Having problems with gas in radiator circuit consisting of 17 rads. Question is how to find the cause of the gas in the system. System is fed by ASHP so has a 45 litre 'buffer' tank - ASHP feeds it with hot water, and the radiator circuit pump draws hot water from the other side. Being a relatively large point of low flow, any gas bubbles rise to the top of this tank and can be vented via manual vent at the top. But tank is still collecting volumes of air two weeks after filling. All radiators are hot and not collecting anywhere else particularly. System has about 250 litres of water and 3 bottles of X100 used in fill. I suspect this means there is a leak, although the pressure doesn't seem to be dropping. Half the system is new and was low-pressure air tested then high-pressure wet tested successfully. The other half has new radiators on old piping. All radiators have now been in the system more than 10 weeks except 2 - one steel and one aluminium. One pressed steel radiator that was replaced was half-filled on the bottom channel all the way across with magnatite (I cut it in half to check). This was perhaps 10 years old and the system had previously had a leak for some considerable period of time, resulting in the entire piping downstairs being replaced (which was pressure tested as above).