[into_infinity]

You generally still need electricity for radiant heating - there's a pump moving water through the radiators.

But I think it's pretty easy to understand why it's unpopular. First, AC is popular in the US, so a system that can reuse the same ducts is a lot less expensive than something entirely separate. Secondly, it's a lot simpler and cheaper to service. If your hydronic heating system freezes or develops an airlock or springs a leak, you might be looking at five-figure repair costs.

[jmclnx]

Ever see radiator in action ? The ones I have uses steam. Water would boil in the furnace and pressure would force the steam up to the radiators. Then the steam would condense, heating the apartment and the water would flow back down to the furnace to be re-heated again.

In the old furnace, every so often you would have to add water, depending on how cold it is outside.

The house was build over 110 years ago, so electricity was not too common back then. I probably think the source of heat back then was a coal furnace.

[bpye]

Oh, I understand that when AC is present, but in my experience in the PNW - AC is uncommon, but forced air is still preferred for heating.