat the top of the thermal store with E7 during the shoulders seasons? What if you used used full price electric to do so on the few days it is needed?
    •  
      CommentAuthorSteamyTea
    • CommentTimeDec 8th 2015 edited
     
    If the top of the tank holds 100lt and it needs to be raised by 20°C
    Then:

    0.00116 [kWh/(kg.K)] x 100 [kg] x 20 [K] = 2.32 kWh

    I pay about 8p for E7 and 19p for day rate.

    So about 18.5p and 42p respectively.

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