We have a huge old house with 15 radiators all of different sizes. some rads get hot where others are cold (so we turned them off to try to increase temp in the others). We have bled all rads so no air and have just got used to a cold house. old boiler but gwo, pump has been changed also.
The main issue is that a baby is arriving and the room is freezing despite the rad being hot. question being is the rad too small for the room? I have worked out the btu and it needs to be 3196.8. (room size 3mx3mx3m high) the rad i have is a single 96cm by 60cm. should this be sufficient for this size room or should we change the rad or do we need the entire system examined - in other words is there a quick fix or will this be an expensive one!
Working out the btu output of a radiator is not as simple as just the size. Radiators come in a variety of types. Single panel, single panel convector (with fins), double panel, etc. Then then consider output is dependent on water temperature. New radiators generally are more efficient and there are differences between manufacturers.
Check the Screwfix or maybe B&Q websites for their panel radiators and outputs for a given size.
A quick check tells me a 900 x 600mm rad could give out as little as 2000btu for a single panel or as much as 5300btu for a double panel double convector.
Simply fitting a larger rad is not going to solve your problem as it will probably not get hot enough because you already know you have flow problems. The system possibly needs flushing and bigger pump fitting. If the boiler is old, it is probably not producing enough heat anyway.