Air in a closed system.

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I appreciate that lots of people ask this question but I'm having difficulty finding an answer to my exact problem.

We had a new heating system installed when we rebuilt our cottage. Oil fired condensing boiler on the ground floor with all plumbing at first floor level. The system has never really been working perfectly since it was fitted. Please don't suggest we get the original plumber back in. We liked him a lot and I got on with him very well but he died last year and we've since discovered he was an alcoholic!

The heating system has three timer controllers controlling three two port valves allowing us to choose HW, ground floor or first floor or any combination.

The two 22mm pipes rise vertically from the boiler and run between the joists and then along a bedroom wall (but boxed in) to the airing cupboard. The 22mm feed pipe has an air vent 'T'd' off it that I've replaced with a bottle type auto bleed.

The radiators all have bi-directional TRV's but we have no end of problems with many of them hammering.

We had another plumber round and he suggested just turning them to max and using the room thermostat. This rather defeats the object and wouldn't work very well as the downstairs room thermostat is in the kitchen where there is an AGA! We don't often have the radiator in there even turned on. In fact we have to have it turned up high just to get the ground floor heating to work.

Some of the TRV's are on the return end of the radiators and some are on the supply or 'feed' end. I've started changing the TRV's round so they're all on the feed end (even if that means they are not towards the centre of the room wall and in the corner). I've also changed a couple to ones that the local Plumbers merchant claims are better quality...)

I've just had to change one of the motorised valves and this is what's prompted my question. After filling and bleeding all the radiators several times, it's obvious that there's still air in the system. You can hear the 'whooshing and gurgling noises' when the boiler and pump fires up. It's loud and obvious. I suspect that it's in the 22mm return pipe to the boiler. Should I fit another bleed vent to this pipe? All the ground floor radiators are off drops from the first floor and the first floor is in two slightly different levels so I wonder if there's a bit of the return pipe that can never be purged?

The original plumber claimed it was air in the system that caused the hammering of the radiator TRV's and it would 'sort itself out...' if I kept bleeding the radiators.

I won't discuss the conversation about whether he was going to pressure test the system and his reply that he'd never had a leaky joint yet so wasn't going to. (And the subsequent re-plastering and decorating I've done after cutting out box work to resolder a leaky joint etc...)
 
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I've realised that the vent fitted to the 22mm feed pipe from the boiler is on the coil side of the motorised valve controlling heated water going to the HW tank coil. This of course means its only effective when the HW is 'on'. I might have tried it many times when the HW was 'off'... duh.
 
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