Every arrangement must transfer thermal energy into the occupied space. The useful comparison is between thermal radiation, convection, energy transport through pipes or ducts, and the associated distribution and control.
- An open fireplace gives rapid local radiant warming and a visible source. A chimney carries combustion products away and draws room air with it. Much energy can leave in the flue gases, replacement air can cool other parts of the room, and the temperature distribution can be uneven. The fact that a fire feels hot nearby does not establish a high whole-building heating efficiency.
- Hot-air heating warms and transports the air itself. Ducts can distribute heat rapidly to several rooms and can be combined with ventilation. Poor distribution gives draughts, noise or stratification; warm air delivered near the ceiling can leave the occupied region colder. Air motion can carry dust, while heating air without adding water lowers its relative humidity. Indirectly heated air should be distinguished from introducing combustion gases into the room.
- Hot-water heating transports energy in circulating water and releases it through warm surfaces. Radiators exchange energy by both convection and radiation despite their name. A distributed closed circuit can give relatively even heating and separate room-air circulation from the furnace. It has thermal inertia, needs adequate circulation and control, and is vulnerable to leaks or freezing.
- Steam heating transfers substantial energy when steam condenses: the delivered latent heat is approximately . This can transport heat effectively, but the system needs pressure management, condensate return and suitable traps. Hot surfaces and poor condensate management are disadvantages; temperature control and maintenance differ from those of a low-temperature water loop.
There is consequently no universal ranking from the heating medium alone. Useful delivered heat, ventilation losses, the building's thermal insulation, the emitter temperature and the control arrangement determine the result. The same installed heat input can produce different comfort and different room-temperature distributions.
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