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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.)

StayOnTarget
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  • My first thought is that the flow detector in the pump is not working properly. – jwh20 Dec 08 '20 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... ? – StayOnTarget Dec 08 '20 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 08 '20 at 12:40
  • But then again, perhaps you're worrying about nothing here. You said the system is working fine, right? – jwh20 Dec 08 '20 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 08 '20 at 12:51
  • @jwh20 I think you are right that swapping the pump might be the only way to really know. I have in the recent past opened the bleed valves on one of the heaters and some water did come out, but that only confirms what I already knew... it couldn't quantify anything. – StayOnTarget Dec 08 '20 at 12:52
  • Check the flow when one of the rooms is demanding heat - if all the rooms are at temperature then the flow could correctly be zero. – Solar Mike Dec 08 '20 at 13:07
  • @SolarMike with no calls for heat the pump gets powered off – StayOnTarget Dec 08 '20 at 13:10
  • 26w is tiny if you have a centrifugal pump it could be turning at a rate two slow to really move much water or below the resolution of the meter. If things are working quit trying to fix what is not broken. – Ed Beal Dec 08 '20 at 15:45
  • @EdBeal specs say the pump max is 45W – StayOnTarget Dec 08 '20 at 16:16
  • Again that is a tiny pump if the impeller is not spinning fast enough there won’t be any flow, I have multi stage impeller pumps with sealed vanes and on my pumps below 800 rpm I have no flow at 900 I have a low flow , 1200 a medium then at 1750 full flow this is how we control our pressures so we are not hitting 150 psi when a 3/4 line is opened but when a 4” main is opened we can maintain 120 psi. (120 psi is out target pressure). – Ed Beal Dec 08 '20 at 16:26
  • @EdBeal I do agree its a small pump :) I just meant that at 26 W its not in the low end of its range. Looking at the chart I would *guess* that 26 W should be in the 6-8 GPM range for that pump. But anyway, not 0 or 1. – StayOnTarget Dec 08 '20 at 16:29
  • Pumping curves are not Linear at the bottom end so I am not sure how you equate the wattage to the curves. As the example I gave my pumps pump almost nothing at 50% speed the performance curves are what it will pump with a given head it has nothing to do with % current you may not actually start moving water until 75% of fla and your adjustment in actual flow is above that that’s just how these pumps work. Or they are not positive displacement like a piston pump yes I do have one of those also. – Ed Beal Dec 08 '20 at 18:48
  • @EdBeal I didn't mean it anything like an exact figure; I was just assuming that at approx 50% of pump input power it would be beyond 50% of flow capacity. I don't know exactly how much head there is but its within the orange area of the chart, so I just extrapolated/guessed it would be around 6-8 GPM. – StayOnTarget Dec 08 '20 at 18:50
  • No it really doesn't work that way for motors. No load at all in most cases will draw close to 75% of fla – Ed Beal Dec 08 '20 at 19:19

1 Answers1

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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.

ThreePhaseEel
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Radiance Heating and Plumbing
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