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2. Furnace (central heating) - Wikipedia

   en.wikipedia.org/wiki/Furnace_(central_heating)

   A condensing gas furnace includes a sealed combustion area, combustion draft inducer and a secondary heat exchanger. The primary gain in efficiency for a condensing gas furnace, as compared to a mid-efficiency forced-air or forced-draft furnace, is the capture of latent heat from the exhaust gases in the secondary heat exchanger.

3. Central heating - Wikipedia

   en.wikipedia.org/wiki/Central_heating

   Electric heaters are usually part of a fan coil which is part of a central air conditioner. They circulate heat by blowing air across the heating element which is supplied to the furnace through return air ducts. Blowers in electric furnaces move air over one to five resistance coils or elements which are usually rated at five kilowatts.

4. Thermostat - Wikipedia

   en.wikipedia.org/wiki/Thermostat

   The reversing valve, controlled by the thermostat, causes the change-over from heat to cool. Residential heat pump thermostats generally have an "O" terminal to energize the reversing valve in cooling. Some residential and many commercial heat pump thermostats use a "B" terminal to energize the reversing valve in heating.

5. Forced-air gas - Wikipedia

   en.wikipedia.org/wiki/Forced-air_gas

   Older furnaces sometimes relied on gravity instead of a blower to circulate air. [1]Gas-fired forced-air furnaces have a burner in the furnace fueled by natural gas.A blower forces cold air through a heat exchanger and then through duct-work that distributes the hot air through the building. [2]

6. Zone valve - Wikipedia

   en.wikipedia.org/wiki/Zone_valve

   Zone valves can also be constructed using wax motors and a spring-return mechanism. In this case, the valve is normally closed by the force of the spring but can be opened by the force of the wax motor. Removal of electrical power re-closes the valve. This style of zone valve operates with a perfectly ordinary SPST thermostat.

7. Air preheater - Wikipedia

   en.wikipedia.org/wiki/Air_preheater

   The airflow is then reversed, so that the hot bricks heat up the incoming combustion air and fuel. For a glass-melting furnace, a regenerator sits on either side of the furnace, often forming an integral whole. For a blast furnace, the regenerators (commonly called Cowper stoves) sit separate to the furnace. A furnace needs no less than two ...