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Heating and cooling: Maintain thermal store stratification by design

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    The heating system – 40Kw gasifing wood boiler, 2000lt. Thermal store (TS) and 300lt. indirect DHW tank supplying the family house and guest house. The TS has 3 loads driven by 3 separate pumps and their controls, the DHW, the family house central heating (CH) and the guest house CH.

    Having been dithering about for some time and got the boiler room enlarged to take the TS I have finally got around to installing the TS. But first I read lots about the how. Stratification within the TS is important and I was always concerned that stuffing 60 deg.+ water into the base of the TS is not going to help. The CH return temperature could be managed down by flow control and mixer valves but the DHW return would inevitably be above 60 deg. to get the DHW to 60.

    Inspired by a photo from gotanewlife at http://www.greenbuildingforum.co.uk/newforum/extensions/InlineImages/image.php?AttachmentID=3239 showing the return manifold on his TS I decided that this was a good idea and took it further by having 8 inputs with 20cm between inputs (rather than gotanewlife’s 4) which I hoped would allow the return water to enter the TS at its own temperature gradient rather than enter at the bottom to mix with the colder water.

    Does it work.....Yes. Am I happy with it .....Yes. What would happen without the external manifold.....No idea but I suspect more mixing would occur.

    The figures.
    I did a test, putting thermometers on the manifold return pipes and first ran the DHW pump on setting 1with a gate valve partially closed to slow the flow.(its normal position) with the following results at the start
    TS top temp. 71, manifold temps. 64 56,50,42,38,36,32 with the DHW return at 18 (room temp)
    I ran the DHW sufficient time to get stable temperatures with the following results
    TS top temp. 68, manifold temps. 66 56,52,48,44,37,35 with the DHW return at 66

    I then repeated the test with the gate valve fully open and the pump speed set to 2 and got the following
    TS top temp. 67, manifold temps. 67 58,57,56,56,55,35 with the DHW return at 67

    Clearly the lower speed helps. In daily running (all 3 loads running) the TS temperature gradient will be between 40 and 70 spread over 40cm height with this band moving up the TS as heat is used. I normally charge the TS to 80 deg top to bottom

    The TS was made locally at a cost of 600 GBP (without insulation), there was no additional charge for the additional inputs and there are no coils in the TS as solar thermal would be difficult pipe work and eventually PV (if installed) will be connected to the DHW tank via immersion heater. Wood supplies are from ‘on farm’ forest
      TS return manifold.jpg
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      CommentAuthordjh
    • CommentTimeFeb 10th 2015
     
    Thanks for posting this, especially with numbers.

    Just for clarity, what is the DHW return? How is your DHW prepared?

    I'm impressed that this ladder network works. I spent ages chasing Solvis about UK supply and eventually gave up. I've now bought my thermal store (a Gledhill Torrent), but we've yet to plumb it in so I'm still keen to learn! My plumber persuaded me that a feed & expansion with a mains fill was sensible, but I keep looking at the four (!) pipes that go up to where it will be and thinking about heat loss (this can't be right, there must be a better solution kind of muttering).
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    The DHW is a 300lt indirect tank. The DHW return is the return of the heat exchanger coil in the DHW tank to the TS.

    The DHW tank is located next to the TS and has a thermostat at its base set to 55 deg., there is another thermostat at the top of the TS set to 65 deg. wired in series such that if the DHW tank (at the base) is at 55 deg. or lower and the 65deg. TS thermostat is at 65 deg. or above then the pump will run transferring heat from the TS until either the DHW reaches 55 or the TS drops below 65.
    In practice this gives me DHW at the top of the tank at anything between 80 and 65, depending on the temperature of the TS graduating down to 55 at the bottom of the tank.

    The TS / boiler / CH system is open vented and the DHW is mains pressure (sort of, we are off water grid with a bore hole and so the DHW and cold water are fed from the same pressure system)

    All the pipework is now insulated with 2 layers of pipe insulation and the TS has 300mm of insulation wrapped around it.
    • CommentAuthorgravelld
    • CommentTimeFeb 11th 2015
     
    Posted By: djhMy plumber persuaded me that a feed & expansion with a mains fill was sensible, but I keep looking at the four (!) pipes that go up to where it will be and thinking about heat loss (this can't be right, there must be a better solution kind of muttering).
    This is what motivated me to post http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12960 ... (which I can see you already replied to).
    • CommentAuthorringi
    • CommentTimeFeb 11th 2015
     
    Peter,

    Why did you not use a plate heat exchanger on your thermal store for DHW, rather then a DHW tank?
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    Posted By: ringiWhy did you not use a plate heat exchanger on your thermal store for DHW, rather then a DHW tank?

    Several reasons, first I wanted the additional capacity that a DHW tank gives me. I get about 250lt of usable hot water in addition to the TS capacity and as the system serves 2 houses I felt this important. With a separate DHW tank I can have an immersion heater for the DHW without involving the TS. (either being lazy in the summer and use night rate electricity or later use PV). And lastly I managed to get a second hand DHW tank for 50 quid. Not a main driver to the decision but it helped.
    Basically I wanted the DHW capable of being independent from the heating and with the higher capacity.
    • CommentAuthorringi
    • CommentTimeFeb 11th 2015
     
    Makes sense, if you are paying for heating DHW outside of the main heating season as you get less loses by having a smaller tank for DHW.

    I do wonder (apart form your very cheap DWH tank) if it would be better to use a direct DHW without a coil and then pump the water from the bottom of the DWH tank, into a coil that runs from the bottom to the top of the TS and back into the DWH tank using a slow speed brass pump, with the same control system as you are using.
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    Posted By: ringiI do wonder (apart form your very cheap DWH tank) if it would be better to use a direct DHW without a coil and then pump the water from the bottom of the DWH tank, into a coil that runs from the bottom to the top of the TS and back into the DWH tank using a slow speed brass pump, with the same control system as you are using.

    That would work but a DHW heat exchanger coil in the TS would have cost £150 (and a solar coil would have been £90). The pump I used is was a standard CH pump that came from the old CH system. So yes apart from a very cheap DWH tank...............
    I also considered a 2500lt TS with a 30Kw DHW coil in the top, but apart from tieing the DHW into the TS it would not fit into the boiler room.

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