Got a job today to take out vented cylinder and replace with unvented indirect cylinder (i’m G3 qualified). Only problem is the system has a strange set up. It’s combined feed and vent S plan system. Heating zone valve is on the flow and pump and hot water zone valve on return. My issue is that just after the feed T’s into the vent, it T’s for the cylinder flow. Seems like a strange set up to me. I plan on putting a zone valve on flow into the cylinder which will make it a customised S plan with two zone valves on the hot water. But I’m thinking this system is going to be a nightmare to refil, any thoughts? Never worked with combined feed and vent. Boiler is an old easicom oil boiler. Heating and hot water works fine now, just rubbish hot pressure as gravity. Have attached some pictures