<Li> When thermostat is "satisfied", blower and element shut off . </Li> <Li> Requires very little maintenance . </Li> <Li> Usually more expensive to operate than a natural gas furnace . </Li> <Ul> <Li> Extracts heat from outdoors, either the difficult to install "ground source" or the less expensive "air source" via the refrigeration cycle </Li> <Li> Requires less energy than electric resistance heating and possibly more efficient than fossil fuel fired furnaces (gas / oil / coal). </Li> <Li> Air source types may not be suitable for cold climates unless used with backup (secondary) source of heat . Newer models may still provide heat when coping with temperatures below 0 ° C (32 ° F). </Li> <Li> A refrigerant coil is located in the air handler instead of a burner / heat exchanger . System can also be used in cooling, just as any central air - conditioning system . </Li> <Li> See Heat pumps </Li> </Ul>

Where are warm air vents usually located in a forced air heating system