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General: Help with heat source in low energy home

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  1.  
    • CommentAuthoran02ew
    • CommentTimeSep 23rd 2019 edited
     
    hello all

    just wondering if there is a definitive answer to the heating of a low energy home?
    Briefly.
    This is a detached new build 100m2 GF and 65m2 FF, fastidiously insulated, very airtight almost zero cold bridging with 3g windows and MVHR. there will be UFH pipes in the structural floor slab and towel rails in bathrooms. 2 residents with the grand kids staying occasionally, there is no gas in the area. with south facing roof availability

    What is the most cost effective way of delivering the low space heating requirement? and hot water for 2 that prefer baths,in the morning.


    A
    • CommentAuthortony
    • CommentTimeSep 23rd 2019
     
    What is the heat loss of your home in say October? Mine is 100W and yours could be similar.

    I use a 120W heated towel rail element as the heat source for my underfloor hearing in my lounge if my mother in law is coming , we don’t often need anything, when it is below -3 outside some heat is added with a convector heater, since I lived there we once got -11C , only use convector during very cold weather

    See tonyshouse.info for more info.

    Is yours built yet?

    For hot water use the lowest possible storage temp and solar thermal, and I would go for a Chinese ASHP/combined with hot water cylinder thingy, with immersion backup. Solar PV divert to hw vis a modulator.

    Interested to know your air tightness test results.
    • CommentAuthorsnyggapa
    • CommentTimeSep 23rd 2019
     
    Probably all electric - electric panel heaters (rarely to be used) and electric immersion in an unvented water cylinder.
    Coupled with PV and a water diversion that is free hot water in the summer.

    We used nobo series 8 panel heaters and a wireless control unit - and wired in spare radials from the consumer unit to places where we might want heaters in the future - but never did.

    Basically what Tony says above , but without the ASHP which may or may not be optimal. If you don't use much hot water then it may be overkill on the cost and complications - all electric has an elegant simplicity to it
    •  
      CommentAuthordjh
    • CommentTimeSep 23rd 2019 edited
     
    Posted By: an02ewWhat is the most cost effective way of delivering the low space heating requirement? and hot water for 2 that prefer baths,in the morning.

    First thing to do is to calculate exactly what the space heating requirement is, and also the DHW requirement. I would recommend using PHPP for that.

    Almost certainly the cheapest solution will be electrical resistance heating, with an ASHP being a more expensive but 'better' alternative. You appear to have already chosen to have water storage of some kind since there are solar thermal panels. Adding an immersion heater and using it overnight is the most cost effective way of solving the remaining DHW requirement. Tank size will be determined based on expected usage but FWIW we find 250 L for two of us is sufficient to buffer occasional sunless periods when relying on solar heating. You may want more if regular baths are a requirement.

    You seem to have decided on a wet heat emitter solution, given the UFH pipes, so use a resistance heater or perhaps an ASHP to warm that water overnight and it should keep the place warm until the next night. You could just use a few simple radiant heaters and ignore the UFH as an alternative. A duct heater in the MVHR would contribute to the heating but also to the expense.

    I would add some solar PV to the roof alongside the solar thermal, and expect that most of that will be exported.

    edit: PS there's an awful lot of glazing on the west wall. Think carefully about how you will prevent overheating. Black frames will get very hot in summer, but shouldn't be a problem as such.

    edit2: There also seems to be a flue for a solid combustion fire/stove. That will likely prove excessive and certainly not cost effective.
  5.  
    I'd edit the pdf in the link to blank out the name and address details.

    I'd then look at filling the south facing roof with roof integrated PV (much nicer looking) and have a diverter to an immersion heater in the HW tank, rather than having solar thermal (too much to go wrong).

    Why have a low energy home when you can have a zero energy home?
  6.  
    Posted By: Pile-o-StoneI'd then look at filling the south facing roof with roof integrated PV (much nicer looking) and have a diverter to an immersion heater in the HW tank, rather than having solar thermal (too much to go wrong).


    I'd second this but suggest you look at SunAmp storage too. We have a similar sized house and have a 5kW peak solar PV array. This fills a total of 9kWh heat storage, which is easily enough for a shower and a bath, possibly 2 baths.

    With a slightly larger array and battery combo (especially if you want to drive UFH) you'd be well sized for what you want and, as P-o-S suggests, there's less to go wrong than a solar thermal system.

