We have a conventional CH system about 30-years old, i.e. gas boiler, water pump, and diverter value for radiators vs coil in hot-water tank. Over the last few days the pump has been making a lot more noise and it sounds to me as if there is air/gas in the pump. Two nights ago the system failed with the boiler reporting insufficient flow. In the morning I removed the large screw from the shaft of the pump but hardly any gas came out, just the usual trickle of water. But it restarted again with much less noise. But today it was noisy again and then failed again.
My guess is that the gas must be coming from corrosion. I put inhibitor in the system the last time it had any attention maybe 6 years ago so perhaps it needs renewing at intervals? I also found that water in the header tank was full of brown floccules. So my current thought is that I should drain down and flush with clean water a couple of times.
Does a central heating cleaner such as Fernox F3 do any good? It seems this should be in the system for at most a day or two, so that needs a few more drain-down and refills, unfortunately.