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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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    • CommentAuthorLAS1404
    • CommentTimeOct 31st 2018 edited
    Hello all - we are half-way through a low energy build. We are not on mains gas - not enough garden for GSHP horizontal (can't afford a borehole) - and too peaceful for an ASHP (we can hear shrews rustling in the garden)....

    Our fabric is sufficiently well insulated to meet UK Bldg Regs SAP by using an oil boiler for CH & DHW (Calor won't fit me an LPG tank - not enough space to qualify with "safe" space separations) - hence we have ended up with the oil boiler option.

    Our heat loss calcs I completed show about 4kW. Smallest A rated boiler I can see is 15-21kW. It may well be much over-capacity for the small CH demand. But I guess needed for the DHW demand through a 200 litre cylinder. i.e. the DHW demand far outweighs the CH demand. Has anyone else experience of this? - and novel or smart solutions?

    Also - now I know the fabric heat loss (which is nice and low for a 220 sq.m house) - how should I sensibly size radiators to meet this? I was thinking of just using a rounded rule of thumb and adding say 1.5kW in the main sedentary lounge/diner/kitchen biggest room and 0.5kW per room thereafter. That would add up to about 7.5kW of radiators around the house. Does that sound about right...!

    Many thanks in advance for your comments and opinions - cheers - LAS
    • CommentAuthorbarney
    • CommentTimeOct 31st 2018
    Have you looked at an electric boiler - you can get those down to about 6kW single phase.

    If your heat loss (including ventilation) is only 4kW and you have a 200 litre HWS cylinder then that would be a reasonable system size - the next size is about 9kW if you want some margin

    That said, I suspect the flue noise on an oil boiler is likely to exceed that from a suitably sized ASHP anyway - and you will also have limitations on the position of the oil tank (but not as onerous as those for an LPG tank)

    Select the radiators for the calculated room heat loss (fabric and ventilation) - if you run them on the ASHP then a degree of oversizing to reflect lower flow and return temperatures would be sensible - it's more effective to do that and top up the DHW temperature with direct acting immersions


    If your heating demand is really 4kW (and I presume that is for the silly cold winter) then I would forget about a wet system -rads or UFH and a boiler and I would be looking at electric storage heaters on a carefully chosen tariff and swap suppliers as charges change.

    A headline figure I saw showed an 80% reduction for night time use compared to the most expensive day rate. This sort of reduction challenges the COP efficiencies of ASHP for cost per kWh of heat.

    I would also consider dumping the 200lt. tank for instant heaters at point of use. This avoids storage losses although it can make running a big bath a bit tedious - On the other hand if you retain the 200lt tank, can you get this heated on the same tariff (E7, E9 or E10 or what ever else the suppliers dream up to help them balance their demands) that you use for the storage heaters?
    • CommentAuthortony
    • CommentTimeOct 31st 2018
    Yes, we need to know the inside and outside temperature assumptions that lead you 4kW
    • CommentTimeOct 31st 2018
    Worcester Bosch make a Danesmoor 12/18 which you can configure as a 12kw boiler.its A rated too and ours has been 100% reliable over 4 years.
    It’s the smallest I could find when I was looking.
    • CommentAuthortony
    • CommentTimeOct 31st 2018
    Mine is a 60 Watt towel radiator element, whole house heat loss this month is 100W

    Re heating and hot water demand

    4kW for 24 hours = 96kWh or 16 hours =64kWh and HW will be 2 to 4kWh per day in both cases heating demand is an order of magnitude higher in your case LAS.
    Posted By: tonyMine is a 60 Watt towel radiator element, whole house heat loss this month is 100W

    What were the average indoor and outdoor temperatures for that month?
    • CommentAuthorLAS1404
    • CommentTimeNov 1st 2018
    Thank you all for your valued comments and opinions - gives us more ideas to explore. For ref: my heat loss calcs are based on 20C indoors - -4C outdoors (i.e. 24C difference) based on Devon UK temperatures.

    We have point of use water heating in our existing old 4"x2" timber framed bungalow (with 25mm rockwool insulation in the walls) - and it is very effective (if expensive..!)

    I guess we also need to balance the big-bad world of resale value in the decision process - and what potential buyers expect when we move out in 10yrs or so when our kids are older.

    Take care all - and thanks again - LAS
    • CommentAuthortony
    • CommentTimeNov 1st 2018
    My calcs were done using both TAS and HOT 2000 using local (Thames Valley) average weather data indoors assume 20C

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