How cooling and heat pumps move heat

Everything an air conditioner does rests on one simple idea about heat. Heat is energy on the move, and it moves because of a difference in temperature. It naturally flows from the warmer material to the cooler one until they balance. To cool a room, the system has to give the room's heat somewhere cooler to go.

Two kinds of heat matter here. Heat that changes a substance's temperature is called sensible heat, and heat that changes it from one state to another, such as liquid to gas, is called latent heat. As room air passes over the cold indoor coil, it gives up both kinds. That is how one machine both cools the air and pulls water out of it. Cooling capacity is often given in tons: one ton is 12,000 Btu per hour of heat removed.

Source: U.S. Department of Energy

The work is done by a fluid called refrigerant and a few parts it passes through. Refrigerant is the fluid that soaks up heat and gives it off again at a higher temperature and pressure, usually by changing between liquid and gas. The compressor is one of four essential parts of the basic refrigeration system; the others are the condenser, the evaporator and the expansion device.

In a room air conditioner, those parts are the evaporator (the indoor coil), the compressor, the condenser (the outdoor coil) and a thin capillary tube. The coils themselves are made of pipe or tubing that holds refrigerant. All the parts that are connected to each other and designed to hold refrigerant make up the refrigerant circuit.

Source: International Code Council · ASHRAE · U.S. Department of Energy · U.S. EPA

It helps to follow the refrigerant through one full trip. Inside, the cooling coil uses refrigerant to take heat and humidity out of the air, while a blower moves air over the coil and spreads the cooled air. As the refrigerant flows through the evaporator, more and more of it turns from liquid to gas.

Next the gas goes to the compressor. The compressor turns cool, low-pressure refrigerant gas into hot, high-pressure gas. It raises the refrigerant to a temperature higher than the outside air. The compressor also keeps the refrigerant moving and creates the pressure difference between the condenser and the evaporator.

Outside, the refrigerant passes through the condenser, where a fan blows outside air over the coil; the refrigerant gives its heat to the cooler outside air and turns back into a liquid. The liquid then goes through the expansion device. In a room unit, the capillary tube lowers the liquid's pressure and temperature. With an expansion valve, liquid refrigerant drops in pressure and temperature at the valve and enters the evaporator as a mix of liquid and vapor. Then the trip starts again.

Source: U.S. Department of Energy · ASHRAE · Pacific Northwest National Laboratory

If you stand next to your outdoor unit on a summer day, the air coming off it feels hot. That is the system working, not a fault. The hot outdoor coils release the heat collected inside into the outside air. The outdoor unit of a split system holds the condenser and the compressor. So the warm air you feel is heat that was in your home a few minutes before.

The outdoor unit needs room to breathe for this to work. The Department of Energy advises clearing anything that blocks the outdoor unit's airflow throughout the year. ENERGY STAR says keeping indoor and outdoor units clear of dust, snow, leaves and other debris is vital for the best performance.

Source: U.S. Department of Energy · ENERGY STAR

A heat pump is the same loop with one extra trick. In the cooling season it moves heat from your house to the outdoors, and in the heating season it moves heat from the cool outdoors into your warm house. A reversing valve changes the direction the refrigerant flows, so the outdoor coil picks up heat from outside air and the indoor coil gives heat to your rooms. ENERGY STAR suggests thinking of a ducted air-source heat pump as a central air conditioner that also works in reverse to heat the whole house in winter.

Running in reverse brings one normal winter habit. Frost forms on the outdoor coil while a heat pump heats. Defrosting removes it, usually by running the heat pump as an air conditioner for a short time. In one federal lab test of two ductless heat pumps, the indoor fan stopped during defrost and the units gave no heat. Outdoor units release water when they defrost, which can form icy patches. The heat pump page covers this in more depth.

Source: U.S. Department of Energy · Pacific Northwest National Laboratory · ENERGY STAR

The loop works best with the right amount of refrigerant and the right amount of air. Air conditioners are typically designed to move 350 to 400 cfm of air across the indoor coil for each ton of cooling. Correct airflow strikes the right balance between cooling the air and removing moisture. ENERGY STAR's installation checklist includes confirming that the refrigerant charge and the airflow across the indoor coil meet the maker's recommendation.

Refrigerant itself is a job for a certified technician. The EPA says that if your air conditioner leaks, you should ask your technician to find and repair the leak instead of topping it off. Recharging a system requires Section 608 certification, and refrigerant for home systems is sold only to certified technicians or the companies that employ them. The refrigerants page explains those rules.

Source: U.S. Department of Energy · ENERGY STAR · U.S. EPA