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Variable speed pump options

Fortunat Member Posts: 103
We've started to do variable speed pumping on most of our solar thermal systems. Though I don't yet have any really good data to support it, I'm convinced anecdotaly that the benefits as far as solar yield goes are significant. I also know that the parasitic (electrical) energy savings can be significant too.

On larger systems we've sometimes used high efficiency ECM pumps (like the Wilo Stratos or Grundfos Alpha or Magna pumps) controlled by a 0-10V control level signal.

On small systems and some other projects we use conventional pumps (Taco 00xx, Wilo or equivalent) but control them at variable speed with, so called, triac control, or basically just a voltage chopping semiconductor relay (for the little ones you can do it straight from most new RESOL controllers, the larger need an intermediate relay to handle the current above 1 A).

Here is my question: Assuming both types of pumps do an equally good job of improving the solar yield and both save some electrical energy compared to fixed speed pumping, how much additional energy savings can I expect from the ECM pumps compared to running conventional pumps at variable speed ? Any other benefits?

What are others doing for small to mid sized commercial solar systems (500-1000 sq ft)?

The ECM pumps are pretty nice, but they are spendy and even more so once you need to buy additional adapters to control their speed externally and rub a separate power and control wire to each pump.

I'm interested to hear the opinion and experience of other professionals on this topic.


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  • hot_rod
    hot_rod Member Posts: 15,410
    how much pump

    do you want to control from a current standpoint? Resol is now building their own triac and it is a 2 amp relay. That switches most high head circs like Grundfos 26-99, Wilo Star 32 and any other circ under 2 amp current draw.

    There is an issue with the listing, however. Currently the control is listed for 1 amp, even though the new relay has been added. The control will need to go back through the listing agencies to get the higher rating.

    with the "green" interface from Resol, or Caleffi :) any size pump can be controlled IF it can accept a 0-10VDC. But that sometimes takes an additional interface from the pump manufacturer.

    We need the variable speed ECM circs from Europe that would wire directly to the PWM output on those controls. Keep leaning on the pump manufacturers for those offerings.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Fortunat
    Fortunat Member Posts: 103
    in the meantime


    Absolutely we need those pumps.  On the commercial side we have them already (the Wilo Stratos takes a 0-10V signal for speed control if you add a little card to it).  But for residential and light commercial, still do options.

    That is interesting about the beefed up relay in the RESOL. It seems we always get the RESOL dregs no this side of the pond.

    In the meantime when we have a larger pump to control (that doesn't take a 0-10V speed control signal) we've just been using the relay in the controller to switch an external solid state relay, which in turn switches the pump. It isn't super elegant, but it works fine and gets the job done.

  • NRT_Rob
    NRT_Rob Member Posts: 1,013
    seems to me

    the ECM circs use half the energy of a conventional circ for the same flow.

    So the real question is, how much flow do you really need? If you're doing dT control that could be a simple heat gain averaging calculation for an average amount of circulation you need, and then you can calculate savings from there. That assumes you can control both equally... for residential systems I have no idea how you'd control an ECM pump at variable speed with external control unless 0-10vdc works? does that work on ECM?

    it seems like slowing down pump speed can only decrease solar yield though (higher average temp across the panels). So how can this help, other than if you are trying to harvest a particular temperature water, or ONLY to reduce parasitic loss? I don't see any other benefits...?
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
  • Fortunat
    Fortunat Member Posts: 103
    ECM pumps

    Half the energy for the same flow...that is pretty good. If that is true, then I should be able to back out the expected savings over the course of a year easily enough.

    And Yes, the big ECM's do take a 0-10V signal for speed control. Or atleast the Wilo Stratos does, which is what we've been using for medium sized commercial jobs (500-1000 sq ft of collector)

    And yes, you are basically right that overpumping a solar system actually only helps solar yield (by keeping collectors cool)...that is until you cool the collector so much that the dT controller shuts the pump back off. Which happens a lot.

    What we've found is that systems with single speed pumping where flow rates are properly set to provide a 20-25 degree dT at rated conditions (1000W/m^2) end up cycling a lot both at the beginning and end of a day when the radiation is lower, but also anytime the tank is already hot and so the collector yield is reduced. Variable speed pumping helps.

    First thing in the morning, rather than having the pump run for 30 seconds, then shut off for 2 minutes and repeat, it can start up slowly and ramp up as the sun comes up. If pipe insulation and collector sensor placement were perfect, the two would pretty much result in the same total BTU's eventually coming back into the tank, but since they aren't the variable speed pumping seems to improve yield in these conditions.

    I've read that the yield improvement is 10%, and that feels like the right order of magnitude but I have yet to verify that myself. I'm working to get a pyranometer on our roof so I can switch our shop system back and forth from variable speed to single speed and collect enough data to tell me if it makes a difference.

    The newest generation of RESOL controllers all pretty much have variable speed pumping (for conventional pumps) built in for free, so at this point I can't see any reason not to do it.

    Oh and of course, there is the electrical savings...which is obviously important too,a s you know.


  • NRT_Rob
    NRT_Rob Member Posts: 1,013
    good stuff

    thanks for sharing Fortunat.

    I know the bigger ECMs are built to take external signals. Stupid commercial grade equipment always gets all the cool stuff. but I don't think you can just throw an ECM residential sized circulator on a 0-10. though maybe the alpha on a fixed speed mode....
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
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