Grundfos Water pump STN-3.0 B hot water

[nerak99]

This pump supplies hot and cold to the shower and hot to several other taps. It has been in place for around 10 years with no issues. If I put just cold through then it is OK (from a pulse POV but not for a comfortable shower)

Recently, when the shower is turned on or indeed any of the taps, the pump pulses on and off (i.e the pump switches on and off, not just the water pressure going up and down).

If another tap is turned on then everything runs OK so that tells me that this is not a pump capacity issue.

Grundfos tell me that this is a flow issue and the flow is less than 0.5 ml/minute.
I checked this and the flow is way more than this. 0.5ml/min is really slow, I checked with a measuring jug just to be certain.

Grundfos then tell me that this is because of the use of a mixer tap. Hence I shut off the cold water outlet. (There are four full bore valves to enable pump service without draining down). The symptoms persist.

Grundfos:
"There is nothing else that would cause this symptom. is is 100% a flow issue, You could try a hose directly on to the pump outlet side to see if the same issue persists. "

I wondered if it was the pre charge pressure on the hot water side pressure chamber.

Grundfos:
"The vessel being under charged would not give this symptom, I'm certain it is the flow knowing how the pump works this is the symptom when the flow fluctuates "

Anyone got any ideas. Currently I am considering using bike pump to stick a bit of air in the hot water pressure chamber. (there is a spreader valve on the end) I figure that I only need to put a few tens of ml of air in to raise the pressure to the 1 bar that the pressure chamber needs.

Before I do that, anyone got any bright ideas?

Best wishes for the season everyone.

 

[Abyone]

I have a reatively new STP 3.0b and i have also experienced issues.

When i switch my pump off sometimes it continues to cycle on and off. Grundfos told me to install non return valves on the pump outlet.
( I havent yet but did read about this being a solution specific to Grudfos)

The other issue I have is when i have a mixer tap on with a lot of one and a litle of the other i.e mostly hot and a bit of cold - the pump also cuts in an out. I think its becasue it sees the low flow from the cold and for some reason cant figure out that it can ignore the cold becasue the hot has a lot of flow. I dont know if its something to do with the flow switches or not. But definitley the total flow is greater than 0.5l/min easily and it still cuts in and out - it works fine if both hot an dcold are open fully.

Rather than try and fix these I have decided to replace my pump with a Stuart Turner - i had one before and no issues.

In your case becasue it worked fine before now it is a bit strange
Seems like the issue is on the hot side - could there be a faulty switch on the pump ?
If you open hot only does it happen or only when its a mix ? Does it never happen on cold water ?