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 matters as much as the temperature-removal side.

The refrigerant cycle in plain English

Modern AC and heat pump systems use the same century-old refrigeration cycle, in four 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 runs. The thermostat starts and stops it to hold the setpoint.

The main components

• Compressor (outdoor unit): raises the refrigerant's temperature above outdoor air so it can reject heat. It is the most expensive, hardest-working part, and compressor failure is the most common reason an older AC reaches end of life. Variable-speed inverter compressors adjust to demand, improving efficiency and comfort over old single-speed units that just cycle on and off.
• Condenser coil (outdoor unit): finned copper tubing that releases the hot refrigerant's heat to outdoor air, with the outdoor fan pulling air through it. A coil clogged with pollen, cottonwood, grass clippings, or pet hair cannot reject heat efficiently, which raises run time and electricity use.
• Expansion valve (metering device): a precise restriction that drops refrigerant pressure entering the indoor coil. Modern systems use electronic expansion valves (EEVs); older ones use thermal expansion valves (TXVs) or fixed-orifice devices.
• Evaporator coil (indoor): a finned copper coil where cold refrigerant absorbs heat from indoor air. Because the coil sits below the air's dew point, water vapor condenses on it and drains away. This is how the AC removes humidity.
• Blower (indoor): moves household air across the evaporator coil and through the ductwork. Variable-speed blowers ramp up and down with the compressor.
• Thermostat: compares indoor temperature to your setpoint and tells the AC to run or stop. 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). Most installed systems today use R-410A, now being phased out for 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, humid indoor air feels hot and sticky even at 76°F. An AC that removes humidity well keeps the home comfortable at higher setpoints, which cuts electricity use. Two design factors drive it:

• Run time: moisture comes out of the air slowly as it passes the coil. Short cycles (typical of oversized AC) drop temperature fast but do not wring out moisture, so the home feels cold and clammy.
• Coil temperature: the colder the indoor coil, the more aggressively water condenses on it. A properly charged system holds coil temperatures in the right range for both cooling and dehumidification.

Why sizing and installation matter

The same components can cool quietly for 15+ years or short-cycle, never get comfortable, and die in 8. The difference is a Manual J load calculation sized to the home's heat gain, properly designed and sealed ductwork, the correct refrigerant charge at commissioning, proper airflow across the indoor coil (static pressure within manufacturer specs), and annual maintenance.

Schedule an AC consultation

If your AC is past 10 years old, struggles on hot days, or feels cool but humid, contact A.J. LeBlanc Heating or call 603-623-0412. 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. That water condenses on the indoor coil and drains away through a condensate line. A clogged drain or failed condensate pump backs water up, 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.