rads not getting hot in new extensions

[terrycuttlefish]

the central heating system is a powermax 155x.sealed unit. when running the rads are red hot in the home.the pressure holds at 1.5 bar,no problem.But since they have had a single storey extension done the two rads in the extension(0.900x0.600) got hot when first turned on by the plumber and then stopped working.I can get heat to the rads if i drain off the system from the 2 rads and pull the hot water throw to the rads,but as soon as i close the rads up the rads lose their heat.I have closed all rads down upstairs to push the heat down stairs the rads get hot but not as hot as the other rads and when i turn the rads back on up stairs the 2 rads in the extension loose there heat.The two rads in the extension are connected to the system by 15mm plastic pipe at the upstairs bedroom rads
help needed,
terry

 

[bengasman]

Sounds like the system needs balancing. Do the new rads get hot if you turn of all the other ones?

 

[terrycuttlefish]

if i turn off all the other rads apart from the problem two rads they get hot but as soon as i turn one of the up stairs rads on they start to lose heat.I have balance them and i left the job with all the rads being hot apart from the problem two rads which were warm but it was the best they have been since the extension was put in a few years ago.When i called back the next day the two rads had lost all their heat.

 

[bengasman]

Pump capacity insufficient, flow resistance to great, or balancing not quite right. Could be a combination.