What BTU for a radiator in this room ( in dispute with builder)

[Footgoose]

We bought a new build last year . Bedroom 1 is downstairs , has 3 outside walls and no room on top. It’s always cold . We keep the radiator switched on full to ensure the room does not become damp even though it isn’t in use full time ( daughter at Uni ) . The builders are refusing to admit there is an issue and say the radiator is actually more powerful than required .
The room is always 4 to 5 degrees cooler than the rest of the house . I have taken photographs of a thermometer in the room and main house thermostat as proof but the builders are not responding . So please , if you know about BTUs and radiators please could you tell me what size / BTU rate of radiator should be in this room.
bedoom ( ground floor , room “sticks out “ from front of house .
3 outside walls
concrete floor
no room above . Tiled roof above .
I’ve no idea of thickness of roof insulation but it is insulated and walls are insulated.
window - 2.113cm squared
height of room 1.87m
width of room 3.5m
length of room. 4.55m

i have put these measurements in to an online BTU calculator and it come out as requiring 4488 to 4702 BTUs.

The current rad measures 90mm by 60mm and rates 2500 BTUs which according to the builders is bigger than the regulations require.

with the heating on full until the thermostat cuts out at 20 degrees in the hallway, this bedroom
only reaches 16.2 degrees max.

we didn’t have the central heating on last night as the wood burner had warmed the downstairs up well enough so when I checked the room this morning it was 11 degrees ( rest of house had maintained heat at 18.5 degrees).

When was it built? Building regs Part L changed in June 2022 so if done after this date they would have to adhere to this to be signed off. They should have the results of the air tightness test, all exposed pipe work should be lagged and they should have a heat loss calculation for the whole property.

I would ask building control to come in and inspect the property.

Was about to say this. We had the same problem with a room over a garage, we moved the thermostat into that room and turned all the other rads down. It's a solution but I'm not sure it's the right one and still think our house developer should have done more to prevent the heat loss in the first place.

@Stephisaur . You are right. The ceiling hight is indeed over 2m and not 1.87 cm. 😂

It sounds like the insulation is poor. Sometimes builders don't spec out all the houses in a development to the same standard as the show home.

Is the underpowered radiator getting fully hot? We have one that doesn't no matter what balancing we do. The plumber claims it's because it's the last one in the line on the system. No doubt the builders will continue to claim there's 'no issue' with those rooms and rads.

Ideally the main thermostat goes in the coldest room, and the other rooms all have their thermostats at the right temp for each room and switch off as they get warm enough. If you put the main thermostat in a room that heats up quicker than others, the boiler will go off before other rooms get to temp. I do wonder though- is your boiler up to the job? Can it generate enough heat for the system (including any other jobs like showers and hot water it may have to do)?
The only rooms we struggled with (worrying about insulation an all sorts) turned out to have rads wildly below the necessary BTU. When we replaced them with one that were overspecc'd we found the rooms were fine. We have to keep the thermostats well turned down on those rooms now.

thanks for replies. I do believe the boiler is up to the job , I’m no expert but it looks pretty huge to me , is in its moderately sized “walk in “
cupboard on the first floor . It sits almostdirectly above but at a diagonal from ( gallery landing ) the cold room below ,.