cold room - radiator problem?

My bedroom just doesnt get warm but the radiator is certainly hot and seems to be doing the job. Any ideas how i might try to improve the situation?
The room is 3m x 4m with a 2m long radiator positioned under the window. but the air temp just never seems more than just ok even if i have had it full on all evening. If I stand within 3 feet I can feel the heat coming of the radiator but not further into the room. A samll elctric oil radiator does make a big difference but i am thinking the radiator must be able to make an impact.
The radiator is in a bay and there is an RSJ on the ceiling , would this prove a barrier to the warm air flowing into the room?
Thanks
CHris
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In an earlier contribution to this discussion,

There are a number of factors to consider here. The first is whether the radiator is adequate for the job you're asking it to do. The second is whether it is running at the designed temperature. The third is one you have identified - restricted convection.
To bottom out the first, you need to calculate the heat losses from the room - taking account of the area and thermal characteristics of the walls, windows, floor and ceiling - and then compare these with the rated output of the radiator.
To bottom out the second, you need to check the flow and return temperatures with a pipe thermometer. [If the rad is too hot to touch for more than a few seconds, that is a good sign].
As far as the convection problem is concerned, it is *possible* that hot air is getting trapped behind the RSJ and not escaping into the room - but rather falling onto the glass, allowing the heat to escape through the window. Does it make any difference whether or not the curtains are drawn? You could try lashing up a false ceiling inside the bay, level with the bottom of the RSJ, to see whether that improves things at all.
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Cheers,
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