2.3 Dedicated Outside Air Systems (DOAS)
Dedicated outdoor air systems are provided to pre-condition the outside air which is then supplied to the main HVAC systems in the building. In most cases, the purpose of a DOAS is to address the large loads associated with the introduction of outside air, whether due to low ambient conditions, high ambient conditions or high humidity of the outside air. Depending upon the configuration, the DOAS may include heating coils, cooling coils and possibly a heat recovery component, in addition to the supply and optional exhaust fans associated with the unit. The main HVAC system fed by the DOAS could be any number of HVAC system types but typically will be smaller zonal systems such as water source heat pumps, VRF systems or fan coil units. By shifting the outdoor air load over to the DOAS, these systems may be smaller.
The DOAS system is modeled as a conventional HVAC system if that is how it is configured, with inputs for heating and cooling coils as well as fans. The choice of system type will depend upon the type of system being modeled. The DOAS system could be a simple rooftop DX system, or possibly a chilled water air handler with hot water coils fed from a central plant. It is important that the system be specified with 100% outside air, unless a more sophisticated outside air control scheme has been utilized. Optional parameters that could be specified include an exhaust fan and/or a heat recovery component. Heat recovery would require the specification of the heat recovery effectiveness as well as any pumps or fans associated with the heat recovery component. Often DOAS AHUs include precool-reheat loops (wraparound heat pipes or sensible loops) or mixed dual heat recovery systems. If proposed buildings include these systems, software should be chosen that models these systems explicitly. DOAS equipment often delivers warmer air to zones with chilled beams, panels or other local cooling. Where minimum airflows are high, this can lead to significant savings in reheat.
Each HVAC system that receives outside air from the DOAS will be specified as a conventional HVAC system, except that the source of outdoor air will be coupled to the DOAS system associated with this unit. When specifying the ventilation rates for the zones served by the HVAC system, they will be specified as normal. By coupling the HVAC system to the DOAS system, the energy model will utilize that stream of air as the basis of the outdoor air.
If the DOAS only provides outdoor air tempering for other mechanical systems, the baseline building is not modeled with a corresponding DOAS system. However, in the case where the DOAS also provides general space conditioning to a space (such as conditioned air into a hallway, that is then pulled out by other HVAC systems) then that space will be modeled with the HVAC systems defined in Chapter 6.