Glow-worm Compact 80P cutting out / not modulating

1 Sep 2004
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United Kingdom
Hello all,

I have a mis-behaving 5-year old Glow-Worm Compact 80P combi boiler connected to a closed (unvented) system of 11 rads. I have thrown many hundreds of pounds at the thing to make it give me a constant supply of hot water. After a new pressure switch, a new PCB (I did these both myself, 'relatively' cheaply) and then about four (expensive) visits from various heating engineers, the problem was found to be the wires connected to the air pressure switch in the fanned flue. They were connected the right way round, which, it transpires, is the wrong way round. These boilers left the factory with these wires incorrectly connected. This error is also faithfully reproduced in the circuit diagram in the installation manual.

So, a couple of minutes later, I had a nice, constant-ish supply of hot water. All well and good. Until the winter...

When the heating's been on for anything from half an hour to a couple of hours (it varies) the boiler shuts off completely; pilot light goes out with a deliberate 'clonk'. I have noticed that the pressure goes up quite a lot before it cuts out; I've tried all sorts of (cold) pressure settings, eg. if it's at 1bar when cold, it will go up to just over 2bar and then cut out.

I have sworn repeatedly at it, bled all the rads regularly (hardly any air in them) increased the system pressure, decreased the system pressure, sworn much more, considered throwing the whole thing away and getting a new boiler (££££££!!!!!!!! :( ).......Could the problem be sludge in the system, causing the pump pressure to increase in the boiler as it tries to shove all this through the rads? Would a shot of Sentinel X400 maybe cure all my woes? Or does it sound more terminal???!! Please help, somebody! :confused:

Getting back to the hot water, the boiler flame is either ON or OFF. The manual claims that it should modulate down when the water flow reduces, but this is not the case; it stays on full and super-heats the water to a third-degree-burn-inducing temperature. Should I 'cure' these problems by adding a large amount of dynamite to the system and sending this useless piece of junk to the next life?
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sounds like it could be the gas valve not modulating .On the system have you got trv`s on all the rads also check the burner pressures the o/heat cut off may be to high s/be set at 72 degs DHW temp can be set by adjusting P1 ON PCB ,HEATING IS P2 ON PCB another test is the o/heat stat could be out check milli volts either side of stat s/be equal reading

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