I agree with the overall idea that we can save a lot of energy by reducing the temperature at which we heat buildings, without sacrificing comfort. As soon I drew a little picture, a trip to the storage room upstairs produced an armload of my heart’s desire.

In modern households, hot water can be sourced from an electric kettle, a pot on the cooking stove, or the hot water tap. My father grew up in Montana in the 40’s and his parents were ranch hands (employees on a large ranch).

But two bottles, placed back to back and wrapped in a double-bed sheet, were as good as another human in the bed–still warm in the morning. This energy could heat roughly 900 water bottles per day – enough to keep the whole neighbourhood comfortable. I’m not sure of soapstone vs water but the manufacturing and opportunity costs are surely much lower and it’s less messy and prone to catastrophic failure.

It never burst, over a period of some 20 years, and there was never any talk of its needing to be replaced because of fear that it might burst.Such accidents are rare, because nowadays hot water bottles are made according to quality standards. Croat inventor Eduard Penkala patented the rubber hot water bottle – which he dubbed the “Termofor” – in 1903. This means that PCMs could provide a significant improvement to water bottles, by releasing energy at a more consistent, safer and more useful temperature.