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    • CommentAuthorGreenfish
    • CommentTimeJul 5th 2017
     
    I have a 500l thermal store made by Joule, fed by ST, ASHP, WBS and with immersions. When at 60C the temperature of the top and middle of the store drop by 4 degrees overnight, that is between peak temp at 6pm and say 8am before it is used for DHW (or UFH in winter).

    Is a 4 degree drop in 14 hours excessive, or to be expected?

    It is located in a small room (walk in cupboard) on the ground floor, inside the heated envelope, so in some ways the heat out into the room is welcome. Makes a great "drying room" (extract fan out to house fitted too) for example. But I could insulate the store and pipes more - I have clad the pipes, but the store has what it came with (50mm of PIR?).

    What drop do other store owners get?
    •  
      CommentAuthorSteamyTea
    • CommentTimeJul 5th 2017 edited
     
    Sounds about right (wrong really as the heat loss may be high).

    You can take a stab at the thermal properties of the cylinder by measuring the surface area and taking a stab at the properties of the insulation.

    I reduced the heat losses from my 225 lt store by delaying heating until as late as possible (in the E7 time window), adding extra insulation and using the hot water as early as possible.
    Saved about 2 kWh/day.
  1.  
    Extra insulation on the store will help reduce losses. And it's worth investigating whether you are getting any thermosiphoning along any of the pipes connected to the store
    • CommentAuthorborpin
    • CommentTimeJul 6th 2017
     
    Sounds about right. I reduced the loss significantly by beefing up the pipe insulation close to the tank.

    Timing of heat to the store is a key issue. I know I still get the odd rogue burn, someone has a late shower and boiler triggers in case someone else has one, and the tank then sits at high temperature overnight simply bleeding the heat away. I have the makings of a better control system but have not implemented it as yet. Key thing is DHW demand; the colder the store, the quicker the control system needs to identify DHW demand and trigger the boiler. The more varied your demand (2 teenagers) the more difficult it is.

    Of course a colder store means less heat loss so if you can have a lower max temperature (when using a boiler of course and not harvesting other energy) and react more quickly that should reduce your losses and thus your fuel cost.

    In the next house (ha ha) the 'little' demands for DHW will be supplied separately from the big demands (shower/bath) so I will be able to tell which is asking for heat. I'm pretty certain I could easily supply all the basins and sinks with 10mm pipe even over quite a distance. The occasions when more than one is in use is so small. Added bonus is they will warm significantly quicker.
    •  
      CommentAuthorSteamyTea
    • CommentTimeJul 6th 2017
     
    Posted By: borpinIn the next house (ha ha)
    Mine will be run on dilithium crystals, but you have a good idea there. I would consider two small inline heaters for basin and sink. These would also sit nicely with PV as most small usage is during the day (I would also be looking to not use any combustion).
    • CommentAuthorGreenfish
    • CommentTimeJul 7th 2017
     
    Glad to know the drop sounds about right for the store without additional insulation.

    I do have some thermosyphoning happening from the very bottom of the store. The ASHP connects to it directly, no heat exchanger, so some blending happens when the ASHP runs.

    I absolutely agree on using multiple/separate stores for DHW and heating, and adding inline heating for the small amounts is a great idea. We have a single thermal store due to both lack of knowledge but also a practical lack of space. What we have is workable for sure, but I could dream up better "next time".

    No gas supply, so no boiler, and only leave the ASHP to drive the UFH automatically (limited by times) when we are away in winter. If nature (solar thermal) is not going to heat the store to the level we need then we manually intervene - running the ASHP or having a fire in the WBS. Using the ASHP during the day for greatest chance of the PV contributing, and having a fire (with gorse that would otherwise have been burnt on the hillside) only in winter. So yes the timing of heat to the store is a key issue, but in an entirely different way to Borpin.

    The plant room/walk in cupboard reached 33C yesterday, not sure the electronic control units installed in that space enjoyed it, so maybe adding more insulation is called for.
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