Got called out to repair a leak in a solid floor, and noticed it was a one pipe system 22mm, the section of pipe i cut out had two 15mm centre tees that supplied the lounge rad and inbetween the tees the section of pipework about 100mm apart had some kind of internal reducer, and then carried on with the circuit.The radiator has a trv at low level, and also a lock shield valve at low level at the other end. On refilling all radiators and repaired pipework gets very hot apart from this teed off rad. I did not replace the reducer and was wondering if this would be the problem. Would a non return valve help, the radiator worked fine before. Any suggestions would be appreciated.