I am currently running the radiator out of 22mm pipes that branch out to the radiators with either 15mm or 10mm. I am adding one radiator and thinking of adding another T from the already reduced 15mm pipes (1 metre further down it is still 22mm) to have a 4th radiator from that branch. I have heard the rule of thumb is up to 3 radiators. I can wait for a few months until I convert the garage and T from the 22mm but I need to close the ceiling in that room this week and need to make my decision. It is a fairly big radiator if that makes any difference, vertical 2200W.