CAN ANYONE TELL ME WHAT IS THE MAXIMUM RUN OF 8MM HEP ON A CENTRAL HEATING SYSTEM BECAUSE MY SYSTEM HAS A LOT OF THIS 8MM HEP. AND UPSTAIRS RADS WONT GET HOT, THE RADIATOR NEAREST TO THE POINT WERE THE PIPES ARRIVE UP STAIRS GETS SLIGHTLY WARM. IF I CHANGE THE UPSTAIRS TO 15MM WILL THAT GET THE UPSTAIRS WORKING . HELP ME PLEASE ANYONE