An HVAC system moves heat from one place to another using a loop of refrigerant, a blower fan, and a network of ducts. In cooling mode, it pulls heat out of your indoor air and dumps it outside. In heating mode (with a furnace or heat pump), it adds heat to the air before pushing it through your home. The entire process is kicked off by your thermostat, which sends low-voltage electrical signals to each component in a specific sequence.
The Four Key Components
Every central air conditioning system relies on four main parts working in a closed loop. Understanding what each one does is the key to reading any HVAC diagram.
- Compressor: Located in the outdoor unit, this pump squeezes refrigerant from a low-pressure vapor into a high-pressure, superheated vapor. This pressure increase is what drives the entire cycle forward.
- Condenser coil: Also in the outdoor unit, this coil releases the heat the refrigerant is carrying. As outdoor air blows across the coil, the hot refrigerant vapor cools down and turns into a high-pressure liquid.
- Expansion valve: This small device sits between the outdoor and indoor units. It forces the liquid refrigerant through a narrow opening, which drops its pressure and temperature dramatically. Think of it like the nozzle on a spray can: the liquid comes out much colder on the other side.
- Evaporator coil: Located inside your home (usually in or near the air handler), this coil is where cooling actually happens. The cold, low-pressure refrigerant flows through the coil and absorbs heat from the warm indoor air passing over it. The refrigerant boils back into a vapor, and the now-cooled air is pushed into your ducts.
On a diagram, these four components form a rectangle or oval loop. Refrigerant travels in one direction around the loop, changing between liquid and vapor states as it goes. The outdoor side releases heat. The indoor side absorbs it.
The Cooling Cycle, Step by Step
When your thermostat calls for cooling, the refrigerant begins circling through four stages. Following the loop from the compressor:
Stage 1: Compression. Low-pressure refrigerant vapor enters the compressor, which squeezes it into a high-pressure, high-temperature vapor. This is the “engine” of the cycle, and it’s why the outdoor unit draws the most electricity.
Stage 2: Condensation. The superheated vapor flows into the condenser coil outside. A fan blows outdoor air across the coil. Because the refrigerant is hotter than the outside air, heat flows out of the refrigerant and into the atmosphere. The refrigerant cools enough to condense into a high-pressure liquid.
Stage 3: Expansion. The liquid refrigerant passes through the expansion valve, where its pressure drops sharply. Its temperature drops along with it. The refrigerant is still liquid at this point, but it’s now cold, roughly 40°F or so in a typical residential system.
Stage 4: Evaporation. The cold liquid enters the evaporator coil inside your home. Warm indoor air is blown across the coil by the blower fan. The refrigerant absorbs heat from that air, causing the refrigerant to boil and change back into a low-pressure vapor. The cooled air continues into your ductwork and out through your vents. The vapor returns to the compressor, and the cycle starts again.
This is the same basic refrigeration cycle used in your refrigerator, your car’s AC, and commercial chillers. The only difference is scale.
How Heating Works
Most homes use one of two heating methods: a gas furnace or a heat pump. They appear differently on an HVAC diagram because they work in fundamentally different ways.
Gas Furnace
A gas furnace burns natural gas (or propane) inside a sealed combustion chamber. The flames heat a metal heat exchanger, which is a set of tubes or coils that get very hot. Your blower fan pushes indoor air across the outside of the heat exchanger, warming the air without it ever touching the combustion gases. The exhaust gases vent out through a flue pipe to the outside. On a diagram, the furnace is a separate box from the cooling components, typically drawn near the air handler with the blower fan shared between them.
Heat Pump
A heat pump uses the exact same refrigerant loop as an air conditioner, but adds one critical part: a reversing valve. This valve flips the direction of refrigerant flow so the indoor and outdoor coils swap jobs. In heating mode, the outdoor coil becomes the evaporator (absorbing heat from outdoor air, even in cold weather), and the indoor coil becomes the condenser (releasing that heat into your home). The reversing valve is controlled by an electrical signal from your thermostat. When your temperature setting calls for a mode switch, the thermostat activates a solenoid that moves a slide inside the valve, redirecting the refrigerant.
On a diagram, a heat pump system looks nearly identical to a cooling-only system, with the reversing valve drawn as a small rectangle on the refrigerant line between the compressor and the coils, and arrows showing that flow can go in either direction.
How the Thermostat Controls Everything
The thermostat is the brain of the system, but it works in a surprisingly simple way. A small transformer (usually located in or near the furnace) produces 24 volts of low-voltage power. Two wires carry this power: the R wire (hot) and the C wire (common). The thermostat acts like a set of switches between the R wire and several control wires, each one connected to a different component.
When you set the system to cool, the thermostat connects the R wire to two other wires simultaneously. The G wire completes a circuit to the blower relay, turning on the indoor fan. The Y wire completes a circuit to the compressor relay, turning on the compressor and the outdoor fan. Both kick on at the same time.
When you set the system to heat (in a furnace setup), the thermostat connects the R wire to the W wire, which completes a circuit to the furnace’s heating relay. The blower typically starts on its own once the heat exchanger reaches a set temperature. In a heat pump system, the thermostat also sends the signal to the reversing valve to switch refrigerant direction.
On a wiring diagram, this looks like a central thermostat box with R, G, Y, W, and C terminals, each with a single wire running out to its respective relay. It’s essentially a set of light switches, each one powering a different part of the system.
Dehumidification and Moisture Removal
Cooling does double duty: it also removes moisture from your indoor air. When warm, humid air passes over the cold evaporator coil, the air temperature drops below its dew point. Water vapor condenses on the coil surface, the same way water beads form on a cold glass in summer. That condensation drips into a drain pan beneath the coil and flows out through a drain line, typically to a floor drain or the outside of the house. This is why running your AC makes a room feel more comfortable beyond just the temperature drop: it’s pulling water out of the air.
Where Air Filters Fit In
Before indoor air reaches the evaporator coil, it passes through a filter. This protects the coil from dust buildup and improves your air quality. Filters are rated on the MERV scale (Minimum Efficiency Reporting Value), which measures how well they capture particles between 0.3 and 10 microns.
A basic MERV 1 through 4 filter catches less than 20% of large particles (dust, pollen) and exists mainly to protect the equipment. A MERV 8 filter captures at least 70% of particles in the 3 to 10 micron range and at least 20% of smaller particles down to 1 micron. This is what most homes use. The EPA recommends at least a MERV 13 filter if your system can handle it, which captures 50% or more of the finest particles (0.3 to 1.0 microns), including some bacteria and smoke. Higher MERV ratings restrict more airflow, though, so you need to match the filter to what your blower fan can push through.
On a system diagram, the filter is drawn as a flat rectangle at the return air intake, right before the evaporator coil. It’s the first thing indoor air hits when it enters the system.
How Long Each Component Lasts
The different parts of your HVAC system age at different rates. Central air conditioners and heat pumps typically last 10 to 15 years. Gas, electric, or oil furnaces generally last 15 to 25 years. The furnace outlasts the cooling equipment because it has fewer mechanical components under stress. The compressor is usually the first major part to fail in a cooling system, since it runs under high pressure and heat every cycle. Blower motors, expansion valves, and coils can also wear out, but they’re typically repairable without replacing the whole unit.