Banging heating valves

[S70JPS]

I have been progressively doing up my house and fitted a new heating system around 2 years ago draining down and adding radiators as I finished each room. All this time the system has worked perfectly. As I have now fitted all the radiators I have added 4 litres of fernox with another 4 to go. This was two weeks ago. About a week ago the zone valves started banging each time they operated then one of the valve motors burned out. I have replaced this and the system is working fine apart from the banging each time the valves open or close. I have bled the system several times and can get no air out of it. The set up is a vaillant ecotec system boiler with unistor tank and vaillent whether compensating controls. There are three 2 way valves upstairs downstairs and hot water. It’s the upstairs and downstairs valves that bang. If anyone could tell me how to sort this it would be much appreciated. On a secondary note should the zone valve motors be beefed up as the operate much more than in a traditional set up?

[Rickyy]

Have they been fitted the correct way? Some valves make a banging noise if water flows through them the wrong way.

Syncron motors are easy to replace and won't require upgrading, not sure such a thing exists anyway.

[S70JPS]

Both valves are on the return pipework and to me they are fitted the wrong way round. I think the metal prong on the motor should be facing down (currently up) and the arrow on the valve is the wrong way round. I fitted most of the system but left the boiler room to a gas fitter so assumed his work was ok. What baffles me is that they have been fine until I put fernox in. I can easily turn the valves over as the plumber put isolation valves everwhere including each side of the valves.

[S70JPS]

Can anyone offer advice please I can hear the valves banging now?