Shower pump pulses when shower turned off

[GeoffJ]

Our previous shower pump (Stuart Turner) worked perfectly for around 10 years and then we had problems with starting it. On checking out the old pump I realized that the installation provided for insufficient water-head.

So we have re-jigged the pluming to place a brand new twin 3bar pump by the hot water tank so that there is now plenty of water head.

It all works perfectly except, periodically, when the shower is turned off - the pump will sit pulsing (3 second cycles) with no open taps in the house. It appears to just keep going until I cut the power for a few seconds.

I have thrown together a schematic (attached).

My thoughts are

1) The Grundfos installation sheet says that I need a non-return valve on the h/w vent pipe. There was no one-way valve on the old set-up and it never caused a problem, so I have not installed one (yet?).

2) Checking on this forum for similar problems there are suggestions that non-return valves are placed into BOTH the pump output lines. I can't see the logic for this as all taps are closed

3) I have tried expelling all air - I do not think it can be air but will try again....

Any other ideas/suggestions - Would you recommend 1 or 2 first ?

Regards
Geoff
(Updated - tweaked diagram as the surrey flange was shown the wrong way around)

 

[just pumps]

No.1 First

Opps, missed that it said on the vent pipe, that doesn`t sound right, I`ll say No.2

 

[Gasguru]

1) The Grundfos installation sheet says that I need a non-return valve on the h/w vent pipe.

Where does it say that?....you should never fit any valves to the cylinder safety vent pipe.

 

[GeoffJ]

Where does it say that?....you should never fit any valves to the cylinder safety vent pipe.

On checking - I see you are very right.
I had misunderstood the diagram. Please disregard the proposal for a one-valve in the vent pipe.
Geoff

 

[Gasguru]

I think the idea behind the check valves being on the pump outlets is that in a scenario where there maybe trapped air in the pipework (say from deadlegs/unused outlets) the check valves will stop the air acting as a spring once the pump initially shuts down.
What do Grundfos say regarding the pulsing.

 

[GeoffJ]

HI Gasguru,

I think the idea behind the check valves being on the pump outlets is that in a scenario where there maybe trapped air in the pipework (say from deadlegs/unused outlets) the check valves will stop the air acting as a spring once the pump initially shuts down.
What do Grundfos say regarding the pulsing.

Your comment got me thinking - I have check-values on all the pump connections. If there is air in the downstream side then temporarily closing the downstream check valves when the pump is pulsing will be similar in behaviour to a one-way valve.

Testing found that the cold-side downstream gate-value stops the pulsing. The hot side gate-value has no impact. Tends to suggest there is air on the cold-side

Ran both showers for 2 mins spitting cold water

Tested for pulsing again..
When it does pulse, it now does 3 or 4 cycles and then stops on its own - major result!!

This suggests there is an air bubble on the cold side. I will put a couple of one-way valves into the downstream pipes near the pump.

Many thanks for your help.

From

Geoff James

 

[Gasguru]

Why have you got check valves on the pump inlets...they shouldn't be there.
Never run just the cold or hot taps in isolation UNLESS the pump is specifically designed to do so...most don't. You run the risk of popping the seals on the closed size due to pressure increase from friction heating of the water.
Have you got the cold feed to the pump connected to the cistern on the opposite side to the float valve to avoid air entrainment.

Have you read pump manuals...randomly sticking check valves around the system is not the answer.

 

[GeoffJ]

Why have you got check valves on the pump inlets...they shouldn't be there.

When diagnosing the old pump I got fed up with draining down to do tweaks. With the gate valves it is now very easy to swap in/out the pump etc.

Never run just the cold or hot taps in isolation UNLESS the pump is specifically designed to do so...most don't. You run the risk of popping the seals on the closed size due to pressure increase from friction heating of the water.

I can see you point and I am willing to take guidance - the pumps appear to be recommended for "bathroom use" (including taps and not just the shower - the previous Stuart Turner pump was fine)

Have you got the cold feed to the pump connected to the cistern on the opposite side to the float valve to avoid air entrainment.

No - but as I have said the old pump worked for years with no problem. I only installed the new pump by the HW tank as the head of water was below the spec for any pump I can buy today.

Have you read pump manuals...randomly sticking check valves around the system is not the answer.

Well yes - the manuals are simplistic. I have read recommendations on this forum suggesting check valves on the out put side of the pump near the pump. It seems logical that they would dampen any bounce. I posted here to check if that is the way forward. I guess it is not?

Regards
Geoff

 

[Gasguru]

I guess when you say check valves on the inlets you meant to say gate valves.
It's worth trawling different manufacturers websites for technical guidelines..AFAIR Salamander have a very good guidebook.

 

[conny]

I had a salamander in my last house and all four flexible pipes had those in-line screw valves for ease of removal.