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Thread: Lowest voltage for Grundfos boiler pump

  1. #1
    DIY Member dwassner's Avatar
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    Default Lowest voltage for Grundfos boiler pump

    My Lochinvar Knight has a setting "boiler pump minimum voltage". In the manual it states that as firing rate decreases, the speed of the pump can decrease as well. The range is 2.5-10 volts and is currently at 3.5 volts.

    First, is there an advantage to using this setting as they state it is for lowering the delta t, and my understanding is that I want a higher delta t?

    Second, I have a Grundfos UPS 1558-FC, so what is the lowest that the pump will safely handle? Will too low of a voltage hurt it?

    thanks in advance.

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    The pump needs to run fast enough to ensure that the water does not boil as it passes through the heat exchanger. If the firing rate drops, you can slow the water down, since there's less heat to absorb. If the things are all lined up properly, the outlet temp might remain the same. You'd notice a difference in the return water temps, but that may not be an issue other than efficiency (which should increase as the delta-T increases). If this is the same pump that circulates water in the radiators, the delta-T from the outlet to inlet will increase as you slow the flow since there will be more time for the heat to be extracted in the loop. If I'm understanding things, the only thing being minimized is the change in temp (delta-T) of the outlet of the heat exchanger, not the system's delta-T. THen again, I may not be understanding things properly!
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    DIY Member dwassner's Avatar
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    The manual specifically refers to the delta t and nothing else. It is the same pump that pumps the water through the radiators as the boiler is plumbed directly into the system.

    http://lochinvar.com/_linefiles/WH-SER%20Rev%20G.pdf here is the link to manual. The parameter is on p.26, 4th paragraph in the left column.

    Any idea on what you would set the minimum voltage to?

  4. #4
    Retired Defense Industry Engineer jadnashua's Avatar
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    The thing of import is the gpm. The specific boiler will have some minimum number range of gpm at different firing rates. Compare that with the spec sheet of the pump in question.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  5. #5
    Master Hot Water Mpls,MN BadgerBoilerMN's Avatar
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    You have not stated what type of radiation the boiler is serving. The boiler controls the voltage, there is no minimum.

    Where is your heating contractor?

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