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Heating and cooling: ASHP recommendations and top up DHW

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  1.  
    • CommentAuthorGreenfish
    • CommentTimeOct 12th 2015
     
    Currently have UFH driven from a 500l thermal store, with DHW heated by exchanger coil in TS. Have 4sqm of solar thermal feeding the TS (and 3kW PV can drive immersion - manually switched), and 8kW wood boiler stove (3kW to room + 5kW to TS) using free fuel sourced from site. House is well insulated and air tight new build. No maions gas.

    Been through our first heating season (while decorating so not normal occupancy) and all was OK. But was we go through warm damp autumn we are thinking of improvements....

    Issues:
    1) If we are away in winter, or feeling unwell, or for whatever reason don't feed the WBS the house eventually gets cold and it is quite an effort to get it warm again.

    2)Inbetween times (like now) house is warm but not enough solar thermal on cloudy days, so we need to light a fire just for some DHW.

    To solve 1) thinking of adding a ASHP - say 8kW Ecodan. We have space for it, and warm winters in Cornwall means it will be effective. Use will be to top up the other heat sources, especially between times. Any recommendations?

    No idea how to solve 2). Some kind of inline top-up water heater would be great. Something to lift the low energy heated water a bit when needed. Any suggestions?
  2.  
    Why not use the ASHP for DHW?
    • CommentAuthorGreenfish
    • CommentTimeOct 12th 2015
     
    Posted By: Nick ParsonsWhy not use the ASHP for DHW?

    Indeed, that would work!

    My UFH comes off the TS from 75% up and returns at the bottom. With the WBS and TS size the higher feed to UFH works best (I have a choice of a lower feed if I want). ASHP could link into the in and out, already have stopcocks so no need to drain the TS, but in at the top and out at the bottom. If the UFH is running then it would blend into that otherwise it is heat into the TS towards the top which gives DHW. Not that we use much DHW, but nice to have the option of more on a cloudy day without lighting a fire. Does that sound viable?

    So what ASHP is going to have good COP and survive a coastal location? Any one experiences of a similar set up?
    • CommentAuthorringi
    • CommentTimeOct 12th 2015
     
    A cheap to install solution would be to heat the top of the TS with E7 for Inbetween times, and have E7 you can turn on to heat the complete TS for the short lengths of time you are away or ill.

    ASHP will be low COP for hot water, as the TS needs to heated to something like 10c over your target DWH temp.
    •  
      CommentAuthordjh
    • CommentTimeOct 13th 2015 edited
     
    Posted By: Greenfish1) If we are away in winter, or feeling unwell, or for whatever reason don't feed the WBS the house eventually gets cold and it is quite an effort to get it warm again.

    How often does this case occur? If it's not very often then perhaps an ASHP is overkill and a fan heater or two would be a much cheaper and faster way to reheat the house.

    2)Inbetween times (like now) house is warm but not enough solar thermal on cloudy days, so we need to light a fire just for some DHW.

    You may have enough solar if you add a diverter to your PV system to power an immersion. We've only had one day so far this year (we started in June) when we didn't have enough hot water and I turned on the booster for an hour. It depends how much hot water you use of course. There are two of us with a 250 L TS and it seems to just last three days.

    Our diverter powers the lower immersion, and I have it set at 75°C. The mains booster powers the upper immersion and is currently set at 60°C but I may change it as we gather experience. There's a time switch on it so I can use E7 if it turns out to be beneficial rather than manually switching it as required.
    • CommentAuthorringi
    • CommentTimeOct 13th 2015
     
    Posted By: djhOur diverter powers the lower immersion, and I have it set at 75°C. The mains booster powers the upper immersion and is currently set at 60°C but I may change it as we gather experience. There's a time switch on it so I can use E7 if it turns out to be beneficial rather than manually switching it as required.


    Just having the top timed to operate every night on E7 may not be too bad, as most days it will not come on due to the tank being over 60.

    Even better get the diverter to first power the top immersion, then the bottom, with a timer set to also put the top on E7 (I am assuming that on most days you don't empty the tank of hot water after the sun has gone down, e.g. most hot water is used in the mornings, leaving "space" for the day's pv.)

