How Does Air Conditioning Work?

An air conditioner does not produce cold air. It removes heat (and moisture) from the air inside your home and dumps that heat outside. The mechanism is a closed loop of refrigerant that changes between liquid and gas as it absorbs and releases heat, driven by a compressor and managed by a thermostat. For southern New Hampshire homes, where July and August humidity is the main comfort challenge, the moisture-removal side of what an AC does matters as much as the temperature-removal side.

Here is what each component is doing inside a typical central AC or heat pump.

The refrigerant cycle in plain English

Modern AC systems use the same basic refrigeration cycle that has been around for a century, with refinements in equipment, refrigerants, and controls. The cycle has four main stages:

1. Compress: the compressor raises the refrigerant's pressure (and temperature) as a gas.
2. Condense: the hot, pressurized refrigerant releases its heat to outdoor air and condenses to a liquid.
3. Expand: the liquid passes through a restriction that drops its pressure (and temperature) dramatically.
4. Evaporate: the cold, low-pressure refrigerant absorbs heat from indoor air and evaporates back to a gas.

The cycle repeats continuously while the system is running. The thermostat starts and stops the cycle to maintain the setpoint.

The main components

Compressor

Located in the outdoor unit. Compresses the refrigerant gas, raising its temperature above outdoor air temperature so it can reject heat. The compressor is the most expensive single component and the hardest-working part of the system. Compressor failure is the most common reason an older AC reaches end of life.

Modern variable-speed inverter compressors run at different speeds based on demand, which improves efficiency and comfort compared to old single-speed compressors that just cycle on and off.

Condenser coil

Located in the outdoor unit. A network of finned copper tubing that lets the hot refrigerant give off heat to the outdoor air. The outdoor fan pulls air through the coil to speed up heat transfer. A coil coated in pollen, cottonwood, grass clippings, or pet hair cannot reject heat efficiently, which raises run time and electricity use.

Expansion valve (or metering device)

A precise restriction that drops the refrigerant's pressure as it enters the indoor coil. Modern systems use electronic expansion valves (EEVs) that adjust in real time. Older systems use simpler thermal expansion valves (TXVs) or fixed-orifice metering devices.

Evaporator coil

Located in the indoor air handler or above the furnace. A finned copper coil where the cold refrigerant absorbs heat from indoor air as the blower pushes air across it. Because the coil surface is below the dew point of the air, water vapor in the air condenses on the coil and drains away as liquid. This is how the AC removes humidity.

Blower

The indoor fan that moves household air across the evaporator coil and through the ductwork. Variable-speed blowers ramp up and down smoothly with the compressor.

Thermostat

The control system that compares the indoor temperature to your setpoint and tells the AC to run or stop. Modern smart thermostats add scheduling, occupancy sensing, and humidity control.

Refrigerant

The working fluid in the closed loop. Older systems used R-22 (retired from production in 2020). The dominant refrigerant in installed systems today is R-410A, which is now being phased out in favor of lower-GWP refrigerants like R-32 and R-454B starting in 2025.

Why humidity removal matters in NH

Southern New Hampshire summer dew points routinely climb into the high 60s and low 70s. At those dew points, indoor air with high relative humidity feels hot and sticky even at 76°F. An AC that removes humidity well makes the home feel cool at higher setpoints, which both improves comfort and reduces electricity use.

Two AC design factors affect humidity removal:

• Run time: moisture is removed from the air slowly as it passes the indoor coil. Short cycles (typical of oversized AC) drop temperature quickly but do not have time to wring out moisture. The home feels cold and clammy.
• Coil temperature: the colder the indoor coil, the more aggressively water condenses on it. Properly charged systems maintain coil temperatures in the right range for both cooling and dehumidification.

Why sizing and installation matter

The components above can be assembled into a system that cools efficiently and quietly for 15+ years, or assembled into a system that short-cycles, never quite gets comfortable, and dies in 8 years. The difference comes from:

• Manual J load calculation that sizes the equipment to the home's actual heat gain
• Properly designed and sealed ductwork that delivers conditioned air where it is needed
• Correct refrigerant charge at commissioning
• Proper airflow across the indoor coil (static pressure within manufacturer specs)
• Annual maintenance to keep all of the above in spec

Schedule an AC consultation

If your AC is past 10 years old, struggles to keep up on hot days, or feels cool but humid, contact A.J. LeBlanc Heating. Serving New Hampshire families since 1928.

Frequently Asked Questions

What is refrigerant?

The working fluid in a closed-loop AC system. It changes between liquid and gas as it absorbs and releases heat. R-410A is the most common refrigerant in installed NH systems today; newer installations use R-32 or R-454B.

Why does my AC drip water?

The AC removes humidity from indoor air. The water condenses on the indoor coil and drains away through a condensate line. A clogged drain or failed condensate pump causes water to back up, which is the most common cause of indoor water damage from AC.

Does an air conditioner produce cold?

No. It moves heat from inside to outside. The cooler indoor air is the result of removing heat, not adding cold.

Why is my AC blowing warm air?

Common causes include low refrigerant (likely a leak), a dirty air filter restricting airflow, a frozen evaporator coil, a failed capacitor on the outdoor unit, or a thermostat set incorrectly. A technician can diagnose quickly.

How long does a central AC last?

12 to 15 years with annual maintenance. Without maintenance, often 8 to 10 years. The compressor is usually the failure point.