Used to have a 2.0Kw oil filled in the conservatory, (4m x 3m x 3m high)
We always had a draught from there as no doors to close it off. Once heater plugged in it soon warmed up and draughts stopped. After turning heater off it still retained heat for quite some time. Now had an extra radiator added to the C/H system and heater is put away in the loft.