new radiators cold old radiators hot

[sim64]

we replaced a back boiler with a combi and kept the original pipework.The new boiler is in the upstairs bathroom and one radiator downstairs was moved to another wall and an extra radiator added both having new pipework. When the heating is on, upstairs heats up very quick then two original radiators downstairs.The moved radiator and the new one take about an hour to heat up, at which point the bathroom radiator goes off. The firm that installed it say its needs a power flush,is this true? if so should it not have been powerflushed when installed. I feel totaly ripped off. Any answers wold be gratefully recieved.

[plumbbob]

Of course, it may need flushing, but has the system been properly balanced??

It may even be a small amount of air trapped left over from the modifications.

Have you tried turning off all the hot rads and see how that effects the cold rad?

[sim64]

it has supposedly been balanced but im not to sure ,ill try again myself.
When the upstairs rads are turned off all downstairs rads heat up at the same time including the 2 cold ones. They then all stay on ok when the upstairs ones are put back on