Grundfos Alpha pump shows 0 GPM but consumes power and heating works?

Question

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).

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:

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:

(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.)

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.