Green Building Tip: Your AC System

Two Laws of Air Conditioning

Central AC systems work by moving heat from one place to another. The air inside the house is pulled through a duct system where it goes through three main stations. At one station heat is extracted from the air. The next two stations work to disburse heat into the outside air. The moving of heat from one place to another relies on a basic law of physics. That is that heat wants to move from a warmer place to a cooler place. Faster moving (hotter) air molecules will move into areas of slower moving (cooler) molecules. If you turn on your oven, for example, the heat will not stay in the oven. It wants to get right out and move around the room. Heat wants to stay balanced.

So, the two laws of AC are: One, heat wants to move into areas of less heat. And, two, the greater the temperature difference, the faster heat will move.

Air Has to be Forced

An electric motor to blows air through the ductwork. That’s why it is called a forced air system. The blower is in the air handler, the unit inside the house. If you have a lot of leaks in your ductwork, you are going have to blow a lot more air, and use a lot more electricity. It will save you money in the long run to check your ducts for leakage.

Secret Sauce of Air Conditioning

AC systems rely on a chemical refrigerant, such as Freon. This chemical has a much lower boil point than water. Just a little heat will make it boil. When something boils it transforms to a gas, taking heat with it. As air moves through the AC system, it goes through three stations - the evaporator, the compressor and the condenser.

A Visit to the Evaporator

Air from your house is drawn back through a “return air” duct and moved across an evaporator. This is a little set of pipes that contain liquid refrigerant. When warm air blows across these pipes, heat is transferred into the refrigerant, because, as we know, heat always moves into something cooler. This small amount of heat causes the refrigerant to boil and change to a gas. Gas holds more heat.

A Visit to the Compressor

After the refrigerant boils and becomes a gas, it moves down a pipe to the outside part of the AC system. Most homes with central AC have a split system – part of it is outside and part inside the house. When the refrigerant gas arrives at the compressor it is put under pressure and compressed. This makes the heat molecules move even faster and become hotter. Then the hot compressed refrigerant heads over to the condenser.

A Visit to the Condenser

The superheated refrigerant is pushed into a condensing unit. In the condensing unit, a fan blows fresh air across the refrigerant coils and heat is transferred to the cooler outside air and disbursed into the atmosphere. This is the final step in moving heat from inside your house to the outside. As the refrigerant gas releases heat it cools off and condenses into a liquid again. Then, it moves back inside the house to the evaporator, and starts its tour though your AC system again.

Inefficiencies in the System

There are several ways in which this system is inefficient:

(1) Leaks in the duct work can account for loss of 10-30% of cool air that is produced. This can be fixed by pressure testing and sealing ductwork.

(2) If the ducts are located in a super heated attic, heat will find a way to penetrate the ducts that are carrying cooled air. Long duct runs through a hot attic are especially inefficient. This can be avoided by not putting the air handler and ductwork in the attic.

(3) Excessive transfer of cooled air from inside to outside can be avoided by better weather stripping and insulation.

(4) There is inefficiency at the point of transfer of air from the condensing unit to the outside. Electricity is required to power the fan to move outside air across the hot refrigerant coils. If the outside air is 100 degrees, heat is not going to naturally move very fast out of the refrigerant coils into the outside air. Hot humid air holds a lot of heat already, and is not very welcoming to the heat that your condensing unit is offering.