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Green Building Bible, Fourth Edition
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Heating and cooling: Height of header tank for vented thermal store?

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      CommentAuthordjh
    • CommentTimeDec 6th 2016
     
    Posted By: ComeOnPilgrim
    Posted By: djhMy thermal store has its immersion thermostat set at 60°C and that heats just the top half of the store and gives us reliable showers all winter.


    I can see the benefits of a store that can be used when it is cooler - presumably it will lose heat far more slowly due to Newton's law of cooling.

    That's part of it. Only heating the top half of the tank also saves energy. It works for us, but I have to admit it is a tactic I developed by experimenting after we installed the system, so I don't know how much knowing it helps in advance.
  2.  
    Posted by djh
    Posted By: Peter_in_Hungary
    70deg. is the minimum useful temp. for DHW
    Nonsense. For actual DHW the UK *UPPER* limit officially permitted is 46°C for bath filling. With an upper safety limit of 60°C for all uses. IIRC

    I meant by "70deg. is the minimum useful temp. for DHW" that is the minimum useful temp of a TS used as a heat source for DHW, not that the DHW itself should be 70 deg.

    From ComeOnPilgrim
    @Peter, sorry for the newbie questions, but can you explain why PV now has an advantage over solar thermal?

    The cost of PV has now fallen to a point that it is as cheap or cheaper than solar thermal (ST)per kW produced. PV is simpler to install (cables are easier to install than pipework) If you install ST for the autumn/spring you will be over supplied in the summer. once your tank is full of hot water you can't use any more, install for the summer then you have a shortfall at other times, whereas with PV you can always use electricity.
    • CommentAuthorringi
    • CommentTimeDec 6th 2016 edited
     
    Posted By: Peter_in_HungaryI meant by "70deg. is the minimum useful temp. for DHW" that is the minimum useful temp of a TS used as a heat source for DHW


    It does depend on the quality of the DWH coil, how large the area of the coil is, how well it is spread in the tank etc.
    • CommentAuthorringi
    • CommentTimeDec 6th 2016
     
    The other issue with solar thermal is that when the tubes overheat they can be damaged, so if you install it for autumn/spring, you need to do someone with the unneeded heat in summer or shade the panels. So unless you have a swimming pool to heat.....

    Solar PV is self regulating, when you stop taking power from it, it stops giving you power.
    • CommentAuthorEd Davies
    • CommentTimeDec 6th 2016
     
    Posted By: Peter_in_HungaryThe cost of PV has now fallen to a point that it is as cheap or cheaper than solar thermal (ST)per kW produced.
    In the context of a discussion of DHW that's a reasonable point but it's worth saying that solar thermal can still be cheaper per kW for low-temperature (e.g., space-heating) applications. I completely agree with PiH and Ringi's other points.
    •  
      CommentAuthordjh
    • CommentTimeDec 6th 2016
     
    Posted By: Peter_in_HungaryPosted by djh
    Posted By: Peter_in_Hungary
    70deg. is the minimum useful temp. for DHW
    Nonsense. For actual DHW the UK *UPPER* limit officially permitted is 46°C for bath filling. With an upper safety limit of 60°C for all uses. IIRC

    I meant by "70deg. is the minimum useful temp. for DHW" that is the minimum useful temp of a TS used as a heat source for DHW, not that the DHW itself should be 70 deg.

    But my TS is at 60°C and still perfectly satisfactory. That was my point!
    •  
      CommentAuthordjh
    • CommentTimeDec 6th 2016
     
    Solar thermal also requires less space than PV, if you are short on suitable roof or other space.
  8.  
    Right, thanks to everyone for their comments and advice! I think I'll probably switch to PV and a smaller(!) thermal store. That should also help the original issue with the head height!

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