Ventilation must be controlled to match occupancy down to some minimum level associated with unoccupied space. In VAV systems, the outside air delivered to each zone has to be compared to the need, and controlled accordingly.

  • Use occupancy or carbon dioxide (CO2) sensors.
  • The controller must calculate actual outdoor airflow to each control zone based on the percent outside air (OA) in the supply air per formula from ASHRAE Standard 62.1, “Ventilation for Acceptable Indoor Air Quality.”
  • Consider using an OA/exhaust-air heat-recovery system
  • Obtain free cooling by using economizers except in DOE climate zone 1. Use airside (rather than waterside) economizers. Control is based on enthalpy sensors.
  • Use exhaust (rather than return) fans when necessary to maintain building pressurization during economizer operation. Use AMCA Class 1A low-leak dampers and weather-resistant louvers with certified ratings to match worst-case weather conditions.
  • Use the multiple-zone recirculating-ventilation system design approach from ASHRAE Standard 62.1, which allows ventilation air to be controlled from measured CO2 levels in key zones and return air, rather than the sum-of-peak zone population at design.

Specify Controls for Low-Energy VAV

Lowering design cooling temperatures saves a bundle on ductwork. To minimize the energy penalty of this approach, raise the temperature at part load. Control fans from a pressure sensor placed near the end of the longest run. The closer to the end this sensor is placed, the more efficient the fan selection and operation can be.

Static pressure

  • Even if there is only one zone, vary the airflow with a change in the demand for cooling. Single-zone systems require no control dampers, but the air handler (or packaged unit) should vary flow with demand.
  • For multiple-zone VAV systems, control the VSD from a static-pressure sensor located near the end of the duct run. Use multiple sensors for ductwork with multiple branches.
  • Locate the static-pressure sensors close to the last VAV diffuser in the duct run.

Supply-air temperature (SAT)

  • For systems with multiple zones, control cooling and heating with a discharge-air thermostat for constant SAT, and reset to satisfy the zone with the strongest call for cooling at the highest temperature possible.
  • Design cooling SAT at 50°F and reset to 58°F to 61°F depending on climate zone (and the need for latent cooling). For SAT reset in humid climates, use at least one zone humidity sensor to disable reset if humidity exceeds 60 percent.
  • Perimeter/skin-system heating SAT must be as low as possible but no lower than 80°F.
  • Changeover between heating and cooling may be manual, with a room thermostat or multiple voting room sensors. Locate the room thermostat in the room of “greatest need” or maybe the most important room.

Optimal Start

  • Use a system controller that waits as long as possible before starting the system so that the temperature in each zone reaches occupied setpoint just in time for occupancy.

Tune the System by Re-engineering for What “is” Post installation

Design tools available to the air-system designer are coarse. System effects in ductwork, and installation adjustments cannot be estimated properly Re-engage with the project during test and balance—uncover the system challenges that compromise performance. Change some things—make it right.