Your HVAC
How air conditioning works
Follow heat, moisture, air, and refrigerant through a central split air conditioner without turning the explanation into repair instructions.
An air conditioner does not make cold air — it moves heat. Refrigerant picks up heat from indoor air at the evaporator coil and releases it outside at the condenser coil.
Source: U.S. DOE — Energy Saver 101, Home Cooling infographic, accessed 2026-10-16
Four loops work together
1. House air moves across the indoor coil
The return side draws room air through the filter. The blower moves it across the evaporator coil and back through supply ducts. A dirty filter, blocked return, wrong blower setting, or damaged duct can change this air loop even when the refrigeration equipment runs.
2. Refrigerant carries heat
Low-pressure refrigerant in the indoor coil absorbs heat and changes state. The compressor raises its pressure and temperature. At the outdoor condenser coil, refrigerant releases heat to outdoor air and condenses. An expansion device lowers pressure before refrigerant returns indoors.
DOE summarizes the same central process: the indoor cooling coil removes heat and humidity, the compressor moves refrigerant, and the outdoor coil and fan release collected heat (DOE). Refrigerant is contained in a sealed circuit; it is not fuel that routine operation consumes.
3. Water leaves through the condensate system
When the indoor coil surface is below the air’s dew point, water condenses on it. That water should fall into a pan and leave through the drain. How much moisture is removed depends on indoor conditions, coil temperature, airflow, and runtime. A cold house is therefore not automatically a dry house.
4. Controls decide when equipment operates
The thermostat requests cooling, while control boards and safety devices coordinate the indoor blower and outdoor equipment. Modern systems may run at one stage, multiple stages, or variable capacity. Long operation at low capacity can be normal; rapid cycling or failure to control temperature needs context.
Where the main parts usually are
| Part | Typical location | Job |
|---|---|---|
| Filter and return | Return grille or indoor cabinet | Protect equipment and remove particles |
| Blower | Indoor air handler or furnace | Move air through coil and ducts |
| Evaporator coil | Indoor unit | Absorb heat; condense moisture |
| Compressor | Outdoor unit | Circulate and compress refrigerant |
| Condenser coil and fan | Outdoor unit | Reject heat outdoors |
| Expansion device | Near indoor coil | Control refrigerant flow and pressure drop |
| Condensate pan and drain | At indoor coil | Carry condensed water away |
Split systems place indoor and outdoor components in separate cabinets. Packaged systems put major components in one cabinet. DOE also classifies some ductless mini- and multi-splits as central air conditioners or heat pumps (DOE).
A symptom does not name the failed part
“The AC runs but the house is hot” can involve the air loop, refrigeration loop, controls, condensate effects, ducts, or a house load above what the system can handle. Observe what runs, what the thermostat shows, whether airflow changed, and whether water, ice, noise, or odors are present. Do not jump from a cycle diagram to opening equipment.
Sources
- DOE — Energy Saver 101 home cooling, accessed 8/12/2026
- DOE — Consumer central air conditioners and heat pumps, accessed 8/12/2026