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    Does anybody know the formula? I was thinking of around 10% of the size of the thermal store. It will be an unpressurised store.
    • CommentTimeJun 13th 2017
    It depends on the total volume of water 'in circuit' i.e. including both the thermal store itself and any directly connected boiler or radiator circuits, but not any water in secondary circuits isolated by a coil etc. The exact rules should be in the installation handbook for your thermal store.
    10% is a rule of thumb. You can get away with a lot less if it is unpressurised. I have a 2000ltr TS and about another 300lts in the boiler rads and pipes. My expansion tank is about 200lts (I'm not sure exactly - it came second-hand), but its size was not by design but I re-purposed my mains pressure DHW tank when I installed my TS and indirect DHW tank. How ever the tidal flow in the expansion tank is only about 1/4 or so of its volume so the difference between system off and system hot is about 40 - 50 lts. I know the approx tidal flow as there is a sight tube up the side of the tank so that I can check the system levels to know when to top up.

    Somewhere on google quotes the expansion of water as 0.000214 / deg C so for my system given temps ranging from 20 -90 deg =70 deg change then 0.000214 X 2300lts X 70deg = 34.45 So it seems that the calculation agrees with the actual experience given I don't know the the exact volumes of anything!

    Just to add to the confusion the coefficient of expansion of water varies over the temp. range.

    Otherwise as always be guided by the manufactures info. which will err on the side of caution.
    I've used the attached example in the past. Don't know if it's up to date.
    Thanks everybody, that's extremely helpful!
    • CommentTimeJun 14th 2017
    Posted By: PeterStarckI've used the attached example in the past.

    Isn't that to size an expansion vessel for a pressurised system though? Not an unpressurised feed & expansion tank like the OP wants?

    Again, I believe the correct size will be listed in the tank's instructions. No need to calculate from first principles. And a Benchmark plumber will use that information to satisfy the regs.
    • CommentAuthorGreenPaddy
    • CommentTimeJun 16th 2017
    It would appear from Mr google (engineers toolbox) that water specific volume rises from 0.001 to just over 0.00104 m3/kg from 0 to 100oC. So lets say 5%. Must allow for all water in entire system. Must allow for tank having water in it already when system cold, prob the point where the ball cock shuts , and it can't be right to the brim when the system is hot. Non melting components of course. So look at around 5%, for the 'working' capacity of the f&e tank. Then allow a bit more. When it's fullest, it could be at boiling point. This is a potentially dangerous installation remember!!
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