If you work in the HVAC industry, you are concerned with the process of conditioning air and moving air from one area to another. It makes sense that you need to know how air flows in ducts and what factors affect the flow.

A good understanding of airflow is needed for different specialties in the HVAC industry, such as:

• Service work

• Testing, adjusting, and balancing

• Energy management

• Indoor air quality work


Think of what HVAC means - heating, ventilating, and air conditioning:

Ventilatingis bringing outside air into a building.

Heating and air conditioningmeans heating, cooling, and cleaning air, and regulating its moisture content.

Conditioned air must be delivered to selected areas of a building and then removed from those areas and returned for re-conditioning. Conditioned air is usually transported through ductwork.

The process of moving air in ducts distributes energy:

• In cold weather, heat taken from an energy source such as gas or electricity is added to air. This heat is delivered to the conditioned space.

• In hot weather, heat is removed from air by the use of electrical energy. In this case, heat energy is being removed from the conditioned space.


An HVAC system has different components:

• Central air handling system - Generally contained in a mechanical room. It includes the fan to move the air and equipment to condition the air before it is delivered to the conditioned space.

• Boiler - Provides hot water to heat the air.

• Refrigeration unit - Provides a means to cool the air.

• Duct system - Distributes the conditioned air where needed.

Figure 1. Central air handling system. (Click on the illustration for an enlarged view.)

Central Air Handling System
Figure 1 shows a typical central air handling system. This is a schematic drawing that shows the parts of the system and how they are related to each other. It does not show the location of the components in an actual installation. There are many different system variations. You need to understand the basic components of a system and their relationship to each other. Then you can identify the components on any job.

Supply air (SA)is the conditioned air delivered to the building. In Figure 1 the supply air is in the lower right hand corner.

When supply air is delivered into a room, an equal amount of air must be removed from the room. Thisreturn air (RA)is transported back to the central air system for reprocessing. In Figure 1 the return air is in the upper right corner.

Figure 2. Hot water system added to air handling system. (Click on the illustration for an enlarged view.)

Only a percentage of the return air can be reused. The air would become stale if the same air were used over and over again. To avoid this, fresh air is brought into the system through theoutside air (OA)intake. In Figure 1 the outside air inlet is in the lower left.

When outside air enters the building, the same amount of air must be removed from the building through theexhaust air (EA)outlet or other exhaust air systems.

The exhaust air (EA), outside air (OA), and return air (RA) ducts all operate together. An automatic control system operates the damper motors to maintain the proper mix of air. The OA and the EA dampers open together and close together to balance the air entering and leaving the building. As the OA damper opens, the RA damper closes so that the same amount of air remains in the system.

After the air is mixed in the proper proportions, the mixed air is drawn through a filter before it enters the fan and returns to the conditioned spaces.

The system shown in Figure 1 does not provide for heating or cooling the air.

Figure 3. A heating coil.

Heating System
Figure 2 shows how heat is added. Aheating coil(Figure 3) is added to the system before the air enters the fan. It is located after the filter so that there is less chance of dirt clogging the coil.

The coil is similar to a car radiator. Hot water flows through tubes in the coil. Metal fins are attached to the tubes. The heat from the hot water is conducted to the outside of the tubes and to the fins. The air passing across the coil is heated by absorbing heat from the fins and the tubes.

The hot water for the coil is heated by aboiler. A pump circulates the hot water through the coil.

Figure 4. Chilled water system added. (Click on the illustration for an enlarged view.)

Cooling System
Figure 4 shows a chilled water coil added to the system to cool the supply air. The chilled water coil is similar to the hot water coil. Chilled water flows through the tubes to cool the air.

The water is cooled by a chiller, which is a refrigeration unit. A pump circulates water through the chiller where it is cooled and then returned to the chilled water coil.

The system shown in Figure 4 is a complete air handling system. It can:

• Mix the return air and outside air to the correct proportions.

• Filter the air.

• Heat the air.

• Cool the air.

Figure 5. The fan creates pressure that causes air to flow. (Click on the illustration for an enlarged view.)


A water system in a house is usually under a pressure of about 30 psi (pounds per square inch). When a faucet on the water line is opened, the line pressure at the open faucet decreases to zero. The water flows to the low pressure area.

Air is a fluid just like water. It also flows from one area to another because of adifference in pressure:

• In the open air, air flows from a higher pressure to a lower pressure. Wind is air that is moving from a higher pressure area to a lower pressure.

• In a duct, air also flows from a higher pressure to a lower pressure. A fan (Figure 5) creates the higher pressure. The open end of the duct has a lower pressure, so the air flows out.

Figure 6. A water gauge.

All air in an open system is underatmospheric pressure, which is normally 14.7 psi at sea level. Pressure in a duct refers to the pressure that is higher or lower than the atmospheric pressure of 14.7 psi. A positive pressure (+) is above atmospheric pressure. A negative pressure (-) is below.

The amount of air flowing through a duct is regulated by the amount ofpressure differenceand by thesystem resistance. The higher the pressure difference, the greater the air velocity and the greater the quantity of air that will flow from the duct.

Frictionis a resistance which slows down airflow. The flow of air creates friction as it rubs against the side of the duct, and the friction creates resistance to the airflow. Think of blowing through a piece of garden hose 6 inches long. You can feel a good stream of air coming out of the tube. Now try to blow through a hose 50 feet long. Little or no air comes out the other end. This is because friction created by the sides of the long hose reduces the pressure at the open end of the hose.

Figure 7. An electronic manometer.


Since the air pressure in a duct directly affects the flow of air, measuring the air pressure is important. Air pressure in a tire is measured inpsi (pounds per square inch). Air pressure in a duct is measured ininches water gauge (inches wg). Both psi and inches wg measure the same thing - the amount of pressure on a given amount of area. However, inches wg is used for duct because it is suitable for measuring small values. Compare the measurements taken by a carpenter and a machinist. The carpenter uses a rule divided into eighths and sixteenths of an inch. The machinist uses instruments that measure in thousandths of an inch.

Compare the measurements of psi and wg. The pressure in a duct might be 1 inch wg. This same duct pressure measured in psi would be just 0.04 psi. You can see why psi would not be a good scale for measuring small changes in air pressure in ducts.

Figure 8. U tube with equal pressure on both ends. (Click on the illustration for an enlarged view.)


The term inches wg meansinches of water differential in a water gauge. Awater gauge(Figure 6) is the basic device for measuring air pressure in duct. In actual practice, other instruments that are more convenient to use, such as anelectronic manometer(Figure 7), are used to measure inches wg. But all instruments measuring inches wg are based on the principle of the water gauge, which is explained in Figures 8 and 9.

Figure 8 shows a small, flexible, plastic hose connected to a U-shaped glass tube at B. The pressure at the open ends of the tubes (A and E) is equal (because it is only atmospheric pressure). Therefore the level of the water at C and D is the same.

Figure 9. U tube with inches wg pressure reading. (Click on the illustration for an enlarged view.)

Figure 9 shows the open end of the hose placed in a duct in which the air is flowing. Therefore the pressure at A is greater than the pressure at E. This pressure pushes the water level down at C. This means that the water level at D must rise by the same amount.

If the pressure pushes the water down 0.4 inches at point C, the level must rise 0.4 inches at point D. Therefore there is a difference of 0.8 inches between the two levels (Figure 9). The pressure reading on this U-tube is 0.8 inches wg.

Excerpted and reprinted fromAirflow in Ductsby Leo A. Meyer, one of the books in the Indoor Environment Technician’s Library series published by LAMA Books.

Publication date:01/22/2007