I moved into a house last year with a mix of 1 pipe (most of downstairs) and 2 pipe (all upstairs and 1 rad downstairs) - all the one pipe rads were very slow to warm up even if I turned off the two pipe rads which might have shunted the flow. One big problem with my layout was the rads were mostly some distance from the main 22mm loop, often via long pipes (several metres) with lots of bends, so there was no way the heat would move into the rads as you don't get much actual water flowing through the rads on one pipe.
Over the summer I got under the floor and repiped one side of all the rads over to the other side of the house and cut the 22mm loop at the far end which quite easily made it a 2 pipe system - the performance of the rads is so much better! I can't understand the thinking behind the original layout!?
Over the summer I got under the floor and repiped one side of all the rads over to the other side of the house and cut the 22mm loop at the far end which quite easily made it a 2 pipe system - the performance of the rads is so much better! I can't understand the thinking behind the original layout!?