got a combi boiler, 10 years old, every now and again it bangs, 2-4 times, every 20 mins or so. i’ve noticed the pressure gauge fluctuates when this happens, what is it? an air lock? i’ve tried bleeding rads and pump, but it still happens… help

What boiler? Could be a load of things,

If you have TRV’s on the radiators, open them all fully and see if that solves the problem.

many aging systems have a knock.. clean it out with a chemical sluge remover then whack a unit of inhibitor and another of noise reducer .. cured for less than 20 quid and will last another 10 years

many aging systems have a knock.. clean it out with a chemical sluge remover then whack a unit of inhibitor and another of noise reducer .. cured for less than 20 quid and will last another 10 years

Or it could be a perfectly innocent knock from something like the diverter shutting (ideal isar style) or millions of other things.

Not neccesarily a dirt issue at all.

thanx for replies chaps, yes its an ideal isar, put in for the old woman who lived here before us under the warm front grant thingie, not a great job to be honest, a couple of leaky joints, etc

Well that has that then. It’s the diverter shutting off after pre heat/water usage.

Check you have no dripping hot taps or leaks on the hot water side of things, as cooling of the plate hear exchanger due to heat loss, will cause the boiler to go into Domestic Hot Water preheat more often than it should.

Not sure if its relevant to combi setup but my boiler started going bang and my plumber friend found that the ‘rising main’ was furred up, He just replaced that section of pipe and all was well.

If a TRV is on the wrong side of the radiator it can sometimes open and shut with a thud depending on the flow rate.

You have evil spirits trapped in your pipes and will need to arrange for a casting out.

Perhaps this chap could help: