System not running as designed

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  • Rahm
    Junior Member
    • Jun 2013
    • 2
    #1

    System not running as designed

    I had a new solar hot water system installed a couple of weeks ago. The system was designed as a closed loop with a heat exchanger in a storage tank. The hot water heater was supposed to draw from the storage tank and the storage tank was supposed to draw from the well (pressure tank). However, I noticed that the temperature in the hot water heater was dropping significantly lower than the temperature in the storage tank immediately after using the hot water. This should be impossible if the hot water heater is drawing directly from the storage tank. I asked the installer who said:

    "As I was installing the system, I discovered some design limitations that I thought I could overcome. I was unable to both; have the cold water feed the solar tank first, thereby automatically sending preheated water to the electric tank; and have the circulating pump work effectively. The limitations were a result of the lack of useful ports on the electric tank. I, without hesitation, opted to install it with the circulation system instead of feeding cold to the solar tank. The reason being that the system will always try to send hot water from the solar tank to the electric tank whether you are using the hot water or not. So, the result is that when you use the hot water, cold water is entering the electric tank, but as soon as the electric tank temperature starts dropping, the pump will turn on sending hot water from the solar tank to the electric tank, and taking the cold water that just entered the electric tank and sending it to the solar tank. This pump will continue to run until the electric tank is within a couple of degrees of the solar tank, vs. the transfer of heat halting once the hot water is shut off if we were to have piped the cold to the solar tank and not used a circulation pump between the two tanks."

    The installer claims that the system should run with similar efficiency in the current configuration as it would have if it had been installed as designed. I'm not sure I believe him. It seems to me that having cold water enter the hot water heater will make it turn on while pre-heated water, if hot enough, from the storage tank would not. Maybe, in certain situations, it wouldn't make a difference but I think that there are a lot of scenarios where it will make a difference.

    I'm not sure what my options are here, or if the installer really only had the two choices, or if he made the correct choice. I'm hoping that those of you with more knowledge about these systems than I have can weigh in with your thoughts.

    Many thanks.
    Tags: None
  • inetdog
    Super Moderator
    • May 2012
    • 9909
    #2
    Originally posted by Rahm
    I'm not sure what my options are here, or if the installer really only had the two choices, or if he made the correct choice. I'm hoping that those of you with more knowledge about these systems than I have can weigh in with your thoughts.

    Many thanks.
    If you search the solar hot water section of the Forum, you will find instructions that allow you to take a perfectly standard electric hot water heater, with only a top outlet and inlet and a bottom drain fitting and construct a tank which introduces cold water at the bottom and draws water from the bottom to go to the solar heater, introduces solar heated water at the middle, and withdraws water for use from the top. All it takes is a modification to the length of the original cold water dip tube and a the addition of a tee fitting to the drain valve. This could be done with an externally mounted heat exchanger instead of a coil inside the storage tank.
    There are also closed loop systems with heat exchangers that are designed to fit into the tank entirely through the original pressure-temperature relief valve opening.
    I find it hard to believe that your installer was not able to come up with any other solution. (He may have been limited by selecting the wrong components for the system to start with, but that should not be your problem!)

    One definite problem with the installation you ended up with is that the electric heater will always turn on at least for awhile every time cold water is introduced into the system. As designed that would have happened only if the solar heated water was not hot enough or was dropped down in temperature by heavy water use.
    The "advantage" of having both tanks at the same temperature makes sense only if you are making heavy use of water at night and do not have enough stored hot water in one tank to carry you through till day.
    SunnyBoy 3000 US, 18 BP Solar 175B panels.

    Comment

    • Naptown
      Solar Fanatic
      • Feb 2011
      • 6880
      #3
      Originally posted by Rahm
      I had a new solar hot water system installed a couple of weeks ago. The system was designed as a closed loop with a heat exchanger in a storage tank. The hot water heater was supposed to draw from the storage tank and the storage tank was supposed to draw from the well (pressure tank). However, I noticed that the temperature in the hot water heater was dropping significantly lower than the temperature in the storage tank immediately after using the hot water. This should be impossible if the hot water heater is drawing directly from the storage tank. I asked the installer who said:

