Potterton Performa 24 - Poor H/W delivery

[pault123]

I am an installer, who has been asked to repair a friends boiler.
Initially, primary flow pin on diverter wasn't lifting & APS wasn't making, so changed diaphragm & APS, followed by complete diverter valve, only to discover that despite the pump rotor spinning (and heat being dissipated), the pin started to lift only when I changed the pump head.
Next day I get a call that boiler only fires up intermittently. Turns out PCB is faulty and fan was sometimes only coming on intermittently.
Then I discover that H/W performance is poor. Gas rate indicated about 16.5 KW on H/W. Burner pressure goes up to 11Mb then rapidly drops to 2Mb and then cycles between 2- 6Mb.
Resistance on top 2 pins of gas valve show 1.59 & botton 2 pins 2.89 - so assume that is OK.
Changed primary thermistor to no avail, but if I run hot water with thermistor out of pocket, it delivers full gas rate & plenty H/W so can I assume that gas valve is definitely OK ??
I have assumed that primary side is overheating, so have descaled plate exchanger (without any improvement !) & now added X400, which has been in for about a week - but its still the same apparently.
Can anyone give me some useful suggestions, and am I right about assuming a blockage somewhere ??
Sorry for long winded post but too much info is better than not enough !

 

[wnjswozo]

i would call potterton,could be faulty gas valve
been there with mates boilers,favour jobs there great

 

[pault123]

Resistance figures for gas valve should be around 1.5 & 3.0 (I've got 1.59 & 2.89) which I think are close enough.
Don't have a lot of faith in Potterton's tech dept (I used to !) or an hour waiting to speak to someone !! - but will as last resort.
Think the fact that it runs ok with thermistor out would confirm gas valve is OK - but maybe I'm missing something.

 

[Billy Bob Bob]

try this number trade direct line 08443358428,have had gasvalves do this on performas but only once fitted 156 of them(performas/main/105he) have you checked/adjusted min/max pressures?

 

[pault123]

Haven't adjusted min/max pressures but will do when I go back.
The problem with the hot water apparently only started since the PCB was changed, but may actually have started the previous day when the diverter/pump head was changed - its just that the hot water may not have been tested properly that evening !
Owner had been manually operating microswitch to get hot water before I arrived - but HW delivery was fine when operated like that. Just makes me think that some **** in the pipes/exchanger may have caused original problem - & now shifted to cause primary side to overheat thereby shutting the thermistor/gas valve down.

 

[Billy Bob Bob]

yes might have sludge in there i have also had totally blocked plate heats,blocked hydrolic pipes(the small ones) knackered hydroblocks(behind pump) and blocked filters(inlet)and faulty microswitchs

 

[Puddle]

yes might have sludge in there i have also had totally blocked plate heats,blocked hydrolic pipes(the small ones) knackered hydroblocks(behind pump) and blocked filters(inlet)and faulty microswitchs

We all have days like that ,gasman

and thats before lunch!!

 

[stevetheplumber]

not to hot on potterton models but some have a waxstat in them which gives this problem