Question about Grundfos MQ3-35 3/4 HP Pressure Booster Pump

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I have a Grundfos MQ3 35B pump that transfers water from a rainwater tank to the wasking machine, garden taps and flush toilets on demand. This works fine most of the day but if left unused (ie overnight or when away for a day or so) it has shut itself down (red fault light on). When rebooted (switched back on) it mostly works straight away or hums away for 5 minutes or so then works. Any ideas on what sends it to sleep?. 2 storey house - having trouble pushing up hill? Thanks Bill

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  • billnagle54 Oct 05, 2010

    thanks tbreeze47. Your response has got me thinking. The tank water level is very low (not much rain and mains top up floats set very low - not much above the outlet/suction line on the tank). I have filled tank manually from mains and this improves situation but not always. So I will check the non-return valve (on the suction side? - none on discharge side). It may be faulty, incorrectly set or slightly ajar with rubbish or foreign matter. What do you think?

  • billnagle54 Oct 05, 2010

    I have a Grundfos MQ3 35B pump that transfers water from a rainwater tank to the washing machine, garden taps and flush toilets on demand. This works most of the day but if left unused (ie sometimes briefly but commonly overnight or when away for a day or so) it has shut itself down (red alarm light on). When rebooted (switched back on) it mostly works straight away or hums away for 5 minutes or so (as it primes?) then works. Any ideas on what sends it to sleep?. 2 storey house - having trouble pushing up hill? Low water levels in the tank or a problem with the non-return valve? Thanks Bill

  • billnagle54 Oct 06, 2010

    Thank you - I am coming to that conclusion as well. Partly low suction side pressure because the tank is so low and possibly a leaky non-return valve on the suction side.

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The only thing that would cause a problem like that is when the pump is no longer primed - the water has drained out of it and it runs on empty = overspeeds.
You need to replace the valves, they are leaking. Also make sure that the rain tank is clean, dirt and debris might block the valves.

Posted on Oct 05, 2010

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Most likely the water supply to the second story is not holding water for a few hours, and so the pump has to repump it up the supply line until it works. There could be a slightly leaky stop valve or whatever is being used to hold the water in the line when off, but it sounds like it might work at times and not at other times.
Have never messed with something like this, just sounds like what's happening. I read on a blog that this pump takes 5 minutes to self prime from a 26 ft well, so it sounds like sometimes it's having to re-prime.
Good luck, and hope this helps.

Hope this helps.

Posted on Sep 30, 2010

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