How to Model a VRF-to-AHU Coil Configuration

DesignBuilder does not directly support modeling a VRF system feeding an AHU coil, because VRF systems are treated as zone-level equipment, while AHUs belong to central air loop systems. As a result, a direct connection between a VRF system and an AHU coil cannot be modeled explicitly.

Recommended Modeling Approach

A reliable representation is achieved by decoupling the systems and modeling them as coordinated but independent elements. The VRF system is assigned as the primary conditioning system serving each zone, while the AHU is modeled separately as a ventilation system or Dedicated Outdoor Air System (DOAS) supplying air to the same zones. In this arrangement, the VRF system addresses sensible loads at the zone level, and the AHU manages ventilation and latent loads. This reflects common HVAC design practice and ensures stable simulation performance.

Implementation in Detailed HVAC

Within Detailed HVAC, the VRF system is assigned to the relevant zones using VRF outdoor units connected to VRF indoor units. In parallel, an Air loop system representing the AHU is created and configured to deliver air to those same zones. A practical starting point is the Detailed HVAC “VRF with HR and DOAS” template, which can be adjusted as needed to suit the application, such as removing the AHU preheat coil if it is not required.

Important Considerations

Care should be taken to avoid double-counting fan energy when both VRF indoor units and an AHU are included in the model. This can be addressed by neutralizing the fan contribution from the VRF indoor units, achieved by setting fan efficiency to 1 and pressure rise to 0.

If the system configuration requires that VRF indoor units do not provide heating, this can be implemented by setting the heating coil availability to “Off 24/7”.