Balancing a HVAC System with Multiple Diffuser Types

The Problem

  • Standard air flow hoods do not correct for diffuser type when balancing a system.
  • Proportional balancing is extremely difficult when a mixture of diffusers are present, due to the inaccuracy of standard hoods on slot diffusers.
  • Below is a sample of diffusers tested on Dwyer’s Precision Air Calibration Station (Figure A) with a standard air flow hood followed by the Dwyer SMART Air Hood™ Balancing Instrument (Series SAH).

Figure A: Precision Air Hood Calibration Station

The Precision Air Hood Calibration Station has a mass flow relative combined uncertainty of 0.276% of the reading. It was built by Dwyer to ensure the accuracy of the new SMART Air Hood™ Balancing Instrument.

When using a standard hood, measuring a 2’x4’ laminar diffuser yields error percentages within +/- 5%, while taking a reading from a TWR swirl diffuser can yield error percentages upwards of 15% (see Figure B). With such unpredictability, it’s no wonder that proportional balancing of a system with standard hoods can take hours of trial and error.

Figure B: Error % vs Flow using Standard Air Flow Hood

Error percentages for standard air flow hoods vary greatly, increasing proportionally with flow. Dwyer’s SMART Air Hood™ Balancing Instrument solves this problem (as shown in Figure C).

Figure C: Error % vs Flow using SMART Air Hood™ Balancing Instrument

How It Works

  • Dwyer calibrates each SAH to a standard calibration curve on our precision calibration station; this means that all SMART Air Hood™ Balancing Instruments behave the same on a reference diffuser.
  • All diffusers in the Dwyer library (example shown in Figure D) are characterized on our Precision Calibration Station so that when they are applied to the standard SAH calibration, all diffusers are accurate.
  • When the operator selects a diffuser in the PredictAir™ Software Application, that characterization is applied to the standard calibration curve of the SAH. Therefore, the output of the SAH is accurate for the selected diffuser.

Figure D: Examples of Diffusers


  • Standard air flow hoods do not correct for the diffuser type and can be extremely innaccurate on some types of diffusers.
  • Proportional balancing is extremely difficult when there is a mixture of diffusers present (i.e.: window slot diffusers mixed with 2’x2’ ceiling diffusers), due to an inaccuracy of standrard hoods on slot diffusers. The proportional balancing method doesn’t work if the hood accuracy varies from diffuser to diffuser in a branch.
  • Fortunately, the SMART Air Hood™ Balancing Instrument has been used to re-balance offices with multiple types of diffusers, window slot and 2’x2’s, and shown to be very effective. The PredictAir™ Software Application allows the SMART Air Hood to balance mixed diffuser systems in a single pass, using Predictive Balancing.