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Heating and cooling: solar to accu tank. Valve for cold dead-leg

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    • CommentAuthorfreemp31
    • CommentTimeDec 17th 2022
     
    Finally setting up my solar tubes 4 sets of 18 (?) tubes to feed the single coil in my 3000L accu tank which runs my wet u/floor.
    I have opted to mount them at ground level because my roof is E/W aligned, and the S end is shaded by trees, plus the fact that I have 3 hectares of land which is just grass.
    However, to get sun for the maximum day length, the panels will need to be some 40M from the tank, which sounds like a hell of a lot of cold water needing to be circulated from the manifold before any heat gets to the coil.
    A Laddomat (which I have on the accu's primary circuit from the gasification boiler) feels a bit like overkill - especially at their current price - anyone found a reliable solution to this problem?
  2.  
    If you use 15 mm pipe from the panels to the tank then you will have about 5.6 lts of water in the pipe for each leg, a 22 mm pipe gives 12.4 lts in each leg. And of course the plug of cold water will take heat out of the accu. tank and return it to the panels.
    How do you intend to control the flow from panels to tank? Short cycling could cost more in losses than the gain. What is the volume of the water in the panels?

    Perhaps a small very well insulated accumulator tank (small is a relative term !!) at the panels which is switched on (by thermostats) when there is enough hot water at the panels to be worth a pumping cycle, that way you reduce the number of cycles and so reduce the losses.

    Health warning I have not had this problem before so the above is just a thought.
    • CommentAuthorGreenPaddy
    • CommentTimeDec 21st 2022
     
    Not tried it for this particular application, but in general, one simple way to control the minimum temp of flow from a system, is using a differential controller (basically a solar controller).

    Logic would be to have the circ pump outlet linked directly to the return to the solar, with a 3port valve (or 2 2ports), which only opens, to let water go to the cyl when the temp at the pump outlet is close to the temp at the solar tubes (say delta 5oc ON, delta 3oC OFF).

    It's a double logic system, with temp diff (cyl to tubes) to bring on the pump (existing I guess), and then temp diff (circ pump outlet to tubes) to let the water go to the cyl.

    You might even find your existing solar controller (assuming that's what you're using) has the option for two relay outputs, for two arrays, and so use one relay to start the pump, and the second to move from bypass to cyl feed. Need to think carefully about where the comparative temps were taken, so might not work.

    Maybe simpler is just set a timer for the 3port (or 2ports), which you would determine by testing, to ensure the cold slug was circ'd back to the tubes, until warm was avail. Might be say 1 minute, erring on the safe side of excess water recycled to the tubes.

    Hope that sparks some ideas for you?
    • CommentAuthorfreemp31
    • CommentTimeDec 23rd 2022
     
    It will, I suspect depend on how quickly the tubes are passing the heat to the manifold, what the outlet temp is, and if the pump at its slowest speed is driving the manifold temp down so low that it short cycles. this is why the idea of a variable mix temp to the coil is so attractive rather than the straight off/on of a motorized 3-way valve.
    However it might well be that the manifold recovery rate (in full sunshine) is sufficient to deliver enough heat that it doesn't cycle.
    To start with I think I'll just plumb in the controller and the collectors direct to the coil - but with lots of isolator valves to allow mods without a complete drain-down.

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