Banging Apollo boiler

14 Nov 2005
Reaction score
United Kingdom
Please help....

The Apollo boiler has being a problem for quite some time. The CH and the HW both heat up but the problem is when the central heating is on. The initial problem was resolved because the pump overrun stat had failed so the constant banging every time the boiler cycles has gone but there is still a problem when the boiler is running and the 3 way valve operates to heat the HW. The pump stops and the boiler bangs. The boiler also bangs when the CH has been on for some time and the boiler starts with hot water in the system. The boiler cycles off but the pump overrun stat does not switch and the boiler bangs and the overheat stat cuts the gas supply.

All the rads warm up equally, the 3 way valve is clear (removed and cleaned), the system has been cleaned and drained twice, and all rads and air vents have been checked - all ok and venting.

1. I have changed the pump. (installed grundfos)
2. Changed the High/Low stat
3. Changed the pump overrun stat
4. Used two different CH cleaning solutions

How do I know if the high/low stat are working correctly because the water seems to just get hotter & hotter - and the problem is the same in both high & low.

Can and should the pump overrun stat be adjusted?

Could the problem be the room stat cycling the boiler too close to the setpoint?

What could be causing this overheating and the pump overrun stat not to switch when the boiler is hot?

:?: :?: :?:

Please help

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