When ductwork is configured, numerous measurements must be taken. Simply connecting system components and vents with any type, length, or size of tubing won’t cut it. If your ductwork is too small or too large, it can be just as bad as installing the wrong-sized HVAC unit for your home. A duct load calculation during HVAC installation can prevent issues such as premature wear, poor airflow, noise, a lack of comfort, and higher energy costs.

Basic Duct Load Calculation for HVAC Installation

There are various online tools to help perform a duct load calculation. The crucial elements of a duct load calculation are the same for every home, and include:

• Square Footage: Calculate the square footage of rectangular or square rooms by multiplying each room’s length by its width. You can also use numbers from the structure’s blueprint if you have them. For irregularly shaped rooms, split them into sections and add up their total areas.
• Air Velocity: Measured in cubic feet per minute (CFM), air velocity relates to the amount of BTUs per hour required for each room. Therefore, an HVAC load calculation (Manual J calculation) must be performed first to ensure each room receives the correct heating and cooling. Also needed is a Manual T air distribution calculation to determine the correct size for supply registers and return grilles.
  • About 1 CFM of air is needed to heat or cool 1 to 1.25 square feet of floor area (for rooms with direct sunlight or lots of windows, it’s closer to 2 CFMs).
• Available Static Pressure: An HVAC blower’s external static pressure can be found in the model’s specifications. Total external static pressure, measured in inches of water column (wc), is .05” wc for most systems. Deduct pressure drops created by components, such as supply registers, return grilles, and balancing dampers (Manual D suggests deducting .03 wc for each). Use the pressure drop listed for the air filter by the manufacturer; the total deductions give you the available static pressure (ASP).
• Total Effective Length (TEL): This is the measured length from the farthest supply outlet through all HVAC equipment to the farthest return outlet. All lengths of turns and fittings are included. With TEL, every instance of pressure loss isn’t calculated. Instead, the length of a straight duct that would create the same drop is measured (each fitting has an effective length equal in pressure drop to a straight duct of equal length). Add the values for all fittings in the most restrictive run to the length of the straight section in that run. Knowing the TEL, you can calculate the friction rate.
• Friction Rate Loss: To determine the friction rate in your duct load calculation, the available static pressure is divided by the total effective length, and multiplied by 100. This tells you how much pressure drop can be accommodated per 100 feet of duct. A higher friction rate means smaller ductwork can be used; a lower friction rate means larger ducts are required and airflow can be significantly restricted by a faulty component.

What to Look for in Good Duct Design

Equal distances between supply trunks along the supply plenum help even out heating and cooling distribution. As a result, every room should be at about the same temperature. The correct number of registers is important as well; so is the supply trunk being the correct diameter. Otherwise, comfort may be limited and rooms may get too little or too much airflow.

Why Should My HVAC Ducts Be Designed Properly?

If your ductwork is poorly designed, there could be issues such as the AC running continuously, some rooms being too hot while others are too cold, and high energy bills. With improperly sized return trunks, AC coils can freeze up, the desired comfort level will take longer to achieve, and the compressor may be overworked. Occupants of your home may also have frequent sore throats and respiratory ailments if ductwork is poorly designed.

Contact Sky Heating & Air Conditioning

Proper ductwork design isn’t only about size. But we can upsize ductwork during the replacement process, seal ductwork from the inside out, and use antimicrobial blue ductwork to help achieve cleaner air. When it comes to a professional duct load calculation, our HVAC professionals get it right in homes across Portland and The Dalles. We ensure your ductwork is configured and sized to improve comfort, equipment performance, and energy efficiency. To schedule ductwork installation, sealing, or repair, call (503) 673-9083 today.