Loud banging, lots of air in CH system

[andyste]

We have a "conventional" gas boiler with header tank and hot water tank in the airing cupboard. Over the last few weeks we've started having a problem which coincides with the CH timer turning the hot water on, usually in the morning but it's happened once or twice in the evening. It's only done it half a dozen times but I'm concerned something is on the way out, as we never had the problem before last month.

We get woken up by a banging sound from the pipes, which gets louder and louder as it travels around the house getting closer to our bedroom. It's like someone walking round the house banging the walls and floors with a very big stick! It only lasts 10-15 seconds. It's obviously a large lump of air - when the banging reaches the airing cupboard (next to the bedroom) you hear a sudden rush of bubbles for several seconds, presumably as the air finds its way up the expansion pipe.

I'm assuming something is causing air to be drawn down the expansion pipe. Could it be a blockage that comes and goes, or is there something wrong with the pump or the 2-way valve in the airing cupboard? The fact it's only done it a few times is puzzling. I'm taking a guess that the intermittent nature is dependent on whether the heating is on or not at the time the hot water comes on? As I'm usually asleep immediately before the banging starts, I couldn't say.

Any ideas what might be causing this? Thanks in advance.

Andy

 

[joe-90]

It could be a gas build up in your rads. Do they need bleeding from time to time? Is there an inhibitor in the system?

 

[andyste]

It could be a gas build up in your rads. Do they need bleeding from time to time? Is there an inhibitor in the system?

The system was drained and filled with inhibitor back in spring. We've never had a problem with air in rads in the 5 years we've lived here. I bleed them from time to time, but rarely (if ever) get air out of them.

Given the sudden and severe (i.e. loud!) nature of this problem, and the large amount of air we hear bubbling out afterwards, I suspect it's coming in via the expansion pipe, but don't know why it's only just started happening.