zlacker

What about heat per unit of volume? Seems like the selling point is that a pretty small box can service a whole bathroom. Presumably it has a higher density than water and requires less insulation?

>>elric+(OP)
I think this is the most crucial part. External heat pumps are OK - people install air conditioners everywhere already - but most houses/apartments aren't set up for large water tanks. The interior heat storage needs to be comparable in size to the existing gas boiler.