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The underfloor pipework between my boiler and the pump/cylinder is very noisy. The sound is like that with expansion or air in the system but there is no airlock and the noise is too persistent to be expansion. The boiler has recently been serviced and I have a magnatron fitted. The feeling that I have is that there is some debris in the pipework not affecting the heating but noisily being shifted about. A couple of years ago someone injected fernox at the bathroom radiator. Could that have not dispersed properly leaving a noisy residue?

DAVEYD wrote: A couple of years ago someone injected fernox at the bathroom radiator. Could that have not dispersed properly leaving a noisy residue?

Most unlikely.

The probable cause is as you suggested, air. Having air circulating around a system does not necessarily result in airlocks. The bubbles can be quite small and not effect the performance of a system but the noise it makes can be quite noticeable.

Apart from the noise, air is undesirable because it contains oxygen which will corrode radiators and pipework. A reliable indicator of this is if the water is very black with build ups of sludge.

Possibly air is being drawn into the system via the expansion pipe. To test this, dip the expansion pipe in a full cup of water and then turn the heating on. My guess is the cup will be sucked empty.

in addition to what plumbbob says, sludge can make a system noisy, particular if it effects the pump.

if he system is dragging in air like plumbbob says, it is possible the open vent and cold feed pipes are reversed or not otherwise fitted correctly