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add extra storage tanks on a existing solar water heater system

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  • #16
    Misread original post first time. Thought it was not reaching original temperatures, now understand that system is producing extra BTU?. Wanted to look at diagrams to see how extra is stored. Don't want high temperatures for the house water.

    Thinking after the first storage tank then one additional tank could be used to store higher temperatures then have the controller transfer the heat to the main tank when called for? Have to look at sketches... Still real new to this and I notice controllers with multiple inputs and outputs.

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    • #17
      Originally posted by SoCalDon View Post
      Misread original post first time. Thought it was not reaching original temperatures, now understand that system is producing extra BTU?. Wanted to look at diagrams to see how extra is stored. Don't want high temperatures for the house water.

      Thinking after the first storage tank then one additional tank could be used to store higher temperatures then have the controller transfer the heat to the main tank when called for? Have to look at sketches... Still real new to this and I notice controllers with multiple inputs and outputs.
      One solution is to use what is called a tempering valve at the output of the solar-heated tank. It mixes cold water into the output flow as needed to keep the outlet temperature below the set maximum. This does not waste much heat.

      These valves generally do not work well with very low flows, and so running the output of one tank through a tempering valve into a second, maybe smaller, tank with its own thermostat would be a good idea if there are kids around who might be injured by hot water.
      Using a separately controlled heat exchange loop from one tank to the other adds a lot of complexity and does not seem like a particularly good idea to me at first glance.
      SunnyBoy 3000 US, 18 BP Solar 175B panels.

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      • #18
        Thank You, yes now with access to the many resources on this site I have found diagrams with the tempering valves. Here in CA everything is legislated and we have been required for years to install scald valves in showers and baths... But yes I wouldn't want anything higher than planned coming out of any device.

        Been taking a crash course on the thermal solar. I have a client I am adding 7KW to a 6KW PV system I installed last year, on a slope 200' behind his home. While there his Gas Control Valve on his 75gal LPG water heater failed (water heater has been giving him different problems for a while). The home is only 7years old or so and very large.

        From the many options such as solar thermal, electric WH and tankless.

        I recommended to him that we install a LPG condensing tankless water heater with integral circulating pump and tiny buffer tank. Then we can latter install a storage tank in the same closet where the existing WH is when he is ready and send preheated water to the tankless water heater which would then bring it up to final temperature if needed.

        I also desire to recheck his circulating hot water pipes in the attic to verify if they are insulated, I do not recall seeing it last year when I was in the attic installing ceiling fans.

        After the new PV section is up, I believe he should replace his compressors with heat pumps to transfer the LPG load to the PV system.

        Next would be installing Solar Thermal for his pool. I believe he will get a much higher return on investment then the home. The home should save 40% from the new condensing tankless and may save more if the pipes are not insulated in the attic etc... Additionally the Pools control system appears ready for solar, and is located close to the slope we have the PV array on.

        Installing Thermal for the home would be most costly (longest return on investment) I believe with the 2 story tile roof. The attic is very large and would be easy to place a drainback tank. But cost would rise cutting walls open to run circulating lines from the attic to the water heater closet.

        These are just my thoughts to this point.

        Thank You Again for your help.
        Don

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        • #19
          Originally posted by SoCalDon View Post

          I recommended to him that we install a LPG condensing tankless water heater with integral circulating pump and tiny buffer tank. Then we can latter install a storage tank in the same closet where the existing WH is when he is ready and send preheated water to the tankless water heater which would then bring it up to final temperature if needed.
          Danger, Will Robinson!

          A gas powered tankless will have a certain minimum heat output that the flame can be throttled back to and this will not be able to take advantage of actually cycling the flame on and off the way a tank heater could do.
          That minimum heat output translates to a minimum product of flow rate and temperature increase of the water going through the tankless.
          As the input water gets hotter, as from a preheat tank, the minimum hot water flow that will allow the tankless to operate will increase, possibly to the point where the flow rate of a sink or shower will not be high enough.

