rads not heating up

[rockchix]

hi 2 x rads at top of house dont heat up also 2 x rads on ground floor also dont heat up these 4 x rads are all on same side of house have tried swapping rad valves from working rads to non working rads no change
if i open drain on non working rads hot water will eventually come out but once i have closed off drain no hot water will circulate to these 4 x rads any ideas please

 

[GasSafeTony]

hi 2 x rads at top of house dont heat up also 2 x rads on ground floor also dont heat up these 4 x rads are all on same side of house have tried swapping rad valves from working rads to non working rads no change
if i open drain on non working rads hot water will eventually come out but once i have closed off drain no hot water will circulate to these 4 x rads any ideas please

You should search the forums, this question has been answered a thousand times.,