    In winter I would like to see the bottom immersion operating on E7 every few weeks, so you know the complete tanks gets hot enough to kill of bugs.
    •  
      CommentAuthordjh
    • CommentTimeOct 13th 2015
     
    Posted By: ringiJust having the top timed to operate every night on E7 may not be too bad, as most days it will not come on due to the tank being over 60.

    Yes, that's one possible strategy I've been considering.

    Even better get the diverter to first power the top immersion, then the bottom, with a timer set to also put the top on E7

    It's a possibility, though it brings some added complexity. We don't have enough experience yet to know just how the bottom immersion heats the tank. It might already be heating the top first for all we know. Adding some temperature sensors is on my tuit list.

    (I am assuming that on most days you don't empty the tank of hot water after the sun has gone down, e.g. most hot water is used in the mornings, leaving "space" for the day's pv.)

    Not a safe assumption, I'm afraid; we shower in the evenings. Don't understand the habit of going to bed without showering.

    In winter I would like to see the bottom immersion operating on E7 every few weeks, so you know the complete tanks gets hot enough to kill of bugs.

    It's a thermal store, so I don't much care how many bugs are in there.
    • CommentAuthorringi
    • CommentTimeOct 13th 2015
     
    Posted By: djhNot a safe assumption, I'm afraid; we shower in the evenings. Don't understand the habit of going to bed without showering.


    But are you using more then the section of the tank that is heated by the top immersion? If not, when you have enough sun, the top section will always be filled from the bottom section that has been heated for free.

    If I was setting it up and had the space, I would have a E7 heated DHW tank, that has its "cold" input connected to the output of the FT.
  9.  
    How much thermal mass do you have?

    I would suggest running your UFH during the E7 period to charge up your slab and if you have enough thermal mass it should keep the house warm throughout the day.
    • CommentAuthorGreenfish
    • CommentTimeOct 14th 2015
     
    It seems that ASHP is not popular here?

    As discussed elsewhere I agree that electric fan heaters are a good means of providing infrequent quick heat up from cold - that or a party!

    DHW in autmn & spring may be a matter of managing my TS better. After 30 years of on demand gas powered hot water and heating (no tank, just separate boilers) it is a steep learning curve. As the winter approaches I like to put excess or low level heat into the mass of the house, but can dissipate too much out the store if the next day is overcast and end up with a tepid shower. OTOH keeping the store over 40C just in case I need more DHW seems a waste and prevents any low level ST gains that could be warming the slab.

    On sunny days I am fine, so not convinced that PV/Immerson is going to help. I can still over cool the TS can be caught out by a run of dull days. DJH's smaller TS may help him with that.

    Then the favorite E7. I guess I need to do a cost analysis, but my green heart finds it hard to see running the immersions (2kw bottom + 1kW middle) as efficient no matter the tariff. I have not tested how long I have to run them to get a shower out of the 500L TS. All about the delta temps, SHC and flow rate I know, but my head hurts when I try to calculate it. Paying for E7 overnight only to discover that there is sun to heat the slab the next day would also upset me.

    So back to ASHP. It is hard to do a cost analysis, it all depends how much I use it rather than the WBS. If I didn't have the WBS would I go E7 or ASHP to provide the (EPC predicted) 9000kWh yearly demand? No mains gas, not a fan of oil or LPG, GSHP is costly for such a small total, so I think it would have been ASHP (along with the ST and PV).

    Although I have got a WBS, and love it and the physical labour that goes with it, the flexibility of a ASHP as a fall back is attractive. I think it will deliver enough heat to the TS for DHW, if it can lift it to 40C. At 8kW rather than 3kW (immersion) it will do it quicker too. The big question is how intermittently I need it, and that will depend on how lazy I am with the WBS.

    Anyone like to chime in on how good their ASHP is?
    • CommentAuthorringi
    • CommentTimeOct 14th 2015
     
    Posted By: GreenfishIt seems that ASHP is not popular here?


    I think the issue is that it is expensive, and works best when it only needs to be heating the output to about 40c, but a TS needs more to give a reasonable amount of DHW.

    If you were wishing to run your UFH on ASHP all winter and had a separate buffer for your UFH from your DHW, I expect you would get a lot more positive response.

    The WBS seem to be giving you the control issues, as you can't send all the heat from your wood to the tank.
    •  
      CommentAuthorSteamyTea
    • CommentTimeOct 14th 2015
     
    If you need to turn your heating on already, you have a fundamental heat loss problem to overcome first.
    •  
      CommentAuthordjh
    • CommentTimeOct 14th 2015
     
    Posted By: SteamyTeaIf you need to turn your heating on already, you have a fundamental heat loss problem to overcome first.

