Splashing water noise in boiler

Truman · Nov 14, 2014

I've recently had a new combi boiler and new radiators fitted (replaced an open vented system) and when it starts up on a demand for CH, there's a reasonably loud splashing/trickling water sound from inside the boiler. It sounds like someone pouring out a bucket of water! The noise dies down after a minute or so. The boiler is holding pressure OK, no leaks underneath it and radiators are heating as expected, inhibitor was added. The system has been fitted a few weeks now and I had expected this noise to gradually disappear as I thought it was air.

I had mentioned it to the RGI originally upon commission as it did it when it first fired up but he wasn't concerned and just said air will make its way out of the auto air valve. Obviously I can't check this as it's internal to the boiler and I'm well aware I’m not allowed to remove the front cover. I've bled the radiators on a few occasions now and got a little air out, but the boiler 'splashing' noise continues upon start up for CH. If it’s just air then why isn’t it collecting at a high point? The boiler is downstairs so I’d expect air to make its way to the upstairs radiators and stay there.

Anyone got any ideas? Is it common? Could air be getting trapped in the boiler each time it shuts off? I’d have thought they’d be designed not to encourage air to collect in them?

Meady · Nov 14, 2014

It sounds like air and it will come out of the auto air vent in the boiler aslong as he has opened it up to allow it to do so.

Truman · Nov 14, 2014

Well it's been a month now and the noise isn't getting any quieter. I'm wondering if the auto air valve has been opened or left closed. If it was closed then wouldn't the air still make its way round the circuit and upstairs where I could bleed it out of a radiator? The noise only happens on start up so if its air then I presume it moves around the system each time the boiler is fired up, as the noise soon disappears, but then re-appears when starting from cold.