How Does An Air Conditioner & Heat Pump Work? – Basic HVAC Tutorial

Written By: Abbie Stancato

The History of the Air Conditioner

The air conditioner was invented in 1902 by the founder of Carrier Corporation — Willis Carrier. The primary design was for Humidity Control, not Temperature Control.

The Simplistic Definition of an Air Conditioner

An A/C system removes heat from inside your home and transfers it outside. This article is intended to explain the most basic functionality and operation of an Air Conditioner And a Heat Pump.

The Basic Physics Behind The Process…

Pressure and temperature are directly related. When a gas is under high pressure, the temperature of that gas increases. Inversely, reduce the pressure, and the temperature of the gas decreases. An Example: Water boils at 212° Fahrenheit at a standard sea level altitude pressure of 14.7 Pounds Per Square Inch. Go to the mountains where the pressure is lower, and you’ll discover that water boils at a lower temperature. The higher the altitude, the lower the air pressure, the lower the boiling point. The liquid refrigerant inside the system called R410A boils at only 55.3° Fahrenheit. As the boiling point of water is affected by changing environmental pressure, so are the boiling points of refrigerant in regard to the variance of applied pressure throughout an air conditioning system. This is the entire premise by which an Air Conditioner operates.

Inside Unit

Two Basic Parts of an Air Conditioning System

Outside Unit

Most home split system air conditioners have two basic parts: an Outdoor Unit (Compressor – Condenser Coil – Fan) which sits next to your home and an Indoor Unit (Evaporator Coil – Expansion Valve – Fan) that’s connected to a central duct near your furnace.

The Explanation

The thermostat inside your home is set to 75°F. When the temperature inside the home exceeds 75°F, a fan inside the home turns on. Its purpose is to draw air from inside the home, into the ducts leading to the Inside Unit while forcing air through air ducts into the rooms of the home. As air is moved throughout the Inside Unit, it’s forced through a winding tube section called an Evaporator Coil / Heat Exchange Section. This evaporator coil is where cold temperatures are introduced to the air inside your home. It reduces the temperature of the incoming supply air.

So how do the cool temperatures get there? Let’s start with the Outside Unit containing the Compressor, Condenser Coil, and Fan. As the refrigerant travels throughout the AC System, it repeatedly changes from a gas to a liquid state.

The R410A refrigerant is in a gas state as it enters the outside unit. The gas enters into the compressor, which pressurizes the gas. The gas enters the compressor at about 50°F. As it exits the compressor it succumbs to a substantial pressure/temperature increase. The gas increases in temperature to approximately 150°F. If you’ve ever noticed all the very delicate air fins creating the walls of the Outside Unit, you’re looking at the condenser coil. The gas enters the coil and is snaked through the long winding coils throughout the outside of the unit. The fan on top of the Outside Unit draws outside ambient temperature air through the coils to remove heat, dispelling it through the top. Place your hand in the air stream of the fan output and you’ll experience just how much heat is being removed. By the time the gas makes its way through the coils, it has cooled to a liquid state. It is still under high pressure and still relatively hot… About 100°F. I’ll bet you’re wondering, how can it become a liquid when it’s 45°F above its boiling point… It converts to a liquid due to the high pressure exerted upon it at that time! The liquid travels through a tube from the Outside Unit to the Inside Unit.

This 100°F liquid encounters a device called an Expansion Value. This valve is designed to reduce the pressure of the refrigerant… cooling the liquid. Removing the pressure has the effect of lowering the temperature of the refrigerant. The departing temperature of the now liquid refrigerant is approximately 20°F as it enters the Evaporator Coil. The cold R410A Refrigerant travels through the Evaporator Coils and begins to cool the air of your home blowing across it. The coils produce approximately 55°F air. Therefore, the warmer 75°F home air is absorbed into the liquid refrigerant causing it to reach the boiling point where it returns to a low temperature – low-pressure gas state. The gas travels back to the compressor outside, and the process is repeated.

Facts And Maintenance Tips


  • Most A/C Units are only capable of only a 15 to 20 temperature differential – In layman’s terms, if the outside temperature is 95°F, the best you can hope for inside the home is 75°F to 80°F. Therefore, any lack of maintenance will cause your system to be less effective and cost more than necessary.
  • According to the Department of Energy, 78° Fahrenheit is the sweet spot for air conditioners to balance energy savings and comfort when people are at home and need cooling.
  • An A/C system is a closed system. Meaning unless there is a leak somewhere, the fluid inside should always be present and will not dissipate.

Maintaining The Condensing Unit

  • Clean The Condensing Coil and Protect Those Delicate Air Fins… Now that you understand how the system operates, you should understand the need for simple maintenance. Lawnmowers and weed eaters are the two of the biggest foes to your system. Remember, the air is being sucked to the inside of the condensing unit. Grass, cottonwood, leaves, and any direct contact which damages the fins reduce airflow across them is costing you money. A simple water hose and/or shop vacuum can easily remove much of the dirt and debris.
  • Keep pets away… A pet’s urine is very acidic. it can cause serious damage to the coils over time.
  • Protect the incoming (suction line), and outgoing (liquid line) copper lines… A typical residential AC unit has two copper lines which both exit the building’s exterior and run to the Outside Condenser Unit. Only one of these copper pipes, the cold line, needs to be insulated. This is called the suction pipe and is typically the larger of the two. Insulation will prevent outside heat from affecting the temperature of the incoming gas going into the compressor. There is no need to insulate the smaller, warmer copper pipe, often called the liquid line.

Replace Air Filter

  • A dirty filter will restrict airflow across the evaporating coil. Replace filters regularly!

What is a Heat Pump?

We have all heard of Heat Pumps, but what are they, and how do they relate to an Air Conditioner? A Heat Pump is basically an AC system in reverse. I previously noted the heat you feel in the summer when you place your hand in the air stream of the exhaust fan of the Outside Condenser Unit. When the cooler days of fall and winter hit, a Heat Pump system reverses. It sends the cold temperatures from inside the home to the outside unit. That’s why you’ll see ice on the Outside Condenser Unit in the winter months. Heat Pumps are cost-effective because they don’t require a dedicated furnace. The problem with Heat Pumps is that they can’t often handle very cold temperatures without the assistance of an electric heating coil. This method of heating is very expensive during very cold winter days and should be considered when choosing between a Heat Pump and a dedicated furnace.

Who We Are

This article is courtesy of MicroMetl Corporation. Established in 1965… MicroMetl is the leader in HVAC Accessories. Specialist in Commercial HVAC Accessories & Energy Recovery.