What each part of your cooling system does
Your air conditioning system has five main working parts: the thermostat, the compressor, the condenser, the evaporator, and the refrigerant that moves between them. Each one has a specific job, and they work together in a cycle. Understanding what each part does makes it easier to spot problems, talk to a repair person, and decide whether a repair is worth the cost.
The system pulls warm air from inside your home, cools it, and pushes it back in. It does this by circulating refrigerant — a special liquid that absorbs heat easily — through a closed loop. When something breaks, it is usually one of these five parts, and knowing which one helps you understand what the repair involves and how long it might take.
Key Takeaways
- The thermostat is the control center that tells your system when to turn on and off based on the temperature you set.
- The compressor and condenser work together outside your home to pressurize refrigerant and release the heat it has absorbed.
- The evaporator sits inside your home and absorbs heat from indoor air, which is then blown through your ducts.
- Refrigerant is the liquid that carries heat out of your home; it cycles continuously and should never leak or run low.
- A clogged filter, frozen evaporator coil, or broken compressor are the most common reasons a system stops cooling properly.
The thermostat: your system's brain
The thermostat is the device on your wall where you set the temperature. It measures the air temperature in your home and tells the rest of the system whether to run or stop. When the air gets warmer than the temperature you set, the thermostat signals the system to start. When the air cools down to your set temperature, it signals the system to stop.
Modern thermostats can be programmable (you set different temperatures for different times of day) or smart (they learn your patterns and adjust automatically). Older thermostats are manual — you turn a dial and it stays at that setting. The thermostat itself does not cool anything; it is just the switch that tells the compressor and fan when to work.
If your system is not turning on at all, the thermostat is often the first thing to check. Make sure it is set to cooling mode, not heating or off. Check that the batteries are not dead if it is a wireless model. If the thermostat looks fine but the system still will not start, the problem is likely in the compressor or electrical wiring.
The compressor and condenser: the outdoor unit
The compressor and condenser are both located in the large metal box outside your home — this is called the outdoor unit. The compressor is an electric pump that pressurizes the refrigerant, which makes it hot. The condenser is a coil of metal fins that looks like a car radiator; it cools the hot refrigerant back down by blowing air across it with a fan.
Think of it this way: the refrigerant arrives at the compressor as a cool liquid. The compressor squeezes it, which heats it up. The hot refrigerant then flows into the condenser coil, where outdoor air blows across it and pulls the heat away. The refrigerant cools down and becomes liquid again, ready to go back inside and pick up more heat from your home.
The outdoor unit is loud when it is running because the compressor and fan motor are both working hard. If the outdoor unit is not running at all, the compressor may have failed, or there may be an electrical problem. If it is running but your home is not getting cold, the refrigerant may be low, or the condenser coil may be dirty and not releasing heat properly.
The evaporator: the indoor cooling coil
The evaporator is a coil of metal tubing located inside your home, usually in a cabinet near your furnace or in an attic or basement. This is where the actual cooling happens. The cool liquid refrigerant flows through the evaporator coil, and warm air from your home is blown across it by a fan. The refrigerant absorbs the heat from that air, and the cooled air is then pushed through your ducts and into your rooms.
The evaporator coil gets very cold while it is running — cold enough that moisture in the air condenses on it, just like water beads up on a cold glass. That water drains out through a small pipe called a condensate drain line. If this drain line gets clogged, water backs up and can damage your system or create mold.
If your system is running but no cold air is coming out of your vents, the evaporator coil may be frozen solid. This usually happens when airflow is blocked — often by a dirty filter — so the coil gets too cold. Turning off the system and letting the coil thaw, then replacing the filter, usually fixes this. If it keeps happening, there may be a refrigerant leak or a problem with the fan.
Refrigerant: the heat-carrying liquid
Refrigerant is a special chemical liquid that boils at very low temperatures. This property makes it perfect for carrying heat: it absorbs heat easily when it is cold and releases heat easily when it is pressurized and hot. Common refrigerants include R-22 (an older type being phased out), R-410A, and R-32. Your system is sealed and should contain the same amount of refrigerant for its entire life — it does not get used up like gasoline.
If your refrigerant is low, it means there is a leak somewhere in the system. A leak is a serious problem because the system cannot work properly without the right amount of refrigerant, and refrigerant is a hazardous chemical that should only be handled by a licensed technician. You cannot refill refrigerant yourself — it is illegal in most places and dangerous.
