Sound like a balanceing problem ie the radiators down stairs are robbing the upstairs radiators. Turn down the [color=red]lockshield val[/color]ves on each radiator down stairs and open to full the radiator upstairs [color=red]lockshield valves[/color]. If this lets upstairs radiators get hot but downstairs now are cooler, then open down stairs a little more & close lockshield valves upstairs until the system is balanced out. If that dont solve your problem then you may have air upstairs or the system is badly piped or the pump speed/size needs increasing.
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