My gas central heating system was originally installed 30 years ago and is of two pipe vented installed with 15mm copper pipe. The radiators are original and are of a high quality enamelled finish with no signs of deterioration. The boiler and pump were renewed seven years ago. All the radiator valves were replaced a year ago. My system is inspected annually by the gas board.

My problem is that about two months ago air has started to get into the system which collects in the first radiator. It's not massive amounts of air but it's annoying. There are no leaks in the system and air is not being drawn in via the vent. There are no blockages in the pipework as the water is pumped to all the radiators almost immediately and as the system is balanced properly all the radiators heat up very well.

The strange thing is that this air is only getting in while the system is down or on initial start up not while it is running. I can hear the air gurgling in the radiator within a few seconds of the heating coming on in the morning. If I then vent the air from this radiator shortly after it appears, no more air seems to be getting in for the remainder of the time the heating is on because if when the heating switches off I try to vent the radiator, there is no air there.

The air only appears to be getting in when the system is cold.

I've checked all the fittings to make sure the couplings are good.

Anyone got an answer for me please.