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Using variable speed pumps with a geothermal

  • 1.  Using variable speed pumps with a geothermal

    Posted 08-24-2016 16:13

    I am planning on using a Magna1 32-100 on the source side of a water-to-water unit, and a Magna1 32-60 on the load side. I won't be using a flow center on the source side, just the pump. One maker (Geo-Flo) of flow centers with Magna pumps insists the pump power can't be connected to the compressor side of the compressor contactor, and has to be connected to the line side (and then switched with a separate heavy-duty relay). Geo-Flo claims the inrush current pulled by the pump is too high and will cause premature contactor failure.

    Geo-Flo is the only manufacturer/3rd party I have seen talk about this issue. I asked Grundfos directly if wiring the pump to the compressor side of the contactor as is normally done would damage the contactor, and they avoided the question and recommended their add-on (and substantial added cost) pump control module for starting and stopping the pump.

    Has anyone used a Magna pump controlled solely by the compressor contactor? Have you had early contactor failure?

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    Jacquelyn Scherer
    Geo Designer
    The Janes Company
    Mukilteo, WA
    425-267-0202
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  • 2.  RE: Using variable speed pumps with a geothermal

    Posted 08-25-2016 08:20

    We simply use an accessory output (cycled with compressor) connected to the Magna3 start/stop input. That way you can keep the Magna3 permanently powered as intended.

    You can simply add a suitable relay wired into the compressor contactor to provide the voltage free start/stop output for the Magna3 if the GSHP does not provide a suitable voltage free accessory output.

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    Cameron Hutchison
    Geoexchange Australia Pty Ltd.



  • 3.  RE: Using variable speed pumps with a geothermal

    Posted 08-25-2016 08:53

    Just curious, why are you using a variable speed drive on the source side?  Is this a two-speed heat water-to-water heat pump?

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    Lisa Meline
    Meline Engineering Corp.
    Sacramento CA



  • 4.  RE: Using variable speed pumps with a geothermal

    Posted 08-25-2016 08:56

    The Manga style pump uses an ECM type pump motor.  It is like an ECM fan motor that we power up all the time on line voltage side and give it a low voltage signal to run at a certain CFM.  This also holds true for the ECM motor in the Magna series pump.  The controller will allow you to program the desired GPM..  Otherwise the pump would run full capacity and you might over pump the system.  I am told Grundfos does not like to see valves in the piping to restrict the flow and you will have better savings on the pumping watts. 

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    Richard Hiles
    Geothermal Sales Manager
    Carrier Corporation
    Anderson IN
    46013



  • 5.  RE: Using variable speed pumps with a geothermal

    Posted 08-25-2016 10:01

    Hi Jackie,

    Geo-Flo's recommendation regarding wiring a variable speed pump is specific to the Magna GEO 32-140 pump and was based on recommendations from Grundfos, both published information and direct discussions with Grundfos engineering.  Further, we received reports of pitting on some contactors in field installations that were wired this way so the issue is specific to the rating of the contactor used.  Finally, the use of this Magna GEO 32-140 pump with our controls (UPC-GEO), and all geothermal heat pump manufacturers' controls requires constant pump power and modulation based on a low voltage PWM signal.

    Regarding the Magna1 and Magna3, I agree that the issue is less clear since Grundfos does not publish the in-rush current for these pump lines or provide specific direction regarding contactor ratings.   However, both the published Magna 1 and Magna 3 data books say the pump should be started via the power supply no more than four times per hour.  Note that powering this way does cause a delay in the pump’s response as the firmware loads.  As Cameron posted, the preferred method on the Magna3 is to use the start/stop function (contact closure) to cycle the pump, and this is what we recommend.  However, the Magna1 does not have this function.  Your options are to 1) use an external contactor and a control system that limits the start/stop via high power on the Magna1 as stated above 2) use a Magna3 were possible 3) plumb a small bypass that allows fluid flow at all times, and keep the pump powered constantly.

    I’ve requested more direction from Grundfos engineering on the inrush current and contactor rating, and will send you further information on this topic as I get it.  

