That comment got me wondering so I went to wikepedia and found the following: Room heating was normally better done by charcoal braziers than hypocausts. But hypocausts did allow them to exploit any poor-quality smoky fuels like straw, vine prunings and small wood locally available. Hypocausts also allowed them to generate a humid heat for their baths.
The Romans worked almost all the coalfields of England that outcropped on the surface, by the end of the 2nd century (Smith 1997; 323). But there is no evidence that this exploitation was on any scale. After c.200 AD the commercial heart of the Empire was in Africa and the East where the climate severely limited timber growth. There was no large coalfield on the edge of the Mediterranean.
A hypocaust was the original slab heating system. They built a fire in the basement and the hollow walls of the building became one huge chimney. The original biomass furnace.
Who woulda thunk it?