          It may be possible to some degree of preheat (such as from 40F to 90F since the latter could be a credible temperature for normal water input) but be sure to get the correct applicable numbers or graphs from the maker of the tankless heaters you are considering. And your plan for a circulating pump and buffer tank may well work if the buffer tank is large enough. But rapidly cycling the flame of a condensing heater does not strike me as a viable solution either.
          SunnyBoy 3000 US, 18 BP Solar 175B panels.

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          • #20
            Thank You Again.

            I was able to get his existing WH to function for the time being, and switched on his WH in a guesthouse so that they can use that shower. So we have a little more time to consider options.

            The smaller tankless model had a minimum of 15kbtu while the two larger were 19k to 190kbtu. He was interested in an electric tankless unit or tank. And now with the 2nd PV unit he should be in the lowest tier rate or negative. I had sent him some literature on solar tank options. He plans on a 2nd electric vehicle and if we switch to heat pumps I do not know where he will be in total use.

            This tankless heater has the circulating pump and small buffer tank built into the unit. Since it is a large house there is a large reserve also in the piping. I have not used this brand before, the one I wired for another client had different settings on how often and how much it would heat the circulated water.

            He will have to be made aware of the dead zone between minimum turn on and set point for a gas tankless system at low flow rates. I would presume the water temperature at a sink or faucet would be adequate, especially since there is a large reserve in the circulating system which at a low flow rate would mean that the water just leaving the tankless may not even be used at that instance. If the water is hot enough not to trip the heater it should be hot enough for use, but that of course is a matter of personal choice like you had stated.

            Even though it is a large house, right now just the two of them living there so I felt the smaller tankless would be sufficient if he planned on adding Solar, and have an advantage with the lower minimum btu.

            As I work on his system, I am working on mine also. Easier install for me the way I built my house. But I have to do a cost analysis based on lower natural gas and electric rates vs his higher Electric and LPG rates.

            Seems the drainback design is popular? It seems the most appealing to me for a roof mount system. My preffered choice would be to avoid his roof and place panels on his slope next to his PV system. Has a great southern exposure and almost optimal inclination.

            Running it 200' with a 30' drop is a whole different project though to design.

            Have other clients to take care of this weekend, but will continue to research and meet up with him to compare notes next week, while I set his PV panels. Will also check his piping for insulation, my goal is to save his money. If he and I can work up a system that is economical, reliable and durable then I believe I will be installing it for him in the near future.

            Cheers,
            Don

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            • #21
              Originally posted by inetdog View Post
              It may be possible to some degree of preheat (such as from 40F to 90F since the latter could be a credible temperature for normal water input) but be sure to get the correct applicable numbers or graphs from the maker of the tankless heaters you are considering. And your plan for a circulating pump and buffer tank may well work if the buffer tank is large enough. But rapidly cycling the flame of a condensing heater does not strike me as a viable solution either.
              We had a geothermal heat pump installed in our house about five years ago, and as part of that installation it takes waste heat from the heat pump and uses it to preheat domestic hot water in a buffering tank which feeds into a tankless gas water heater. So not so different from this installation.

              Your mileage may vary depending on the manufacturer, but here's how our system works:

              a) The buffer tank temperature can vary between 40F (when we're not running either heat or a/c) and 130F (when heat or a/c are running flat-out).

              b) We have the hot water heater set to 120F. That means that water going into the hot water heater can range from cold to over-heated.

              c) In our system, if the incoming water is below 120F and the flow rate is above the set point for the water heater (0.5GPH) it will always turn on. Sometimes this means that we get overheated hot water.

              d) If the incoming water is at or above 120F, the hot water heater doesn't turn on.

              We've noticed that the buffer tank will cool over the course of several minutes running hot water, so sometimes the temperature of the hot water can vary quite a bit during the course of a single shower. The more hot water which is being used at once, the less overheating we get, so having two people take a shower at once actually helps stabilize the hot water temperature.

              The flips side of this is that we do save a lot on the hot water bill. Some months, the gas water heater hardly runs at all.
              16x TenK 410W modules + 14x TenK 500W inverters

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