I am installing a std boiler with an indirect cylinder, 3 port valve, cylinder stat etc. The existing system is a std boiler doing purely heating only. An AGA is used to heat a primatic cylinder. My question is this :- Is it safe and correct to be able to use the same feed to BOTH cylinders, using 2 individual gate valves, and to also connect the vent pipes together, or do i need to keep them completely seperate? One cylinder would be feeding hot water to downstairs, and the other would be feeding upstairs. I am aware that the primatic cylinder has an air bubble, and that i wouldn't want to potentially cantaminate the indirect cylinder,so maybe if i was to put a non return valve on feed to primatic? Any feedback would be very much appreciated.