Insufficient hot water flow to shower pump?

[Jerry1]

Ok so this problem is now doing my nut in!

The System

22mm Surrey valve on a vented system hot water tank supplying a grundfos/watermill 1.5bar twin impeller centrifugal pump pushing water up to a Hansgrove thermostatic mixer with a 4 setting showerhead.

The Problem:

The hot water flow seems to be insufficient to start the pump. In order to start the flow or pump operating i need the turn the mixer setting to full cold, then when the pump kicks in - turn the temp knob to required temp setting - the pump does not start when the temp knob is left at desired settting.

Tried solutions so far:

I have done some research and suspected it could be a broken surry valve - removed this and found no plastic bubble eliminator at the base of the 22mm pipe inlet and the plastic bleed nipple in the vent hole was missing. Replaced with new one and had no effect!!

The pump was replaced at the same time as the surry valve (problem had been occurring months before this.)

System was bled after new pump/surry valve installed for 5mins to eliminate all air bubbles.

Problem still occurs

Fault finding:

With shower head removed and pipe in basin of bath the pump and water flow does activate pump. It is only when shower head is fitted and placed in holder high on wall that the problems start occurring.

This points at insufficient head limits or minimums - the website for grunfos/watermill says that a min head is 1metre and i think my set up has just this.

Other observations - The previous pump had slightly wider bore connector hoses which i have kept on this new pump, the ones that came with it look thinner - whilst i realize the flow rate may be slightly reduced by using them - would this have any effect on the pump kick in pressure?

All comments/ hints/tips/ criticism welcome!

 

[seco services]

What flow rates are you getting from the hot/cold ?
If the hot flow rate is to slow then it won't trigger the flow switch to kick the pump in.

 

[Jerry1]

The flow rate for the cold is about 7 Ltr/min and the hot is 6 Ltr/min - this is measured with the showerhead off, pump off and full hot/cold selected respectively.

When i turned the hot flow full on - nothing came out. It was only with a bit of hose shaking that the flow started. After this hot flow was normal - including after turning it on and off several times and after putting it in the shower holder on the wall.

When i reassembled the shower head and put it in the holder and switched the hot flow on the water did indeed flow including activating the pump flow which means the head of pressure is sufficient to flow - way above what the manufacturing limits state (0.5 ltr/min)

The problem reappeared when i switched the shower off which shuts off the pump and tried to turn it back on again. ie no hot flow. This problem only occurs after the pump has just operated.

Could the pump be creating an airlock after it has been switched off - as the problem only seems to occur in the last few seconds after switching the pump off - but not sure where it could get the air from?

Or....could it be that the back pressure created to switch it off is too great for the float switch and hot water pressure to over come? - the cold flow is fine - but then it has a greater pressure.

Help appreciated - i feel a solution is close and am confident the answer lies in the pump design. Please correct me if i am way off

 

[Jerry1]

HA! FOUND THE PROBLEM!

There is a float containing a magnet in the hot flow side that operates a relay switch which hovers in position when the pump is on and when the flow decreases it drops out of position and turns off the pump.

This float is getting wedged in the small hole it is designed to cover whenever the shower mixer is switched off quickly - i guess the back pressure is quite strong and pushes the float into the hole like a plug and then when i switch the mixer on again the hot water pressure is insufficient to shift the wedged float.

Solution: turn the mixer off slowly so that the float has chance to "float" out of the on position and not become wedged.

Got there in the end!

Anyone know if there is a modification part that can be bought to eliminate this? or do i just have to keep turning the mixer off very slowly?