Innovative Energy Engineering

Airside Design

Almost all HVAC systems require an air-side component because removal of latent loads (water vapor) and ventilation are required for almost all HVAC applications. Airside components typically are physically larger and much more energy-intensive than water-side components.

Air, unlike water, is a compressible fluid, which impacts its interactions when velocities change. Total pressure is the sum of static and dynamic (velocity) pressure. In diffusers (velocity decreases ) dynamic pressure decreases with the square of velocity. Static pressure increases by the same amount (static regain). In a nozzle (velocity increases), dynamic pressure increases with the square of velocity and static pressure is reduced accordingly. In real world systems with conversion losses and friction total pressure decreases. The lost pressure is converted to small amounts of heat and noise. Efficient designs minimize those conversions.

Pressure can be measured by measuring the rise of water (i.e. inH2O) or mercury (i.e. mmHg) in a tube or by using sensors. When using sensors or liquid in tubes a reference pressure needs to be selected. Static pressure is measured by inserting a tube into the duct (opening 90° to flow direction) and using the atmosphere as a reference (tube open to space). Total pressure is measured using the same setup, except the inlet tube is 0° to the airflow (air pushing into the tube opening). Velocity pressure is measured like total pressure, except the reference is the duct itself (tube inlet 90° to airflow).


Fans provide the total pressure required to move the air. Manufacturers provide performance curves and selection software to select fans for optimal performance. Software also accounts for actual altitude and temperatures, which impact air density and fan performance. Under all operating conditions (flow, pressure, rpm) the fan should be outside the surge region and at highest possible static efficiency (metric: flow x static pressure / input power) and lowest possible break horse power (bhp). Manufacturer-recommended inlet and outlet conditions need to be obeyed or fan performance suffers due to system effect.

Modern fans are driven directly by speed-adjustable motor and use computer-designed airfoil blades for higher efficiency and less noise. Older less efficient designs employ backward curved, backward inclined and forward-curved fans and single speed motors. AC motors vary the frequency with a variable frequency drive (VFD), AC motors change speed with an inverter. Older obsolete technologies use belt-driven fans and speed is adjusted by changing pulleys. Airflow was adjusted by adjusting blade-pitch control or variable inlet vanes, which is an expensive and vulnerable mechanical control.

Centrifugal fans typically are more efficient when high static pressures are required, for example in air handlers where coils, filters and other equipment cause significant pressure drops. Vaneaxial fans typically are more efficient when lower static pressures are required, for example return or exhaust fans. Straightening vanes should be added for improved efficiency.

Housed fans should discharge directly into a straight duct. The first turn should be on the side of the fan inlet or projecting in the direction of fan rotation. When discharging into a plenum system effect can cost as much as 250 Pa (1 in-wc). Plenum outlets should be as far away from the fan as practical to minimize pressure drop. Outlets should have bellmouths for round ducts and rectangular ducts should not be larger than the opening.


Diffusers need to be chosen based on location, air temperatures (heating and cooling operation with different buoyancies), varying flowrates distribution characteristic for the application and location. Manufacturers will advise on throw, noise and pressure drop. Heating and Cooling need to be taken into account due to different flow rates and buoyancies of air. Create Revit families that automatically calculate pressure-drop, noise level and throw to aid in design. Price and Titus provide information.


Equipment and spaces need to be protected from dirt. it is recommended to use 4" pleated filters in angled arrangement to minimize pressure drop and extend replacement intervals. For filters protecting equipment MERV 8 typically is sufficient. For air entering occupied clean areas (i.e. offices) MERV 11-13 is recommended. MERV 11 filters are much cheaper than MERV 13 filters at almost same filtration. Pre-filters are usually not necessary and increase pressuredrop and equipment size unnecessarily. Bag filters also collapse in VAV systems and require excessive space.

Duct Design Guidelines

Standard duct design typically employs rectangular duct for main trunks and round duct for branches. Occasionally oval duct is used in lieu of rectangular duct. Disadvantages of rectangular duct include:

In most cases spiral round duct should be used instead of rectangular duct. In some cases, oval duct can be beneficial. Spiral duct has the following advantages:

For positive pressure ducts downstream of a fan (e.g. supply ducts) the static pressure lost due to friction can be re-gained. Based on the Bernoulli equation, high velocity air contains kinetic energy (velocity pressure) that converts to potential energy (static pressure) and vice versa. To take advantage of this, velocities need to be relatively high (to have enough kinetic energy) and velocities need to reduce as friction-losses occur. Often ducts are sized using the equal friction method. Disadvantages of the equal friction method include:

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