Thanks.
It’s a Santon premier plus, tundish is dry. I’ve attached a photo.
I have tried draining the system then refilling? How else would I recharge the air bubble?
Recently my heating system has started a dull consistent thudding/ knocking noise, sounds and feels like it’s coming from unvented cylinder cupboard, possibly water combination valve - I can feel vibration when holding the valve.
Happens regularly throughout day and night but At night this dull...