4 Responses to “Variable Speed Pumps”


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  1. Some comments on this as it relates to residential systems (20sq. ft. to 200sq.ft.) :

    1. Sizing the return line the same size as the supply will almost always establish a siphon. One way to guarantee an open drop return would be to have a small hole on the return pipe that would suck air in. I’ve never heard of autoflow valves being used in residential.

    2. Practitioners in residential solar almost always use a siphon return and don’t ever see those problems.

    3. If you use two pumps in series to establish the siphon, and then turn one off, it will double the COP of the system. This can be very significant.

    • Dr. Ben


      I am not sure I agree with your first & second statements. In the article I note that establishing a syphon return can be problematical. Sometimes it works and sometimes it doesn’t. It has to do with temperature and density of water, the flow rate, the number of elbows, and the length of horizontal vs vertical piping. In other words, the system geometry and thermal conditions all play a role. A syphon return may be established in the morning and not in the afternoon. For example, a 1 story high installation might work all the time, but a three story installation where the pipe runs 60 ft (almost) horizontally wouldn’t.

      We tried smaller returns than the supply to make sure it was full of water. Sometimes they would air lock. When the supply and return are the same size we got variable results. Making the return bigger than the supply would probably work all the time, but the cost of materials make this option unattractive.

      I am not in favor of drilling holes in the return pipe. We don’t want fresh oxygen coming into the system and accelerating the rust/oxydation of the tank. Invariably, a hornet will coat the think with goo, ruining the operation of it. In fact, we don’t want any holes or devices anywhere in the line. except flow control valves.

      It is good that your systems have worked well, but for all the climates, pump heads, piping runs possible across the nation, I don’t think you can get the same results everywhere. Since my systems are identical from the smallest to the biggest, installed anywhere, I prefer one solution for them all. So far, the results have been good.

      Autoflow valves are used only when there is more than one row of collectors that drains independently. On our small systems, we put a flow indicator/control valve right at the pump. On larger systems, needing two or more flow control valves, we have found the autoflow valves eliminate the manual balancing required. Manual valves are just as good, you just have to set them manually. This can be a real challenge if you have three rows, or 5 as in some systems. The autoflow valves really pay for themselves when you get above two rows.

      We finally settled on same size supply and return and setting the flow rate when the system was in steady state condition. This will take into account either open drop or syphon return. Throttling a pump down unloads it somewhat, reducing the power.

      Using two pumps in series then turning one off means you could have a stall condition if the syphon return breaks and you have to pump the whole head with one pump. If you want to see a steam generator, just try stalling the water in the middle of the collectors on a sunny day. If you actually have a syphon return in a one pump system, the pump power will go down when the load goes down. That is why we set the flow rate when the system is running in steady state. A single pump of the right head will always pump water over the top, just at different flow rates.

      You are right that the COP of a system is important. That is why we carefully select the collector pumps for a system. We reviewed a diagram submitted by an engineer that used a 1.5hp pump for the collector array. We determined that a pump with 1 hp would do the same job, saving up front cost and running cost.

      You note refers to residential systems only. I am referring to all systems from 40sf to 18,000 SF. While we distinguish between residential and commercial systems, the only real difference is who the owner is. The equipment is frequently the same. Our small systems go in residential and commercial installations alike. I have 400 ft2 systems on residences, 120ft2 systems on car washes, and 40ft2 systems on a business office. The big ones may go on an apartment complex, so they are still basically residential. This is why we have developed one system design that we stick with.

      Thanks for the questions.

      Ben Gravely

  2. I’m sure I haven’t done as much experimenting as you have. I’ve done a lot, but only on one-row systems.

    Did you ever try putting the flow control valve on the return side?

    If the maximum pump head is at least 2-3ft more than the system height, you would find:

    1. Siphon is always established.
    2. Siphon is never lost.
    3. Pump power is optimized.
    4. It never airlocks.
    5. Differing supply and return pipe sizes don’t matter as long as velocity is below the acceptable maximum.

  3. Dr. Ben


    Items one to three are discussed in detail above. Our experience is different from what you describe on systems ranging from the hot south to the cold north. Larger return lines will almost always result in an “open drop” (no thermosyphon) system. After a couple of thousand installations, (about 50/50 res/comm) we got rid of most of the bugs.

    We don’t recommend flow control valves on the return side of drainback systems. There is the potential for the valve to create an air bubble at the high point of the return line above the valve. This could vapor lock the system and cause problems.

    When you put the control valve in the supply line, it sees a solid slug of water go through, purging out all the air, so there is no potential for an air bubble or vapor lock. So, there are no potential problems with the valve on the supply side, but potential problems on the return side. I would rather be safe.

    I know it is customary in the HVAC industry to put control valves on the output side of exchangers, which may be the hot side for a boiler exchanger, or the cold side for space heating fin coils. On a solar system, the supply side is always the cold side. Boiler systems are always pressurized, usually with air scoops/air traps, so there is no danger of air bubbles and it probably doesn’t matter where the valve is placed.

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