Water bubbling through boiler on cool down.

[bobdabuildarrr]

Firstly all water and heating works ok...
BUT when the boiler is cooling down, say 30 mins after shut off, you can hear water (like its bubbling and running) moving through the boiler. This lasts for about 30 seconds, then stops.
Within the last 6 months the system has had noise reducer and inhibitor added. All the radiators have been bled (some air removed), but the noise still persists.

Any pointers?

Many thanks
Mark

 

[breezer]

this

 

[ChrisR]

This is after the pump has stopped, yes? Where's the pump in relation to the boiler?
Check for signs of a leak on the input side of the pump - could be drawing air in..

 

[bobdabuildarrr]

yeah, a bit more info for you.
Gas fired boiler, std header tank system.
Boiler is about 5-6 old, but has been regularly serviced.
pump is with hot water tank in airing cupboard, so some distance away.
I am also certain that a radiator has been removed in the kitchen. But no leaks or noises from the pipes, or problems with the system heating at all.

Noise started about 2 months ago, but you only notice it when in the kitchen, next to the boiler, so may have been doing it longer. Since then I added 'noise reducer' to the header tank, which producd a little air, which was bled, but didnt stop this 'rushing water/air' issue.

There is also a change in tone when the heating starts.. Starts higher, then gets deeper. I suspect the 'higher' is the air being pumped back in.

Header tank and ballcock etc work fine.

many thanks

Mark