Proper Sizing HVAC Ducts


A duct system layout plan for central and heating and cooling systems should be designed and duct sizing should be specified in the plans and on CAD drawings. See the ENERGY STAR Checklist HVAC/R 2 Duct Quality Installation for more on duct installation.

Duct systems for distribution of conditioned air should be properly sized using ACCA Manual D at the lowest pressure drop possible. High pressure losses in the duct system create an indoor blower (fan) energy penalty. The up side to today's variable airflow blower motors is their ability to work against high duct pressures resulting from under-sized or improperly sized duct systems. However, there will be an energy penalty.

The static pressure chart in Figure 12 shows how fan energy usage is tied to airflow and external static pressure. If the duct system was designed for a required airflow for heating of 1,065 cfm at a 0.3 external static pressure (ESP in inches of water column [IWC]) it would draw 154 Watts. However, the cooling system requires 1,300 cfm, which increases the static pressure to slightly above 0.4 ESP and increase the operating wattage to slightly over 179 watts. If a high-efficiency filter with a 0.28 IWC ESP pressure drop is added to the duct system, the ESP goes to 0.7 ESP and the operating energy increases to 248 Watts (161%).

To stay inside the original equipment manufacturer’s (OEM) temperature rise criteria, the lowest allowable OEM temperature rise produces the highest efficiency but may cause the space to feel drafty because of high airflow and cooler discharge temperatures. However, lower airflow yields a higher temperature rise, less draft, and less efficiency. Higher temperature rise due to undersized duct systems is a double hit; it lowers combustion efficiency and increases blower motor energy.

Typical indoor blower static pressure and watts chart.
Figure 1. This typical indoor blower static pressure and watts chart from a manufacturer’s product data sheet shows the relationship between external static pressure fan speed and power draw.


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