head banging problem with glow worm boiler

15 Jun 2006
Reaction score
United Kingdom
Glow worm compact 75e combination boiler.

the water is only just getting warm at every hot tap and shower unit, too cold to get a shower. flow rate reduced and hot water gets hot.

burner pressure checked, 8mb
burner pressure should be 14.7mb

increase burner pressure, but found that it was set to maximum, no extra adjustment would take it above 8mb

Replaced gas valve for new one, fault was worse with no burner pressure but think we have a faulty new gas valve.
We suspect that the problem is either gas valve or PCB, has anybody had similar problems?
We have also recently replaced DHW NTC (Thermister)

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Yes and it was the p.c.b in my case.Other possibilities are d.h.w thermistor faulty,gas working pressure low,maybe a blocked d.h.w h/exch.
maka2k2 said:
inlet pressure is 21mbar, working presure is 20mbar

Wow, perfection, that must be a first. Do these use a modureg?
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maka2k2 said:
inlet pressure is 21mbar, working presure is 20mbar

Spot on then. The GV is normally only adjustable via the potentiometer which of course is part of the PCB.

So now you have a new gas valve with no burner pressure at all? have you checked to see if it is getting any voltage?
yes voltage is as manufacturers instruction,according to manual if burner pressure cannot be achieved by potentiometer then can be adjusted on gas valve,on both parts burner pressure can be adjusted down but not up,no more than 8mb........with new gas valve not even allowing any gas through i can only assume it is faulty as when old was refitted we were back to square one with 8mb
Would lean towards the p.c.b ,if you change it adjust the ign. rate,also when you reduce the flow trough the taps what is the flow s/be about 8/9litres.If the plate is blocked the gas will modulate down.On c/h can you adjust b/p to 14mb if you can and plates not blocked prob. pcb
Check the PCB as you can set up the gas pressure on them. On the back panel of the PCB there is 3 holes with a little tool to adjust. One is for heating one for DHW and one for ignition but I cant remember which one is which. There is also an extra potentiometer on the PCB which can also adjust temps on the boiler.
if flow rate reduced we get hot water upto 60 degrees -so plate never considered as this was getting hot fine and maintaining its heat but burner pressure is just same when doing heating
pcb checked and potentiometers have all been adjusted to max but will not achieve more than 8mb
I think if you disconnect the modu reg two wires at the front of the gas valve it should fire at full rate if not it is the pcb as was the case on a 100e i worked on what is the flow rate when you reduce it to get 60degs you will only get a 35deg rise at 8/9litres it will drop as flow is increased especially with that boiler size you really need a minimum 30kw for decent flow rate
has no modureg only 1 wiring harness with 4 wires ,like a multiplug

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