    I'm not sure about that. Last night our house got down to 19.5°C so just below PH control temperature. The recent run of dull days has done that. So I'm thinking about installing some heating.
    •  
      CommentAuthordjh
    • CommentTimeOct 14th 2015
     
    Posted By: Greenfish(EPC predicted) 9000kWh yearly demand

    EPC predictions are not terribly reliable for well-insulated houses are they?

    Then the favorite E7. I guess I need to do a cost analysis, but my green heart finds it hard to see running the immersions (2kw bottom + 1kW middle) as efficient no matter the tariff. I have not tested how long I have to run them to get a shower out of the 500L TS. All about the delta temps, SHC and flow rate I know, but my head hurts when I try to calculate it. Paying for E7 overnight only to discover that there is sun to heat the slab the next day would also upset me.

    Instead of calculating it, you could test it. Turn other things off, then put the immersion on for say an hour and see if its enough. If you just use the middle immersion on E7 then there'll still be the bottom half of the tank still to be heated by solar, and if you set the thermostat on that immersion as low as possible, then there's headroom for the solar to heat the top half hotter as well. That can make a big difference to how long the heat lasts.

    I think the qustion about the ASHP is not whether it will work; it's just whether the extra expense and complexity is worth it for the [small?] use you're going to get from it.
    • CommentAuthorGreenfish
    • CommentTimeOct 14th 2015 edited
     
    If you need to turn your heating on already, you have a fundamental heat loss problem to overcome first.

    No worries Steamy, don't need heating yet :)
    But my efforts to trickle some solar warmth into the slab have left me without DHW. Silly since yesterday was lovely down here wasn't it. But now I need this cloud to clear or put on the immersion or SWMBO will be complaining.

    The WBS seem to be giving you the control issues, as you can't send all the heat from your wood to the tank.

    That is true. It is great in the winter though, and I am burning gorse that would otherwise (wastefully) be burnt in the field.

    I think the issue is that it is expensive, and works best when it only needs to be heating the output to about 40c, but a TS needs more to give a reasonable amount of DHW.

    If my whole TS was 40C then I would be OK for showers (have tried that out), to wash those greasy pans we could either get a little lift with the immersion or boil a kettle. With a TS indirectly heating the DHW there are no bugs to worry about. Or could I not turn up the ASHP temp, to the detriment of the COP depending on weather, to briefly get some hotter water? I'm in warm damp Cornwall, frost is not an issue.

    But I will admit that for once money is not the main issue. I have received a little sum unexpectely, and would like to invest it in the house and comfort. But house is new, so have done all the insulating etc. A tepid shower helpd focus my mind! ASHP as an additional heat source is a little bit of a "guilty pleasure", but maybe when I am older and slower at chopping wood I will be glad I did it?

    EPC predictions are not terribly reliable for well-insulated houses are they?

    No they aren't, I'm sure we use less energy. Since last Oct, with almost normal occupancy, we have imported 1800kWh of electricity for cooking, lighting and some immersion. Ignoring capital investments in all the kit, that is my only cost. We burnt about 1400kg of free wood. Solar + PV on top that of course.

    I will try some testing when conditions allow, easier than the finite element analyisis alternative!

    I think the qustion about the ASHP is not whether it will work; it's just whether the extra expense and complexity is worth it for the [small?] use you're going to get from it.

    Indeed, that is where my thinking was when did the build. But in a moment of unusual extravagance I am now just wondering about it working, and what best to buy.

    Also wondering what control/monitoring kit I need, something I didn't think about with the original system and have regreted a little.
    • CommentAuthorgravelld
    • CommentTimeOct 14th 2015
     
    Posted By: SteamyTeaIf you need to turn your heating on already, you have a fundamental heat loss problem to overcome first.
    Tell me about it :sad:
    • CommentAuthorgravelld
    • CommentTimeOct 14th 2015
     
    Back to the OP, have you read JSHarris's EBuild posts about his ASHP pre-heat and inline, on demand, modulating heater? http://www.ebuild.co.uk/blog/12/entry-232-part-thirty-five-hot-water-and-leds/

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