I too am currently doing a diy upgrade of my central heating at the same time as replacing a few blocked radiators.
My question concerns backfeed of the water in the boiler loop.
I have found a logical place to put the three port valve after the pump then i can divert water through the tank to the radtiators or both there is already a sort of t present as the hot water worked via gravity before .
my question is what prevents the return from the hot water tank backfeeding to the radiators and supplying them with hot water. the feed line has the valve in it to direct flow do i need something to control the flow through the return lines as they are all connected??