      "As I was installing the system, I discovered some design limitations that I thought I could overcome. I was unable to both; have the cold water feed the solar tank first, thereby automatically sending preheated water to the electric tank; and have the circulating pump work effectively. The limitations were a result of the lack of useful ports on the electric tank. I, without hesitation, opted to install it with the circulation system instead of feeding cold to the solar tank. The reason being that the system will always try to send hot water from the solar tank to the electric tank whether you are using the hot water or not. So, the result is that when you use the hot water, cold water is entering the electric tank, but as soon as the electric tank temperature starts dropping, the pump will turn on sending hot water from the solar tank to the electric tank, and taking the cold water that just entered the electric tank and sending it to the solar tank. This pump will continue to run until the electric tank is within a couple of degrees of the solar tank, vs. the transfer of heat halting once the hot water is shut off if we were to have piped the cold to the solar tank and not used a circulation pump between the two tanks."

      The installer claims that the system should run with similar efficiency in the current configuration as it would have if it had been installed as designed. I'm not sure I believe him. It seems to me that having cold water enter the hot water heater will make it turn on while pre-heated water, if hot enough, from the storage tank would not. Maybe, in certain situations, it wouldn't make a difference but I think that there are a lot of scenarios where it will make a difference.

      I'm not sure what my options are here, or if the installer really only had the two choices, or if he made the correct choice. I'm hoping that those of you with more knowledge about these systems than I have can weigh in with your thoughts.

      Many thanks.
      That is probably the most ridiculous set up I have ever heard of.
      Keep it simple put the tanks in series and perhaps introduce a bypass for the summer when solar production is high allowing you to bypass and turn off the electric tank.
      anytime you start pumping between a solar storage and back-up tank you are stirring up both which is not a good thing to do.
      NABCEP certified Technical Sales Professional

      [URL="http://www.solarpaneltalk.com/showthread.php?5334-Solar-Off-Grid-Battery-Design"]http://www.solarpaneltalk.com/showth...Battery-Design[/URL]

      [URL]http://www.calculator.net/voltage-drop-calculator.html[/URL] (Voltage drop Calculator among others)

      [URL="http://www.gaisma.com"]www.gaisma.com[/URL]

      Comment

      • peakbagger
        Solar Fanatic
        • Jun 2010
        • 1566
        #4
        I run mine in series in the winter, with the SHW feeding my hot water heater and in the summer I bypass the hot water heater and run direct off the solar. No need for a circulator on the potable side. It does add some pipe and valves but any competent plumber should be able to rig it up that way.

        Comment

        • inetdog
          Super Moderator
          • May 2012
          • 9909
          #5
          Originally posted by peakbagger
          No need for a circulator on the potable side.
          I take it you are using a heat exchange coil in the solar tank rather than an external one?
          SunnyBoy 3000 US, 18 BP Solar 175B panels.

          Comment

          • peakbagger
            Solar Fanatic
            • Jun 2010
            • 1566
            #6
            I have one of the Rheem tanks with the copper coil wrapped around the tank and then insulated, so it looks like a hot water heater with extra connections on the side. Keeps things real simple, one DC pump for the glycol and a PV to drive it. I have DC differential controller from the guy in maine (artech?). It has a backup single electric element but I have never hooked it up.

            I check the anode every two years and drain about once a year as I really don't want to buy another one as they are pricey.

            Comment

            • LucMan
              Solar Fanatic
              • Jul 2010
              • 628
              #7
              No need for a pump between the two tanks, cold water goes to the bottom of the solar tank, HW outlet of ST goes goes to cold water inlet of electric tank, HW outlet of electric tank to fixtures.
              If you have sufficient solar capacity you could use the pump to circulate between the two tanks only when the solar tank is above 130 degrees, this would require a separate differential controller or one that can sense the temperature of both tanks and some repiping. This option most likely will be outside of capabilities of your installer.

              Comment

              • Rahm
                Junior Member
                • Jun 2013
                • 2
                #8
                Thank you all for your responses. I'm trying to get the installer to make alterations to the system. At a minimum I'll get him to change it so that the solar tank feeds the electric tank but, if possible, I'd also like the two tanks to circulate when the solar tank is hot enough. I'll post again after I've worked it out with the installer. Thanks again.

                Comment

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