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solar pool heat temperature

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zacmobile
zacmobile Member Posts: 211
I installed a solar pool heating system about 4 months ago and customer is not so much complaining but a little perplexed that they can't maintain the pool temperature at 92°F I thought that seemed a little unreasonable as I know the recommended

industry standard is 1-2 deg above the ambient room temperature. they are only able to hold it at 86°F during the day and are losing about 2-3°F overnight.

the pool is indoors & the system was designed by the manufacturer (viessmann) about a year and a half ago.  originally they called for 4 25sq ft flat-plate collectors,  (I bumped it up to 6 because of DHW requirements thank goodness) with a max pool setpoint of 86°F. collectors are facing due south at 45° and get full sun from dawn (about 6AM) to about 5PM when they get shaded by an adjacent roof. pool is about 25,000 gal uninsulated fiberglass in dry fine sand with a thin bubble wrap cover and ambient room temp is about 79°F. yesterday the collectors were supplying 160°F to the pool HX and returning at 122°F.



anyways, my question is: do you think their desired pool temperature sounds reasonable? is the amount lost overnight normal?

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  • hot_rod
    hot_rod Member Posts: 22,131
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    is that the temperature

    they requested? If so the design should be built around 92F. The loss overnight sounds reasonable not knowing the ambient air temperature around the pool at night. It sounds like they are more concerned about reaching the 92F.



    Any way to know the flow rate through the HX? Glycol on the solar side I assume.



    Here is the math to calculate the HX performance find the two pages, 46 & 47 at this link



    www.caleffi.us/en_US/caleffi/Details/Magazines/pdf/idronics_6_us.pdf



    hr
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Fortunat
    Fortunat Member Posts: 103
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    Heat exchanger or design?

    I think HR is onto something.



    Why are your collectors running at 160 degrees when the pool is under 90?  Since collector performance is fairly strongly dependent on differential temperature to ambient, getting the collectors to run as cool as possible will get you more BTU's into the pool.



    So step 1: check your heat exchanger sizing. I think you should be able to get the collectors to run at roughly 120 degree exit temperature, which should help a bit...but I doubt it will get you into the 90 degree range.



    The second question is what is the design you are using? You say this system heats DHW also?



    Finally...really?  Viessman reccomends 100 sq ft of collector to heat a 25,000 sq ft pool? That seems absurdly small, especially if the target temperature is up in the 90 degree range. Did they do some performance modeling to support that?

    RETScreen is kind of limited in its ability to model indoor pools, but I think you'll find it usually suggests something on the order of 50% of the pool's surface area. From the sound of things, you might have close to 15%.



    Best of luck,



    ~Fortunat

    www.revisionenergy.com
  • zacmobile
    zacmobile Member Posts: 211
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    heat exchanger

    I kind of suspected the heat exchanger might be undersized too. I did not supply it, it was installed by the pool guys who told me it was a "200,000 btu"exchanger. it is about 4" in dia. w/ 1" connections on the primary side. I have no idea what they are running for flow on the pool side, it is 2" though so maybe 30 gpm? the temperature difference on the secondary (pool) side is only 1.5 deg F.



    my calculations using the caleffi formulas give me a heat transfer of 35,454btuh so that would mean the pool flow is closer to 40-45gpm. the original collector design output was 46,770 so it does seem to be botlenecking a bit



    the heat exchanger has a small leak on one of the welds and the owner said he was going to request they change it. I think I will ask them to put a bigger one in when they replace it.
  • Fortunat
    Fortunat Member Posts: 103
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    I changed my mind...i think it is flow limited

    What is the flowrate in the solar loop? or alternatively, what is the temperature of the return water to the collectors when in pool heating mode.



    Even though that '200kBTU' heat exchanger is nothing close to that under the conditions you are operating, it should still be plenty to handle 100 sq ft of flat plate solar collectors.  Forgive my unit switching (insolation numbers are stuck in my head in

    metric units), but 100 sq ft is 9.2 sq meters and assuming 1,000 W/sq m

    of insolation and a total collector loop efficiency of 65%, your only

    talking about 5,980 watts peak or roughly 20,000 BTU/hr peak (where is

    your 47,000 number from?).



    Assuming you are talking about a HEX something like this one: http://www.triangletube.com/TriangleTubeProduct.aspx?CatID=2&PID=12

    I think it should be plenty. Though I tend to prefer flat plate heat exchangers for solar loops, we've succesfully used the MF-200 on systems with up to about 300 sq ft of collector area and we've maintained reasonable collector temperatures (maybe 25 degrees above pool temp).



    That is a 200,000 BTU heat exchanger....so long as you have 180 degree boiler water at 8 GPM and 78 degree pool water at 65 GPM flowing through it. Heat transfer is more or less a linear function of dT (actually a linear function of Log Mean dT, but in this case the arithmetic mean is awfully close to the log mean).



    I don't know what your collector loop flow rate or dT is, but assuming incoming temps of 160 as you said and maybe a 25 degree delta, and assuming the pool is humming along at 85or so, your heat exchanger average dT is roughly 62 degrees.



    The sizing condition for the HEX is probably an average dt of 90 degrees. So you should still easily be able to move all the solar heat to the pool through that HEX, assuming you have enough flow.



    so anyway...i would like to revise my hypothesis and say your issue may be insufficient flow in the collector loop. Or more accurately, your primary issue is a woefully undersized solar array for the particular load, but the secondary issue may be flow related.



    hope that is helpful...let me know if I screwed up any math (which tends to happen later in the day...)



    Cheers,



    ~Fortunat

    www.revisionenergy.com
  • hot_rod
    hot_rod Member Posts: 22,131
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    could it be a HX

    intended for use with a boiler, 180F input temperatures? Most of those tube and shell pool HX are intended for high delta T applications. Get the data from the manufacturer and see what the performance is with different source temperatures.



    Collector efficiency goes up as operating temperatures drop.



    hr
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
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