Dedicated Outdoor Air System

Background

Dedicated outdoor air systems (DOAS) decouple the heating, cooling, dehumidification and humidification of outdoor air from the space air-conditioning system. Common HVAC systems such as fan coil units, chilled beams , variable refrigerant flow units (VRF) and heat pumps – ground and air require a DOAS to meet ventilation requirements.

A DOAS may not be any more complicated than a supply fan and an exhaust louver. However for many buildings the control and monitoring of conditioning (filtration, heating, cooling, humidifying and dehumidifying) will be a requirement.

Control Sequences

A DOAS can be controlled to:

  • deliver outdoor air to offset local exhaust such as toilet exhaust
  • building pressurisation
  • sequence with chilled beam cooling demand
  • demand control ventilation (DCV) using population counting or indoor air quality measurement in healthy building strategies

Diversity Factor. Centralizing a DOAS to a single unit will often provide the greatest energy and maintenance savings using a demand-based control system. The key is to also size capacity of DOAS for design conditions based on full population of the building. Population diversity throughout the day means the DOAS ventilation will be directed and balanced to only where it is needed in the occupied space. We find the ability to take advantage of the diversity factor to reduce HVAC capacity is limited in systems other than a demand based-control-system.

Heat Exchanger. Demand-based-control will often provide far greater energy savings throughout the year and have a better return on investment than a heat exchanger for DOAS systems in most buildings and cities in Australia.