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Solar water heater electric backup overheat trip

joseph annon
joseph annon Member Posts: 51
I have a customer with a SDHW system.

It is a 80 gallon tank with a 4*8 flat plate collector.

System is 6 yrs old.

Problem. From beginning the customer has complained about the reset button on the thermostat for the electric backup thermostat popping.Company that installed the system went back and installed a control to run the pump in the evening to cool the tank. This does not appear to have helped.

I replaced the thermostat and that solved the problem for about 4 months.

It has started up again.

Any ideas on solutions?



  • hot_rod
    hot_rod Member Posts: 19,289
    how hot

    are they running the tank? Check the solar control for the storage max. setting. I've found if you try and run 160F or more it will pop the reset on the electrical element located on top.

    I'm not sure what temperature the newer resets are set for, but if the bottom tank sensor see 160F, often times the upper portion of the tank may exceed 180F.

    I have also heard of T&P valves seeping on solar tanks that are set too high.

    Put a temperature gauge up high on the tank and see what it reads.

    You may need to lower the tank temperature setting on the solar control a bit.

    The night time cooling function is good for times when the home is un-used to lower the tank temperature, but you may start the next morning with a cooled down tank. This lowers your solar benefit, and may kick on the element because the tank was pulled down too much.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • joseph annon
    joseph annon Member Posts: 51
    Pv Direct pump

    I will look at putting in a switch to interrupt the PV power to the pump.

    There were problems with T&P weeping. solved that issue with a higher quality T&P. Same temp and pressure rating just a higher max BTU rating also about 5X the cost of basic T&P.

  • hot_rod
    hot_rod Member Posts: 19,289
    classis example

    of why PV powered solar thermal may need temperature control. Just because the sun is shinning doesn't mean the tank needs, or can take energy.

    A lot depends on the collector to tank capacity ratio, and how much DHW the home uses everyday.

    There are a few PV powered differential controls available on the market.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • SWEI
    SWEI Member Posts: 7,356
    PV direct

    We used to put a snap switch on the tank and wire it in series with the one on the collectors.
  • michael_34
    michael_34 Member Posts: 303
    ac/dc relay switch


    How about using a ac/dc relay switch to control the pv pump :http://static.schneider-electric.us/docs/Machine%20Control/Relays%20and%20Timers/Relays-General%20Purpose/Relays-Miniature/8501CT0301R708%20Relays.pdf.

    That way the PV pump can run on a differential like an AC pump would. I thought about this a few times because I noticed the pv pump would start early and run late causing a cool down in the storage tank.
  • Larry Weingarten
    Larry Weingarten Member Posts: 2,966
    I'd like to second...

    ... what Hot Rod said   "A lot depends on the collector to tank capacity ratio, and how much DHW the home uses everyday."  To me, a single 4 by 8 foot collector is undersized for an 80 gallon tank unless you're in someplace hot like Phoenix.  This suggests the owners don't use much hot water.  A non technical fix would be to tarp off about half of the collector in summer.  I'd start though be getting a better idea of how much hot water is really used and how much time is spent away from home.

    Yours,  Larry
  • Kevin_in_Denver_2
    Kevin_in_Denver_2 Member Posts: 588
    12VDC Differential controller

    This one was developed for this application:

    Superinsulated Passive solar house, Buderus in floor backup heat by Mark Eatherton, 3KW grid-tied PV system, various solar thermal experiments
  • SWEI
    SWEI Member Posts: 7,356
    Two snap switches

    One on the collector, closes @80F or so.

    One on the tank, opens @160F or so.

    Wire in series with pump and panel.
This discussion has been closed.