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Thread: Solar heating: flow v temp?

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    Solar heating: flow v temp?

    Hi,
    I'm just installing a solar heating system and need some advice please.
    I have an Intex pump 2000GPH and have the solar T'd off and back in from the pump outlet, into 1 1/2" poly pipe
    for a distance there and back of about 160' total with a rise in ground level of about 4', with 5 coils of 3/4" poly pipe each of 160' all linked from a central manifold also of 3/4".
    At the moment I have 4 coils connected to see what the flow would be like with this pump (which is about 1/2 to 3/4 hp).
    I am surprised to see that the flow is strong even after going through 4 coils.
    [attachment=0:17mjzbxk]solar 004.JPG[/attachment:17mjzbxk]

    Now, I have read conflicting opinions on flow v temp. Some people seem to say that flow should be slow but not so slow that the coils are able to warm up to the touch.
    Others say flow should be fast and that even a slight temp difference is better from larger flow than a greater temperature from a smaller flow.
    I have also read that only a small flow should be diverted to the solar coils whilst allowing the pump to move the majority of the water in the normal way so as to
    reduce back pressure on the pump and therefore not shorten its life.
    This would require a non return valve on the return line otherwise the water would not circulate in the solar system (ie if the main valve is open).
    I have a Jandy non return which I could fit, but the spring on it is probably too strong for such a small flow to push it open.

    As the pump seems to be functioning near normally on full flow through the solar system (at least with 4 coils at the moment), should I worry about back pressure and reducing the flow
    to increase the temp?
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    Mod Squad JohnT's Avatar
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    Re: Solar heating: flow v temp?

    There is no debate. Highest flow performs best.

    Pressure us not really an issue to the pump, but it may be to other system components, so most systems are designed with a max pressure specified across the solar and the flow is adjusted to remain below that pressure.
    TFP Moderator
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    Re: Solar heating: flow v temp?

    Now that we have some sunny days I am testing the solar system and am not sure whether or not I need to add more solar collector capacity.
    The average for full sun with an air temp of around 70-73F is a temp rise at the return of around 3.5F on average, with a flow reduced to about half normal (normal flow now increased significantly due to previously unknown blockage at return eyeball): guesstimate of around 700-1000gph and a daily pool temperature increase of around 3.5F.
    Would there be any benefit of adding extra collector capacity or am I near the point of diminishing returns?
    I have 2 rolls of those rubber collectors with the narrow tubes, which I haven't fitted. I am wondering that if I add them, all that I will do is just slow the whole system down with reduced flow without much gain in temp.
    thanks

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    mas985's Avatar
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    Re: Solar heating: flow v temp?

    Quote Originally Posted by UltraIntex
    Would there be any benefit of adding extra collector capacity or am I near the point of diminishing returns?
    I have 2 rolls of those rubber collectors with the narrow tubes, which I haven't fitted. I am wondering that if I add them, all that I will do is just slow the whole system down with reduced flow without much gain in temp.
    thanks
    Definitely there would be benefit. The more solar surface area, the higher the heat gain. But if you can plumb them in parallel with the old panels, you can reduce the head loss and flow rate concerns. You just want to make sure you have approximately equal flow rate per square foot of panel.
    Mark
    Hydraulics 101; Pump Ed 101; Pump/Pool Spreadsheets; Pump Run Time Study; DIY Acid Dosing; DIY Cover Roller
    18'x36' 20k plaster, MaxFlo SP2303VSP, Aqualogic PS8 SWCG, 420 sq-ft Cartridge, Solar, 6 jet spa, 1 HP jet pump, 400k BTU NG Heater

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    Re: Solar heating: flow v temp?

    Quote Originally Posted by mas985
    You just want to make sure you have approximately equal flow rate per square foot of panel.
    Thanks for that. Is there an issue of mixing collector types as the flow rate difference will be quite substantial?

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