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Our home uses a Grundfos pump to compensate for the lack of pressure from the water main. In the last few months, the pump’s alarm light has regularly turned on, which requires us to manually stop and start the pump to get water through again.

It seems to happen more often in the evening.

Of note is that small pebbles are visible on the bathtub floor after we drain it.

A possible cause is that the limescale preventer we use* once dissolved into the water tank, resulting in a large influx of salt still present on the tank floor (see picture). I also attach a picture of the pump when it's not in alarm mode.

* https://www.buildbase.co.uk/fernox/supe ... 33-2830133

Hi mchl,
Most likely cause is that the pump is running dry either because the water level in the supply tank has dropped or that the inlet filter ( if there is one ) is blocked. Check for a filter and look in the tank next time it happens.
Regards S

stoneyboy wrote:Hi mchl,
Most likely cause is that the pump is running dry either because the water level in the supply tank has dropped or that the inlet filter ( if there is one ) is blocked. Check for a filter and look in the tank next time it happens.
Regards S

Thanks, I will go with the inlet filter hypothesis - and will check that there’s one - because there is always water in the tank. I wonder what may block the filter. Perhaps it’s just old and needs to be replaced?

Hi mchl,
Most likely culprit is limescale in the filter (if there is one).
I mention water levels because in areas with low mains pressure, in the evening when many people are drawing water the flow can be reduced or even be none at all. If levels in your water tank drop the pump may be drawing air even though the water level is above the tapping point supplying the pump.
Regards S

That makes sense, thanks for explaining.
I will arrange for plumber visit because the user guide lists 5 possible issues (including the ones you mention) for this alarm.