A duct run is not just a duct run. Two pieces can have the same dimensions and still cost very different amounts if their pressure classes differ.
That is one detail estimators can easily overlook when working through a mechanical set. Duct pressure affects how the duct is built, including material thickness, reinforcement, joints, sealing, and testing requirements.
SMACNA pressure classes commonly range from 1/2 to 10 inches of water gauge, with positive- and negative-pressure applications.
Where the Bid Can Change
Not all duct runs are created equal. Before carrying quantities into the estimate, look at:
- Pressure class: Record the pressure class where it changes between one section of duct and another.
- Pressure requirements: Different runs in a system may require different pressures.
- Reinforcement: High-pressure ductwork may require more reinforcement or heavier construction.
- Sealing and testing: Some pressure classes have less sealing and leakage.
- Plans and specs: Work with drawings, schedules, details, and specifications. A symbol on the plan does not always convey all the information.
A common estimating mistake is measuring the duct accurately and then pricing it as if every run has the same construction. That's where a seemingly complete takeoff can still produce the wrong number.
HVAC takeoff software can speed up the measurement side, but it shouldn't replace the pressure-class review. The estimator still needs to connect each duct quantity to the requirements that govern how it will be fabricated and installed.
For a deeper look, Beam AI's HVAC duct pressure class guide breaks down the common classes and what they mean for estimating. The quantity tells you how much duct you need. Pressure class tells you what that duct may cost.
