Knocking noise from pipes/boiler even tho' boiler is off

[goalc]

Hi, I hope someone can help....

Every night we get a loud bang somewhere in the heating pipes. This happens about every 5-6 minutes, and sounds like someone has hit a pipe with a hammer. It continues for about 5-6 hours after the boiler/heating has switched off, so i cant imagine it is a pipe contracting all that time (and also it happens at very regular time intervals) . Has anyone any ideas what may be causing this ? Is it air trapped somewhere ?

FYI, we also have an odd situation where as soon as the boiler kicks in, there is the same loud knocking noise that goes continously until the boiler kicks off (it sounds like pipes expanding, but never stops and happens immediately that the boiler comes on and then stops immediately it goes off, to be replaced by the 5-6 minute sound).

Do you think this may be connected ?

We have an old (about 15 yrs) Potterton Netaheat Electronic (thats all its says on it) boiler, Grundflos pump, and micorbore system, with 15 rads (2 of which are heated towel rails).

thanks for any advice.....

 

[wouldbplummer]

have you had a go at bleeding sistem for air sounds like you could have a pocket traped somewhere

 

[chrishutt]

I think you need to get a boiler engineer in. It sounds very odd to get a noise like that when the boiler is off, so I think you may have something very odd happening with the boiler - possibly the pilot light staying on continuously.

Do not attempt to do anything to the pilot light or anything that involves taking off the main cover on this boiler - it's potentially very dangerous. The noise when the boiler is on indicates localised overheating in the heat exchanger which may require flushing and cleaning.