Signs of a refrigerant leak include the system running but not cooling, ice forming on the outdoor unit, or a hissing sound coming from the pipes. If you suspect a leak, turn off the system and call a repair technician. They will locate the leak, repair it, and refill the refrigerant to the correct level.
The air filter and ductwork: keeping air moving
The air filter is a flat or pleated screen, usually located in a slot near your furnace or in a return air vent. It catches dust, pollen, and other particles so they do not get into your system or circulate through your home. A clean filter is essential for good airflow; a clogged filter makes the system work harder and can cause the evaporator coil to freeze.
You should check your filter every month and replace it when it looks dirty — usually every one to three months, depending on whether you have pets, allergies, or construction happening nearby. Filters are inexpensive and straightforward to replace yourself; the size is printed on the frame, and you can buy replacements at any hardware store.
Your ductwork is the network of metal or flexible tubes that carry cooled air from the indoor unit to each room and return warm air back to the system. Leaks in ducts waste energy because cooled air escapes into walls or attics instead of reaching your rooms. Ducts can also get blocked by debris or crushed if something is stored against them. If some rooms are much colder than others, a blocked or leaky duct is often the cause.
Common problems and what they mean
A system that runs but does not cool usually has one of three problems: a clogged filter blocking airflow, a refrigerant leak, or a frozen evaporator coil. Start by checking and replacing the filter. If that does not help within a few hours, turn off the system and let the coil thaw overnight, then turn it back on. If it still does not cool, call a technician — there is likely a refrigerant leak or a failing compressor.
A system that will not turn on at all may have a thermostat problem, a tripped circuit breaker, or a failed compressor. Check that the thermostat is set to cooling mode and the temperature is set lower than the current room temperature. Check your electrical panel for a tripped breaker labeled for the air conditioner and flip it back on if needed. If the system still will not start, the compressor has likely failed and needs replacement.
A system that is very loud or making unusual noises may have a failing compressor, a fan blade hitting something, or a refrigerant leak. Hissing sounds often mean a leak. Grinding or squealing usually means a motor bearing is wearing out. These problems get worse over time, so it is worth having a technician look at it sooner rather than later.
When to call a repair technician
You can handle some cooling system problems yourself: replacing a filter, checking that the thermostat is set correctly, clearing a blocked return air vent, or cleaning debris from around the outdoor unit. Everything else — refrigerant leaks, compressor problems, electrical issues, or coil damage — requires a licensed technician.
Refrigerant work is especially important to leave to professionals. Handling refrigerant without a license is illegal in most places, and improper handling can damage the environment and create safety hazards. A technician will have the tools to find leaks, the license to handle refrigerant, and the knowledge to refill your system correctly.
When you call for service, describe what you have noticed: whether the system is running or not, whether any air is coming out, whether you hear unusual sounds, and how long the problem has been happening. This information helps the technician come prepared with the right parts and tools.
Frequently Asked Questions
How often should I replace my air filter?
Check your filter every month and replace it when it looks dirty or clogged. For most homes, this means every one to three months. If you have pets, allergies, or live in a dusty area, you may need to replace it more often. A clean filter keeps your system running efficiently and prevents the evaporator coil from freezing.
What does it mean if my outdoor unit is not running?
If the outdoor unit is completely silent and not running, check that the thermostat is set to cooling mode and the temperature is set lower than the current room temperature. If that is correct, check your electrical panel for a tripped breaker. If the breaker is fine, the compressor may have failed or there may be an electrical wiring problem — call a technician to diagnose it.
Can I refill refrigerant myself?
No. Refrigerant work requires a license in most places, and handling it improperly is dangerous and illegal. If your system is low on refrigerant, there is a leak somewhere. A technician will find and repair the leak, then refill the system to the correct level. Attempting to refill it yourself will not fix the underlying leak and can damage the environment.
Why is my system running but not cooling?
Start by replacing your air filter — a clogged filter is the most common cause. If that does not help, turn off the system and let it sit for several hours so the evaporator coil can thaw if it is frozen. If the system still does not cool after these steps, there is likely a refrigerant leak or a failing compressor, and you will need a technician to diagnose it.
What is that water dripping from my outdoor unit?
Water dripping from the outdoor unit is normal — it is condensation that has drained from the indoor evaporator coil through the condensate drain line. If you see water pooling around the indoor unit or dripping from your ceiling, the drain line is clogged and needs to be cleared by a technician or a handy homeowner with a wet vacuum.