I am getting a new boiler fitted and want to change a couple of radiators whilst I'm at it. My hall/landing currently has a radiator with an output around 4700 BTU which according to online calculators should be about right but it doesn't heat the room as much as I would like. It could be because my house is an old nofines ie has solid concrete walls. Would there be any problem fitting say a 6500 BTU or will it cause the system to be inefficient. If so would fitting a thermostat valve be the solution?