2 Responses to “Evaluating Hot Water Meter Data from The Healing Place”


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  1. My team is in the process of commissioning a 1410 Vac tube closed loop drain back system.
    I bring this to you for three key parts.
    They may be of value to your Sept.9 commercial program.
    (1) Sizing large Drain back piping with moultable arrays.
    (2) Using variable speed pumping (and keeping it balanced flow)
    (3) Vac tubes OK to use in Drain back?

    I am honored to be the Lead Solar Thermal Consultant on a job for the “Bill and Melinda Gates Foundation”. Its the new World Headquaters in downtown Seattle,Wa.
    I would like to share this with your program.
    Let me know
    Tim connolly, AAS Solar Thermal, NABCEP Cert

    • Dr. Ben

      Looks like you have a nice job on your hands. Let me see if I can answer your questions.

      1) Sizing drain back piping. The simple answer is to size the piping for the flow rate exactly as you would any hydronic system. Pick a reasonable velocity, such as 5 fps and size the piping according to the flow rate through each branch. The return lines should follow the same patterns as the supply lines. People tried and failed to create syphon return by making the returns smaller. The trouble was, some systems worked and others did not. The variables are too great to get any certainty of syphoning in the designs. We finally dropped any effort to get syphon return and just made the piping symmetrical.

      2) I don’t have any experience with variable speed collector pumps on commercial size drain backs systems. The theory of variable speed pumps is that efficiency can be increased overall by maintaining a constant delta T across the collector array. However, I haven’t run across any commercial size solar pumps that have variable speed controls. There are a lot of rules about the proper use of variable speed pumping in hydronic systems. It can be done wrong, causing a loss of efficiency and high power consumption. Since most solar systems are ignored after installation, I tend to think simpler is better. However, I am always looking for studies on this subject.

      3) Balancing the flow through different branch circuits is another separate area of design. On large arrays, we want each collector to have the same flow rate through it. Actually, we want each collector to have the same delta T across it. We could always take an infrared camera and look at an array to see the consistency. Maybe some day I can do that.

      In the mean time, balancing complicated arrays with manual valves can be exhausting. In the old days, we were very careful to pipe the array symmetrically and let the flow do what it will. We are still careful about symmetry, but now there are autoflow valves with spring loaded pistons that will automatically maintain a constant (+/-5%) flow through a row of collectors. They are bought with a specific flow set point, and cannot be changed in the field, except for swapping out the flow cartridge.

      4) The use of evacuated tube collectors in drain back systems is entirely under the specification of the manufacturer. There is no general rule. If the manufacturer approves drainback operation, then it is acceptable. The piping of arrays follows the same pattern as flat plate collectors, except the supply and return are at the same level, so a little adaptation would be required.

      I hope this helps answer your questions.

      Dr. Ben

      PS – Let me know if you’re interested in integrating a Fluid Handling System into the project. I’d be more than happy to provide a quote.

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