Air conditioning and heat pump systems are part of everyday life—but their operation is often misunderstood. This guide breaks down the basics in simple, practical terms so anyone can understand how these systems keep indoor spaces comfortable.
A Brief History of Air Conditioning
The modern air conditioner was invented in 1902 by Willis Carrier, founder of the Carrier Corporation. Surprisingly, the earliest systems were created to control humidity, not temperature. Cooling was a secondary benefit that later became the primary focus of residential and commercial comfort systems.
The Simple Definition
An air conditioner removes heat from inside your home and transfers it outside.
A heat pump can do this in reverse—moving heat into your home during cooler months.
This tutorial explains the fundamental physics and operation of both systems.
The Basic Physics Behind Cooling
A/C systems rely on a simple relationship:
Pressure and temperature are directly related.
- Increase pressure → temperature increases
- Reduce pressure → temperature decreases
This principle explains how refrigeration works. Just as water boils at different temperatures depending on altitude, refrigerant boiling points change based on pressure.
For example:
- Water boils at 212°F at sea level
- Refrigerant R410A boils at 55.3°F under standard pressure
By controlling pressure, air conditioning systems manipulate the state and temperature of refrigerant to absorb and release heat.
The Two Main Parts of a Residential A/C System
Most home A/C systems include:
Outdoor Unit
- Compressor
- Condenser Coil
- Fan
Indoor Unit
- Evaporator Coil
- Expansion Valve
- Blower/Fan
The thermostat controls when the system turns on and off.
How an Air Conditioner Works – Step by Step
Let’s walk through the cooling cycle:
- Thermostat calls for cooling
When indoor temperature rises above the thermostat setting (e.g., 75°F), the blower fan turns on.
- Indoor air is pulled through the evaporator coil
Warm air travels through ducts and across the evaporator coil, which contains cold refrigerant.
- This coil cools the air
- Cooled air is distributed back through the home
- Refrigerant absorbs heat and boils
As warm air passes over the evaporator coil, the refrigerant absorbs heat and turns from a cold liquid to a low-pressure gas.
- Warm gas travels to the outdoor unit
Once inside the compressor:
- The gas is squeezed (pressurized)
- Pressure increases
- Temperature rises dramatically
The gas temperature jumps from ~50°F to around 150°F.
- Heat is released outdoors
The hot gas enters the condenser coil—the finned coil on the outside of the unit.
A large fan draws outdoor air across the coil to remove heat.
You can feel the hot air being expelled from the top of the unit.
- Refrigerant condenses into a hot liquid
As it travels through the condenser coil, the gas cools and becomes a liquid again—still under high pressure, at approximately 100°F.
- Refrigerant passes through the expansion valve
The expansion valve reduces pressure, dropping the refrigerant temperature to roughly 20°F.
It then enters the evaporator coil and the cycle repeats.
Important Facts & Maintenance Tips
Temperature Limitations
Most A/C systems can only achieve a 15–20°F temperature difference between outdoor and indoor conditions.
If it’s 95°F outside, 75–80°F indoors is realistic.
Energy Efficiency
According to the U.S. Department of Energy, 78°F is the recommended thermostat setting to balance comfort and energy savings.
Closed System
Refrigerant does not evaporate or dissipate unless there is a leak.
Basic Maintenance Tips
1. Clean the Condenser Coil
The outdoor unit draws air inward, which means:
- Grass clippings
- Leaves
- Cottonwood
- Dirt
…get trapped on the coil.
This blocks airflow and reduces cooling performance. Clean using:
- A garden hose
- A soft brush
- A shop vacuum
2. Protect the Coil
Keep pets away—urine is highly corrosive and damages coils.
3. Insulate the Suction Line
The larger copper pipe (suction line) should be insulated to maintain refrigerant temperature.
4. Replace Air Filters
A dirty filter restricts airflow, decreases performance, and increases energy costs.
What Is a Heat Pump?
A heat pump is essentially an air conditioner that can reverse its cycle.
- In summer → it removes heat from inside and releases it outside
- In winter → it removes heat from outside air and moves it indoors
This is why outdoor heat pump units may accumulate frost or ice in winter.
Pros:
- Very energy-efficient for mild climates
- Eliminates the need for a dedicated furnace
Cons:
- Struggles in very cold temperatures
- Often requires supplemental electric heat—expensive to operate
Who We Are
This educational content is brought to you by MicroMetl Corporation, established in 1965.
MicroMetl continues to lead the industry in commercial HVAC accessories and is committed to supporting technicians, engineers, and contractors with practical, accurate HVAC knowledge.
