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HVAC School - For Techs, By Techs

Jan 18, 2018

In today's podcast episode, Bryan talks to west-coast commercial tech Jim Loring about pneumatic controls and variable air volume (VAV) systems.

People sometimes confuse pneumatics and hydraulics. Hydraulics use liquid to provide pressure; conversely, pneumatics use air to provide pressure. Pneumatic controls use a bit more energy than other controls, but they are less costly all around. Nowadays, direct digital controls (DDCs) provide greater energy savings than pneumatics. However, pneumatic controls were a precursor to the DDC technologies we use on actuators today, and they are still a prevalent technology.

The air compressor is a critical component of pneumatic controls. That is because pneumatic controls require clean, dry air. Air compressors have an auto-drain and auto-dryer to help purify the air for peak performance. However, while air compressors are basic, their maintenance practices are often overlooked.

Variable air volume (VAV) units vary airflow throughout the building via zones. Each zone has a damper and a thermostat. The thermostats control the dampers, which control airflow to the zone and move via actuators. In a pneumatic control system, the air pressure release or gain at the thermostat moves the dampers. Thermostats also have to bleed off some of that air via direct or reverse-acting controls. Bypasses help regulate static pressure when dampers close.

Thermostats can help modulate the dampers; they don't merely open and close. The modulation occurs within a certain pressure range on a VAV system. (For example, 8 PSI would close the damper while 13 PSI would leave the damper wide open.) In addition to damper modulation, velocity controllers help control the air velocity based on signals from the thermostat.

Jim also covers:

  • Common air compressor problems
  • Pressure-reducing valves (PRV)
  • Restrictor tees
  • Direct-acting vs. reverse-acting controls
  • Heating and cooling in VAV systems
  • Damper position

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