Tips for VRF and DOAS System Modeling in HAP

Tips for VRF and DOAS system modeling in HAP

In HAP, both DOAS and VRFu2019s terminal units (or indoor units) are modeled as one system. Also, you need to define the number of zones in the General tab of the Air System Properties window. See an example below:

The DOAS needs to be defined in the DOAS Components tab as shown below:

Terminal units or indoor units are defined in the Terminal Units section under Zone Components tab:

Tips for Report Generation

For VRF systemu2019s sizing, both System Sizing Summary and Zone Sizing Summary are needed. The System Sizing Summary provides information for selection of the DOAS and Zone Sizing Summary provides information for selection of Terminal Units or Indoor Units.

Types of DOAS for VRF systems

There are a few types of DOAS for VRF systems:

  1. Packaged outdoor or indoor units
  2. Outdoor air handling unit with chilled water coil
  3. Indoor air handler working with either DX coils or chilled water coils

Selecting a packaged outdoor unit to serve as a DOAS of a VRF system is a common practice. In this configuration, the DOAS has its own controller which controls the capacity of the DOAS to meet the discharge air temperature setpoints.

Daikin Rebel is a great choice for small to medium size applications. For this project, select a packaged rooftop unit as the DOAS of the VRF system. This packaged rooftop unit can be either Daikin Rebel or any other packaged rooftop unit that can meet the required capacity.

Air Balance of Building

Pressurization is desired in residential and commercial buildings, but over-pressurization must be avoided. Consider allowing 200 CFM for pressurization of the entire building for this project. In other words, the amount of the buildingu2019s total exhaust should be 200 CFM less than the amount of fresh outside air supplied through the DOAS.

The exhaust fan of the ventilation system can be a part of the DOAS. The exhaust fan of the DOAS can provide exhaust for multiple applications at the same time.

For this project, select a packaged rooftop DOAS with an exhaust fan as shown below. Make sure to provide the required restroomsu2019 exhaust via the DOASu2019s exhaust fan.

Notes on Indoor Units Airflow

In the DOAS ventilation method, the difference between indoor unitu2019s calculated airflow and actual equipmentu2019s airflow is not important. This is because the ventilation air is provided by the DOAS at the correct rate and indoor units only operate to provide heating or cooling. So, as long as the indoor units have the correct cooling capacity, then their airflow does not need to exactly match the calculated airflow.