Too much heat required for the amount of rads tee'd off that 15mm pipe. There's a limit to how much heat you can force down a pipe.That's probably the issue.Thanks for all your replies (including the funny ones! )
I just teed off of a couple of 15mm's I found going to and from the kitchen rad.
Any chance you can tell me "the issue"? ......pretty please!
I would suggest you want to be teeing off the primary heating circuit where it's either 22mm or 28mm.