head banging problem with glow worm boiler

Apologies getting boilers mixed i am wondering if the c/h thermister is faulty it will affect the b/p by modulating the gas valve down .Try checking that otherwise it has to be a pcb fault a faulty c/h thermister could affect the dhw as it regulates the heat in the primary h/exch
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I changed the c/h thermister like you suggested, I checked burner pressure and it read 11mb, i thought it had cracked it but then realised that all settings were set to maximum, I changed the thermisters back, and still the burner pressure is 11mb. This means there has been a 3mb rise from the last reading.

Still scratching my head, im now looking at replacing the pcb but wonder if anybody have any alternative suggestions.

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You said the voltages on the gas valve were correct - but what have you got? I can't find a figure in the MI. Worth a call to GW. Called GW recently about a problem which could have had several causes. They wouldn't admit a "common fault" but directed me to one of the half dozen which was a bit obscure - and they were right.
Please tell us - useful to know these things!

(NB be careful when measuring voltages like that. Unless it's pure smooth DC your meter might give a different reading from someone else's. If you put it on a low range AC you'll see if there's any "ripple" on the DC which can give the different reading)

You also need to check the resistance of the gas valve coils. If they're too low, it can drag down the voltage coming off the pcb,and make you think the pcb is faulty. Again, GW ought to know, though you may have the old valve to compare it with?? If you suspect that, it's worth measuring the voltage coming off the pcb with no gas valve connected, which would normally be a bit higher.

That should give you a 90% certain diagnosis on whether its the gas valve.
If you think it's sticking a bit, tap it with the handle of a hard plastic screwdriver and see if the pressure alters much - you'd expect a jitter on a good valve, but not a jump of a couple of mbar say.

To decide if its the pcb you might have to check everything connected to it, and you get the same sort of slight uncertainties with readings, so 100% confidence is evasive.
If you have already changed the gas valve it is unlikely to be faulty as well though not impossible. I had a similar problem and like you changed the gas valve no change, so opted for the pcb which cured the fault as the land/lord has a few compacts i kept the valve for spare I think from memory the gas valve s/be 50v ac cold thermister down to 24v very hot measured across the black and white wires.On start up it s/be aprrox 130v across red and blue to the valve. the thermisters are 10/12k ohms.If the c/h thermister is broken the b/p will be 1/2mb in dhw mode untill it locks out,in c/h mode it will be full rate untill the over/heat kicks in and locks out.The ohms on the gas valve coil are 1900/2000 one side and 2300/2500 the other. These are approx values from memory.I hope this helps.
I have not looked at the details for this boiler but my initial reaction is that the new gas valve was not correctly set up when it was fitted.

Nevertheless I dont think that the gas valve is at fault anyway.

My guess is that the PCB is probably Ok but just needs setting up properly. There might be some flow restriction problem though. Have the temperatures within the boiler been checked on DHW delivery?

blue and red wires at gas valve 120v ac, black and white 26v approx. If correct and inlet pressure ok suspect gas valve.

If wrong check dhw sensor across purple and purple wires:
20 - 25 deg 12.5 - 11 kohms
25 - 30 = 11 - 10 kohms
30 - 35 = 10 - 5.5 kohms
35 - 40 = 5.5 - 4 kohms

If ok check ch sensor

20 - 25 = 10 - 7
25 - 30 = 7 - 6
30 - 35 = 6 - 5
35 - 40 = 5 - 4.1
40 - 45 = 4.1 - 3.5
45 - 50 = 3.5 - 3.1
50 - 55 = 3.1 - 2.1
55 - 60 = 2.1 - 1.8
60 - 65 = 1.8 - 1.4
65 - 70 = 1.4 - 1.2
70 - 75 = 1.2 - 0.9

Then control pcb presuming of course that the hot water gas pressure adjuster has been adjusted correctly.

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