    Thanks,

    Tom

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    Tom Wyer
    BSME/MSME
    President, Geo-Flo Products Corp.
    www.geo-flo.com



  • 6.  RE: Using variable speed pumps with a geothermal

    Posted 08-25-2016 11:41

    Thanks for all your replies. I'd been hoping to use a Magna 3 pump for the superior control options that several of you mentioned, but regrettably it seems the Magna 3 line has been discontinued. My local suppliers and several other online suppliers all told me the pumps were unavailable for order, and then Grundfos directly confirmed the discontinuation to me. So as far as Grundfos goes, my only reasonable option here is the Magna 1.

    The reason I'm using variable speed/ECM in the first place is to lower my pumping power, as the property constraints (and extremely high cost of vertical bores in my area; over $30 a foot!) are forcing an earth loop design that would require a Grundfos 26-116 or equivalent. There's a two year payback on the Magna 1 pumps in this system design. (Incidentally, I am using a two-stage water-to-water unit, but to my knowledge there's no cost-effective way to control the Magna 1 line at different speeds for the two stages.)

    The Magna 1 has a pump curve setting that closely matches my system flow and head loss (at 60% pump efficiency), so I shouldn't have any overpumping. I do have 2 valves in the piping on the load side, but they are high Cv (30+), so the added head loss is minimal.

    Thank you for the clarification on Geo-Flo's electrical design for the Magnas, Tom. I'm confident this system will almost never cycle more than 4 times an hour (pipe in a high mass pour, two-stage unit, correct ASHRAE unit sizing, no safety factors in the heat loss/heat gain calculations). Your mention of a delay in pump operation on startup does give me pause... if the delay is excessive I'll get low flow lockouts, so hopefully it's less than 30 seconds. Regarding the 3 options for the system, I'd rather not plumb a bypass, and the Magna 3 line isn't available. Your final option, if I'm reading it right, is simply to use a relay or contactor that can handle the inrush current, and ensure the pumps rarely (or is absolutely never the requirement here?) cycle more than 4 times an hour?

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    Jacquelyn Scherer
    Geo Designer
    The Janes Company
    Mukilteo, WA
    425-267-0202



  • 7.  RE: Using variable speed pumps with a geothermal

    Posted 08-26-2016 07:21

    Hi Jackie,

    The small (32 Series) Magna3s have not been completely discontinued, but you are correct that they cannot currently be ordered.  Grundfos placed a temporary manufacturing hold on these 32-XX pumps, but the other larger Magna3 pumps are available (40-, 65-, 100-, etc.).  We’ve heard that they will become available at some point in the near future.

    The Grundfos Magna GEO 32-140 pump coupled with a UPC-GEO controller allows two stage pump operation using either flow rate based control or delta-T control.  The control accepts 24VAC input for first and second stage calls from the heat pump, and outputs a PWM signal to modulate the pump (1-10V outputs are also possible, but the 32-140 accepts PWM).  The flow rate or delta-T inputs are user determined and input at setup/installation.  The controller allow shows the pump watts, delta-T, flow rate, and HE/HR.  I have this setup at my house on a 3-ton 2-stage unit and use around 20-30 W in the summer on first stage with a delta T of 11 F and ~ 6 GPM.  This is very low due to my loop configuration, but does illustrate that some good energy savings are possible.

    Thanks,

    Tom

    ------------------------------
    Tom Wyer
    BSME/MSME
    President, Geo-Flo Products Corp.
    Bedford IN
    www.geo-flo.com



  • 8.  RE: Using variable speed pumps with a geothermal

    Posted 09-14-2016 06:55

    As a follow-up to this thread, be sure to use a relay/contactor with silver tin oxide contacts to start/stop the Magna1.

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    Tom Wyer
    BSME/MSME
    President, Geo-Flo Products Corp.
    Bedford, IN
    www.geo-flo.com



  • 9.  RE: Using variable speed pumps with a geothermal

    Posted 09-20-2016 10:44

    As an addendum to Thomas' information, I've located suppliers of relays and contactors that use silver tin oxide contacts, and are available from several retailers. American Zettler makes the contactors, the XMC0 line (although some sizes seem hard to source), and Schneider Electric makes the relays, the Zelio RPF line. I'm sure there are other manufacturers; just wanted to leave this here for anyone having trouble finding the parts.

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    Jacquelyn Scherer
    Geo Designer
    The Janes Company
    Mukilteo, WA
    425-267-0202