I have some heating zones which run off a Grundfos Alpha pump which is set to the "auto" mode.

Heating seems to work fine - rooms heat up normally and this doesn't seem to take an unusually long time.

However I noticed that when the pump is on and shows power usage, it also shows 0 flow. That doesn't seem to make sense.

The various runs to the heaters are between approx. 20-40' one-way distances through 1/2" EVOH PEX. There is a zone valve for each loop and the shared pump is triggered on/off by a controller (Taco).

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There is no appreciable noise when it is running.

I switched it out of "auto" mode as a test, and then it does show 1 GPM of flow:

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which still seems really small?

I know the alpha is supposed to be efficient, but how to reconcile almost-no-flow with heating working OK?

I don't have any other direct way of measuring flow rate.

Pump curve:

enter image description here (source)

(So it does go to 0 in the 'alpha' range, but that doesn't tell me much... AFAIK any centrifugal pump could spin, consume power, and move no water; but that doesn't seem to be the case here.)

  • My first thought is that the flow detector in the pump is not working properly.
    – jwh20
    Dec 8, 2020 at 12:31
  • @jwh20 I'm not sure what the mechanism is for that. I know in the boiler I can here a small noise when there is flow, I picture some kind of "water wheel" device that actually spins / moves (though that's probably not the right visual). But it could be a reed sensor or something... ? Dec 8, 2020 at 12:34
  • The only way to know for sure it to remove the pump and test it. You really don't know what's going on inside those pipes right now.
    – jwh20
    Dec 8, 2020 at 12:40
  • But then again, perhaps you're worrying about nothing here. You said the system is working fine, right?
    – jwh20
    Dec 8, 2020 at 12:41
  • If heat is making it to the room that is served by the pump, then you know that there is flow. The fact that the flow indicator shows no flow tells me that the flow indicator is wrong since we know there is flow. If this presents a problem, remove the pump and fix the flow indicator or replace the unit. Seems simple enough to me.
    – jwh20
    Dec 8, 2020 at 12:51

1 Answer 1


The good news is it is working. I've noticed that pump is both a bit optimistic (claiming flow rates of, say, 7GPM when all my other data says 6 or less). But it also rounds to a whole number. So you might be getting 0.49GPM, and it rounds to zero.

26 watts is a LOT of energy for 1 GPM with this super efficient pump. I suspect there is air in the lines. Assuming you have automatic air elimination in the system, I suggest you set it to speed III, where it uses 45 watts, and see if you can get that flow rate up. If so, leave it there for a week to push that air/water mixture to your automatic air elimination, then go back to your auto setting.

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