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  • Thermosiphon Help

    Hey I could use some advice
    I have a small solar water heating system I build myself this fall, to heat my trailer (and hopefully DHW come the summer). but the loop that bring the heat from the storage tank to heat the trailer, isn't. I am trying to use thermosiphonic action to circulate the water in this loop. but no heat seems to be being transferred. I attached a simple diagram of it. my storage tank is a 55 gal. drum, the lines are 1/2 CPVC, water-to-air heat exchange is a radiator from a 90s Honda Civic. my storage tank as be running 80-90*F but at the exchange it is only 50-60*F (room temp). so it seems that it is not thermosiphoning. does it matter that the tank waters surface is below the heat exchange? what did I do wrong?

    Thanks!

    solar water heater 2.jpg

  • #2
    The hot water has to be below where you want to heat.

    The tube size has to be large enough to have little resistance for enough flow to be usable.

    I have two loops working fine in my house, so it really does work, with the proper conditions.
    Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister

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    • #3
      How are you priming the system?

      The force of hot water rising is greater than the force of cold water sinking. There is nothing to help the hot water rise to your radiator so they must be on the same level or radiator lower. Then the outlet from the radiator would also need to run down hill or be on the same level as bottom of the tank. this gives one a very small window of elevation to install everything. By looking at your diagram you don't appear to be in that window.

      WWW

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      • #4
        Originally posted by Wy_White_Wolf View Post
        How are you priming the system?

        The force of hot water rising is greater than the force of cold water sinking. There is nothing to help the hot water rise to your radiator so they must be on the same level or radiator lower. Then the outlet from the radiator would also need to run down hill or be on the same level as bottom of the tank. this gives one a very small window of elevation to install everything. By looking at your diagram you don't appear to be in that window.

        WWW
        So if I were to pump the hot water through it at first, it might then continue on its? Or do you think that my system is fatally faulted?

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        • #5
          Originally posted by Mike90250 View Post
          The hot water has to be below where you want to heat.

          The tube size has to be large enough to have little resistance for enough flow to be usable.

          I have two loops working fine in my house, so it really does work, with the proper conditions.
          my hot water is below, I wondering if 1/2" might be to small even for a small system. do you know what size tubing your system uses?

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          • #6
            Originally posted by donjeare View Post
            my hot water is below, I wondering if 1/2" might be to small even for a small system. do you know what size tubing your system uses?
            Use the size that is on the radiator (1 1/4")? Use a minimum amount of elbows, and use long radius sweeps. Connect the supply to the top of the radiator not the bottom and or insulate the supply up to the radiator.

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            • #7
              Your thermosiphon may be working but it probably isn't producing a very large flowrate. Without knowing more about the particulars, I'd GUESS that you may be circulating 10-12 gal./hr., say ~ 100 lb./hr. or so. So, say you get a 20 deg. F. water temp. drop through the radiator. That's 2,000 BTU/hr. or so. Not a whole lot in the heat transfer business. Part of what may be going on is that it may be working just fine, or as well as can be expected, but as well as can be expected may be so low that it isn't up to expectations.

              I'd agree with Lucman's suggestions, but even if you double the flowrate by decreasing the flow resistance, the heat output may not increase all that much. I'd also throw some fiberglass insulation around the barrel and put a couple of inches of rigid foam insulation under and over it as much as possible..

              One way to perhaps get a SWAG measure on things: Measure the barrel temp. change over say, 10 hours when warm, with no input (after sundown) and the thermosiphon shut off. Do the same under similar conditions with the thermosiphon on and measure the temp. diff. as before. The diff. in the (temp. differences) X (the barrel's water thermal capacity) will be a real rough approx. of the thermosiphon's heat transfer.

              So: A 55 gal. drum has about 450 # of H2O = thermal capacity of ~ 450 BTU.

              Say over 10 hrs., with no flow, the water temp drops from 100 to 90 deg. F. so the loss = (450) X (100-90) = 4,500 BTU.

              Now, repeat the experiment under as close to the same conditions as possible with the thermosiphon running and again, no solar input.

              Say the following is measured on the repeat: Water temp. starts at 100 deg. F. 10 hrs. later water temp. = say, 85 F. --->>> temp. diff = (100-85) = 15 def. F., --->> the water lost (450) X (15) = 6,750 BTU.

              Sooo: The diff., 6,750 - 4500 = 2,250 BTU is an approximation of what the thermosiphon removed.

              The #'s are examples only, but the method works (I've done it, or at least something quite similar) to a first approximation depending on particulars. Insulating the tank before you do such a measurement will probably reduce the error.

              If the barrel temp. diff., start to finish (that 15 minus 10 deg. diff. = 5 deg. in the example above), one trial to the next is zero (or less), the thing probably isn't working. I kinda' suspect what's going on is that the thing is working as well as can be expected, but that ain't much, and therefore can't be measured or felt to any great degree.

              Your brain is working just fine on the concept. Mean old Mother Nature got in the way with a lot of details. Been there, done that more times than I can remember. Lots of fun.

              If you want to get a bit fancy, measure the temp. at various places along the thermosiphon path and, with some number manipulation, get a guess at flowrate, or measure the air temp. diff. through the radiator. That last one will take some additional measurement (SWAG) of the air flow rate through radiator however, and such measurement are hard to get accurately without a little more instrumentation.

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              • #8
                I'm using 3/4 " stainless in my fireplace loop, gets my 80 gallon tank over 100f rather quickly if I don't use hot water.

                My generator uses 1" for it's coolant tank.

                Thermosiphon forces are really low, it's the weight difference between hot water and cold water, that makes it work. There is VERY little difference, so even a little bit of resistance will impede the operation.

                The rising run cannot have any level spots or any dips in it, same for the return, it must be a continual slant or vertical - any horizontal or loops will STOP the flow dead.
                Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
                || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
                || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

                solar: http://tinyurl.com/LMR-Solar
                gen: http://tinyurl.com/LMR-Lister

                Comment


                • #9
                  Originally posted by Mike90250 View Post
                  I'm using 3/4 " stainless in my fireplace loop, gets my 80 gallon tank over 100f rather quickly if I don't use hot water.

                  My generator uses 1" for it's coolant tank.

                  Thermosiphon forces are really low, it's the weight difference between hot water and cold water, that makes it work. There is VERY little difference, so even a little bit of resistance will impede the operation.

                  The rising run cannot have any level spots or any dips in it, same for the return, it must be a continual slant or vertical - any horizontal or loops will STOP the flow dead.
                  I believe you. At least some of that may be due to a relatively high temp. diff. involved in a combustion process. Also, there's the matter of your surface area, and also usually low fin efficiencies in radiators tike the OP is using.

                  Also, good point on the piping slope. I left that one out. Haste makes waste and omissions.

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                  • #10
                    A closed pressurized system seems to help also with the Thermosiphon.

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                    • #11
                      thanks for all the good info, sounds like the best fix may be to put a small pump on it. it seems I can not get good thermosihponing going without rework/placing a good portion of the loop.

                      I do have the storage container insulated with about 4" fiberglass insulation board inside a alum. skin and standard foam pipe insulation on the lines.

                      Comment


                      • #12
                        Originally posted by donjeare View Post
                        thanks for all the good info, sounds like the best fix may be to put a small pump on it. it seems I can not get good thermosihponing going without rework/placing a good portion of the loop.

                        I do have the storage container insulated with about 4" fiberglass insulation board inside a alum. skin and standard foam pipe insulation on the lines.
                        For anything you think I may have puked out in the way of info, you're most welcome. A small (fractional HP) pump will increase the flow quite a bit. Taco, Grundfos, lots of other names are available.

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