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For a building that is centrally heated by local fuel combustion (i.e. heating within the building rather than district heating), one may distribute the heat throughout the building using as a energy carrier either water or air. Which type is more efficient? On what circumstances does it depend? Water carries a lot more energy per unit mass, but with air heating we can dispense of radiators that transfer heat from water to air, and air heating might more easily allow for heat exchangers, on the other hand you'd still need a separate system for hot water. Are there studies comparing the overall efficiency between the two, given typical household use in a single family home?

Of course, there are many technologies more sustainable than either, with passive houses as the gold standard, but my question here is assuming we are still in a phase of central heating by local fuel combustion (such as natural gas).

  • An excellent question. I know that the answer is hydronic (boiler), and so do the top several results on Google -- but so far I can't find any hard numbers to back this up. I'll keep looking... – LShaver Jan 13 at 16:29
  • @LShaver Interesting, for the person who brought my attention to forced air in the first place claimed it was more efficient than with water, but that was just a random person in a non-technical chatroom. – gerrit Jan 13 at 18:36
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The big win for hydronic heating is that it's easy to zone. So you can easily have the bathroom warmer than the bedroom, have no heat at all in the guest room and the pantry.

The second win for hydronic heating in the floor: People will set the room temp lower if their feet are warm. This drops the room temp by several degrees.

A third potential win is that you can move your heating to the cheapest time of day. It's not unreasonable to store a day's worth of heat in a large tank of water. At present this is only useful if you are using electricity.

With clever design, you can eliminate one heating unit. A well designed house can be heated with a simple hot water heater plus a mixing valve. The hot water tank is set to the higher temp used, and a mixing valve is used to create water of the desired temp for the other use.

Downsides: Hydronic heat doesn't respond quickly to changes in demand. In floor had a substantial mass of floor to heat. Radiators general circulate by convection and so don't warm up a room quickly.

You may still need a duct system to get reasonable amounts of fresh air. This use can be MUCH smaller ducts, and typically can be run with what amounts to a large bathroom fan.

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