How to Design HVAC Duct Sizes

Perform a room-by-room load calculation with HVAC load calculation software. To obtain all the information to enter into the program, you may need to have your home evaluated by a home energy auditor. Software takes you step by step as you enter pertinent information such as ceiling, floor, skylight and window insulation in each room.

Calculate the whole-house heating load. An HVAC technician or the load calculation software can help you determine the entire home’s heating load, which is a combination of heat gain from appliances and heat loss from air infiltration in every room of the home. An example of a reasonable figure for whole-house heating load is 150,000 British thermal units (Btu) per hour.

Determine the cubic-feet per minute (CFM) of air needed for each room, which might be between 50 and 150 CFM. Take each individual room’s load divided by the whole house load and multiply the result by your heating equipment’s CFM. This is important for obtaining an even temperature throughout the house.

Examine the heating system’s specifications to discover the external static pressure of the blower. Deduct pressure drops caused by filters, dampers and registers to determine the duct system’s available static pressure.

Determine the total effective length of the duct work by using a measuring tape to measure the farthest supply vent to the farthest return vent along the path that the ducts will be installed.

Calculate the friction loss rate by multiplying the available static pressure by 100 and dividing the result by the total effective length of the duct work. Your result should be somewhere between 0.05 and 0.08.

Select duct size with a duct calculator. You can determine the diameter and air velocity of each branch by entering the rate of friction loss and the CFM. Keep in mind that air velocity through duct work should never be higher than 700 feet per minute (FPM). If the velocity of the “ideal” duct-work diameter is too high, select one size higher until the velocity is lower than 700 FPM.