Sizing AC Transformers for HVAC Applications

Understanding the volt-ampere requirements of an HVAC control unit requires installers to accurately size the AC transformer they intend to use. This enables the system to ensure adequate power is provided to all the control components within.

HVAC control systems require stepping down mains power to 24VAC for use by 24VAC systems. This process requires installers to size the total VA load, not just the nominal load.

Image

Installers must calculate the steady-state load and account for inrush from contactors/relays/actuators before selecting a transformer with sufficient margin to ensure the control voltage remains within acceptable limits during both start-up and normal operations.

Consider grouping your components into these three groups when sizing your AC transformer.

• Continuous Loads
  • Components such as thermostats, controllers, sensors and pilot lights.
• Intermittent Load
  • Components such as relay coils, contactors, solenoids and valve actuators.
• Inrush Demand
  • Various components require a higher VA draw at pull-in than at hold. These can be components such as coils and actuators.

Calculating Sealed and Inrush VA

Calculating the total power demand of the components within the continuous and intermittent load groups is known as sealed VA, also called steady-state VA.

Once the sealed VA has been determined, installers can determine the inrush VA required for start-up. For example, the TX100-240-24 may be a suitable solution for an HVAC installation with a sealed VA of 22VA and an inrush VA of 90VA; however, for more conservative installers, the TX160-240-24 may provide the added headroom to eliminate the risk of dropouts entirely.

If you’re an OEM designing power systems for industrial HVAC systems and still have some questions, the PSA team is here to help! Send any questions to our team today via our contact page here or call the team directly on 1800 632 693.