HVAC Basics: Central heating and cooling

By Anne Fonda

In a nutshell, a central heating, ventilation, and air conditioning (HVAC) system heats or cools your home by blowing heated or cooled air through your ductwork. While the process seems simple, achieving the perfect temperature (and humidity) in your home requires a complex interplay of various HVAC equipment. 

Understanding the central heating and cooling processes will help you achieve a higher level of home comfort and better maintain your HVAC system.

A central air conditioning system works by taking heat and humidity from your home and releasing it outside, to ultimately drop the temperature in your home. This happens during the refrigeration cycle.

1. A fan from the indoor portion of your AC pulls room temperature air into the return air vent system. 
2. This air is blown over the evaporator coil. The evaporator coil is filled with cold liquid refrigerant that absorbs the heat from the air, which turns the refrigerant into a gas. 
3. The newly cooled air is blown back into your home via supply ducts and supply registers. 
4. The refrigerant continues flowing into the compressor, which pressurizes the gas and sends it to the condenser coil. 
5. The condenser coil releases the heat from the refrigerant into the outdoor air. 
6. When the refrigerant leaves the condenser coil and releases most of its heat outside, it’s still too hot to re-enter the evaporator coil. The expansion valve works by decreasing the pressure of the refrigerant and cooling it down even more before sending it back to the evaporator coil.  
7. The process starts again. 

There are two main types of central cooling systems – the split system and the packaged system. 

A traditional split system is composed of a thermostat, an outdoor AC unit or heat pump, an indoor unit (air handler or furnace), copper refrigerant lines that connect the outdoor and indoor units, and ductwork throughout the home. While a split-system AC unit only cools, a heat pump can reverse the flow of refrigerant to heat the home as well. 

A packaged air conditioner is the other type of central cooling. A packaged unit works the same way as a split system AC or heat pump, except the whole unit sits outside the house; there is no equipment inside the home. The packaged unit takes air from your house through the return air vents and ductwork, cools it, and then sends it back to the house through the supply ductwork. A packaged system can house both the heating and cooling systems. 

Explore central cooling systems

Central heating systems can work in the same general way as central cooling systems by moving air around to change the temperature in your home. Instead of moving hot air out, central heating systems either burn fuel to heat the air (gas furnace) or extract heat from the outdoor air (heat pump). 

There are two main options for central heating systems: heat pumps and furnaces.

All central heating processes start the same way central cooling processes do: a thermostat registers the temperature and signals the HVAC system to start heating. 

Heat pumps reverse the refrigeration cycle from cooling to heating as the seasons change and you switch to heat mode on the thermostat. The heat pump absorbs heat energy from the outside air and transfers it into the home to warm it up.  

Furnaces, on the other hand, function in a totally different way than other HVAC products. While the thermostat still kicks off the heating process, furnaces burn fuel to produce heat. The heating process works like this, with some differences based on the efficiency of your furnace. 

1. The thermostat signals the need for heat. 
2. Fuel comes into the furnace through a pipe, and an ignitor lights the furnace burner. 
3. The furnace burner heats up the heat exchanger. 
4. Cold air from your home is warmed as it passes over the furnace heat exchanger. 
5. A blower fan inside the furnace directs the newly warmed air through the various supply ducts and registers in your home. 
6. Flue gases from combustion are piped out of the furnace through a vent and exit your home. 
7. More cold air is directed from your home into the furnace via the return ducts. 
8. The process repeats itself until your home is warmed, at which point the thermostat signals the furnace to stop. 

Anne Fonda, Content Writer

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A Content Writer with Trane Technologies, Anne Fonda researches topics and writes for Trane® and associated residential HVAC brands. She works in collaboration with Trane Technologies subject matter experts, offering easy-to-understand, informative content on complex topics. Her goal is to help consumers make informed decisions on the products and services they need.

She has written for HVAC and other service provider websites for over 16 years. Before transitioning to web content writing, Anne had a 14-year stint as an award-winning journalist. She graduated cum laude from the University of Missouri-Columbia School of Journalism.

When she’s not working, Anne enjoys playing word games, reading, gardening, spending time with family, and visiting gardens and museums.

Expert review by Kimberly Sexton, Senior Systems Engineer