A tradeoff has always existed between the water temperature at which hydronic heat emitters are sized and their cost. The higher the supply water temperature assumed by the designer, the smaller the required heat emitters and the lower their installed cost.
While I agree that heat pumps do move heat from one location to another, I strongly disagree with the premise that air-source and geothermal heat pumps are not renewable-energy heat sources.
Sidearm water heaters were developed decades ago so that boilers could provide both space heating and domestic hot water. Is there an application they can be used for today?
Interest in wood-fired hydronic heating systems continues to rise along with the price of conventional fuel. What kind of solutions are available and how do you handle them?
