Combi dhw/shower problem

[ShowerPower]

Hi Boiler folk.
I need advice.
We have recently had a customer report a fault on a thermostatic shower connected to a roughly 10-15 year old Potterton combi. The shower has been working without problem for about 12 months. The fault is that the shower will run correctly for about 45 seconds then then runs cold for approximately a minute, then warms back up (repeat this sequence ad infinitum). Initially we suspected the thermo cartridge had failed in the shower, but a completely new shower produced the same result. We have also tried a different make of shower on the feeds - same symptoms. The boiler appears to operate ok when dhw taps are turned on full, but if you turn on the hot tap in the kitchen it needs to be about half on before the burner ignites.
The customer has a Corgi Homeplan in place however their engineer states the boiler is functioning perfectly ok.
Am I missing something? Can anyone tell me what the minimum flow rate at a tap should be before the boiler ignites? Help please. Thanks.

 

[Dan Robinson]

Could be a blocked plate heat exchanger or an imbalance in the supply.

Perhaps a single check valve on the shower inlets if the shower is working.

 

[picasso]

What model of potterton ? at that age I would guess it has a dhw diaphragm and its probably stretched .

 

[ShowerPower]

Thanks for the replies chaps!

Further investigations reveal:
The boiler is a 30HE & about 7 years old.
The boiler engineer has "apparently" replaced the diaphragm in the diverter.

On testing yesterday it took 8lpm at the kitchen tap before the boiler ignites. This seems pretty high?
As the shower had worked for a year prior to this recent problem I'm convinced something has degenerated in the boiler. Trying to get the Homeplan engineer to accept this is like trying to push a watermelon up your nose!
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