Shower Pump comes on by itself for a few seconds - short cycling

Saruman

I've a Grundfos STN-3.0 B pump in the apartment beside the hot water tank.
It periodically comes on by itself a a couple of seconds. Sometimes, it happens every 5 minutes which is very annoying. It has to be switched off at night to allow sleep.

It's a good few years old, I believe 7.

Anyway I've done a fair bit trying to rule out any leaks anywhere. None can be found anywhere except one which I'll get back to as it's not part of the pump circuit.

What I've done so far is:
Closed off incoming connections and then opened up hot and cold taps/shower to drain all the water out then open up again and switch on pump. When I do this, it usually increases the intervals, often there's no short cycling for hours but as soon as taps start to get use and water flows, the the intervals get shorter until eventually it can be every few minutes.


**That's an older photo, the damp concrete is from taking off the accumulator tank, it's perfectly dry now**

At one stage, I took off smaller, hot water accumulator tank and drained it of water. When I pump air into it using a bike pump, water sprays out through the threaded end. Would that indicate the bladder is compromised and it should be replaced?
To add to that, when I try to pump air into the tank when full of water, the pump doesn't seem to push down so no air seems to go in.

The larger cold water accumulator tank at the back is fair inaccessible so I haven't taken that one off yet.

One other thing, if I close off the outgoing pipes from the pump, the pump still short cycles so to me that indicates the issue is not a leak from a tap or cistern that's just not visible.

My options as I see them(in no order) are.
-Buy a whole new pump as that comes with all the bits that might have failed.
-Buy the bits that might have failed like accumulator tanks, NVR, pressure sensor etc and hope they resolve it. Looks like that's about half the cost of a whole pump.
-Disconnect the pump, take all the bits off and try to clean them, perhaps soaking in citric acid solution to remove limescale.

My only other option is to get a pro out but the issue I have is simply that I have never found a reliable plumber. Every time I've had a plumber out, they've said whatever job is probably going to be very expensive and then never get back to me with a quote. I always figure it out and fix myself then.
If I knew for certain the issue was with the pump, I could get a pump expert out to fix if needed.

I did once have to get a pump expert out to repair the pump itself as it stopped working. PCB board failed.
The short cycling was a minor occurrence before that but it seems to be worse these days than ever before.

Now, just back to the leak. There is a miniscule leak from one of the pipes from the water tank leading to what I'm pretty sure is radiators/heating system.
By miniscule I mean a single drop an hour or so from one of the threaded nuts. Sometimes nothing happens at all. It dries up before it even has a chance to drip again. I've tightened as much as I can so I suspect I'll need to disconnect, clean it and use plumbers tape or else just replace the connection that's leaking.
It's not part of the pumps water circuit but is there any chance that this could still be the cause? Would a tiny drop in water pressure from the main water tank cause a pump to short cycle?

Does any of that make sense?

Saruman

Just in case anyone comes across this and decides they want to know the outcome.

That drip was in fact part of the heating circuit so nothing to do with the pump.

There was 100% no leak on the system causing the short cycling. I'm fairly certain the issue was either with one of the sensors or the accumulator tanks but for some reason, replacement part for that pump are very difficult to find. I only found one website which sold them as complete parts kits and the cost would have been not far off the cost of a new pump.

So I took a punt that I was right, and the issue was with the pump or at least the sensors and took the opportunity to replace the whole pump with a slightly quieter one. I got a Stuart Turner Monsoon 3 bar (negative head). I got it from screwfix who it turns out only sell the UK version from their website meaning the push fittings don't match Irish plumbing so I had to get adapters and 22mm pipe to adapt. Anyway after lot of cursing due to the very tight space I replaced it all and it works absolutely perfectly. No short cycling and it works quieter than the old pump and even comes on instantly without a pause of a second or so. That means, not splashing/surge of water when opening the tap.

The old pump itself works perfectly, it was just one of the sensors causing it to come on for no reason but figuring out which one without easy or economical access to replacement parts made it impossible. The other reason I picked the ST pump was I can easily purchase individual components if I ever need to.

HerrKapitan

I had similar issues to this. In my case it was the reverse flow valve. It was worn and was letting a trickle of water back causing the pump to run for a few seconds, every minute or two.

Mike3549

https://www.boards.ie/discussion/comment/117674675#Comment_117674675

Thank you!!!! Non- return feken valve!!!!

Sorry for resurrection of zombie thread, just got this problem myself. Though it was a main board on grundfos Stn 1.5, but my supplier didnt have any and thought it could be a pressure switch.

In my case pump turns on every 10-20 secs, couldn't find any leaks. But i noticed that if i turn off the valve on the supply side of hot water, it stays off. Check both NRVs and the hot one is slightly open all the time.