If the HW zone valve is normally closed, sounds like the air is getting into the HW circuit via the return pipework. That might be connected with the return pipework layout you described earlier, hard to say without knowing the details. But it doesn't alter the fact that the air shouldn't be there anyway.One further observation I should have mentioned above - the air seems to hide in the DHW circuit. If I manually open that zone valve, I hear the air rush into the return and filter, before hearing it again coming out of the flow,