[freebeard]

Perhaps a sand battery heated with/around the exhaust ... in cars...

A phase change material would have less weight, but for a stationary situation it might show a better ROI.

en.wikipedia.org/wiki/Russian_stove

The Russian stove[1] (Russian: русская печь) is a type of masonry stove that first appeared in the 15th century or earlier.[2] These stoves combine the functions of a traditional stove, oven, and fireplace into a single unit, and serve a broad range of purposes, including cooking (boiling, baking, and smoking), drying plants and mushrooms, providing interior heating and ventilation, bathing, and providing a warm place to sleep (many units include a sleeping berth atop the stove).[3] They can be found in traditional Russian, Ukrainian, Romanian, and Belarusian households.[3] Such stoves burn only firewood.

The only thing they're missing is a[n] hot tub kludged onto the side.