    For what it's worth, we went with electric panel convectors as an alternative to wet rads/underfloor and they worked well in the colder months, but it is highly dependant on your predicted heat loss.
    • CommentAuthoran02ew
    • CommentTimeSep 23rd 2019
     
    Posted By: tonyIs yours built yet?

    Due to start in a couple of weeks




    Posted By: tonyWhat is the heat loss of your home in say October? Mine is 100W and yours could be similar.

    Total daily heat loss power averaged in October 631w

    Posted By: djhYou appear to have already chosen to have water storage of some kind since there are solar thermal panels

    yes, my original plan was to ST but i shouldn't think there would be problem to change to PV

    Posted By: djhou seem to have decided on a wet heat emitter solution, given the UFH pipes,

    IMO they are the easiest way to deliver all over heat at low temps,
    • CommentAuthortony
    • CommentTimeSep 25th 2019
     
    Yes and any heat source can be used to feed them
    •  
      CommentAuthordjh
    • CommentTimeSep 26th 2019
     
    I said: "PS there's an awful lot of glazing on the west wall. Think carefully about how you will prevent overheating."

    FWIW, I just came across a simple overheating design tool that might give you a rough idea. It's at https://goodhomes.org.uk/overheating-in-new-homes

    Thanks to PH+ magazine for pointing it out :bigsmile:
    • CommentAuthortony
    • CommentTimeSep 26th 2019
     
    West facing glazing is the worst scenario for overheating problems, solar heating at the end os a hot day can make building uncomfortable to sleep in
    • CommentAuthoran02ew
    • CommentTimeSep 26th 2019
     
    Point taken about solar overheat, and maybe the evening overheat will be a real problem hear as the master bedroom is a mezzanine partially over GF. we provisioned for external sun louvers on that elevation but may have to consider some sort of adjustable type to almost block out the low sun?

    Just to confirm, in your opinion is it worth bothering with a heat pump? or just go for an electric boiler or some kind of electric water heater for the UFH and immersion heater for HW? bearing in mind the morning water demand.

    The panels shown on the roof and log burner, are just to give us options.
    • CommentAuthortony
    • CommentTimeSep 26th 2019
     
    Heat pump will be cheaper to run for both heating and hot water but has higher capital cost.

    Solar is a must have
    •  
      CommentAuthordjh
    • CommentTimeSep 26th 2019
     
    Posted By: an02ewwe provisioned for external sun louvers on that elevation but may have to consider some sort of adjustable type to almost block out the low sun?

    I would think you might need to, yes. Certainly design one in so it can easily be added afterwards if required. PHPP will give a good idea of the need. One design I saw had vertical louvres instead of the more usual horizontal ones. The idea is that they can completely block out direct sunlight whilst still offering a good view in some directions.

    Just to confirm, in your opinion is it worth bothering with a heat pump? or just go for an electric boiler or some kind of electric water heater for the UFH and immersion heater for HW? bearing in mind the morning water demand.

    For our house, I didn't bother with a heat pump because of the capital cost and ongoing maintenance cost (including replacement units every decade or so). In fact I didn't initially fit any space heating at all, just put some radial circuits in because I didn't know what we would need. We just use a duct heater and a radiant heater, both on timeswitches to run overnight to use our E7 tariff. I've never known how the extra embodied energy of a heat pump affects the actual carbon savings.
    • CommentAuthoran02ew
    • CommentTimeSep 27th 2019 edited
     
    Posted By: djhWe just use a duct heater and a radiant heater, both on timeswitches to run overnight to use our E7 tariff


    I quite like the sound of this, maybe a small electric water heater 3kW (ish) timed to heat up the floor slab over night through the UFH, and an immersion heater in the DHW cylinder again timed to heat the cylinder in the small hours ready for morning demand, both taking advantage of the E7, then maybe PV on the roof to offset the increased kW/h charged on peak E7. Simple and cheap to set-up
    •  
      CommentAuthordjh
    • CommentTimeSep 27th 2019
     
    Yes, that was my thinking. There's next to no solar (PV) available when you need space heating so that can be disregarded. DHW is needed throughout the year so we have one immersion powered by PV and another on a timeswitch that heats the water in winter.

    To my mind, one advantage of the strategy of designing to concentrate all the grid-powered heating demand into the overnight period is that there are another 17 hours available each day that could be used in the case of extreme weather.

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