Application of air source variable refrigerant flow in cold climates

VRF diagramVariable refrigerant flow (VRF) systems use variable speed, split heat pumps to provide space heating and cooling to a building's conditioned areas. Air source VRF systems perform best in moderate climates, as they typically lose capacity and efficiency at low ambient temperatures—or moderately low wet bulb temperatures where defrost is required—and may be supplemented by an additional heat source.

In colder climates, this often necessitates the addition of a supplementary heater within a partially-enclosed mechanical room housing the outdoor units. Because of their lower capital cost as compared to water or ground source alternatives, the majority of VRF systems being implemented in cold climates today are air source. However, significant questions remain regarding how to design and operate air source systems at low outside air temperatures.

White paper and VRF analysis tool

Our white paper outlines the optimal control strategies for designing and operating air source VRF systems in cold climates.

In addition, a tool was developed (for free download) that allows designers to better understand the optimal design parameters of air source VRF systems.

How to download the tool:

  1. Go to
  2. Click “Sign in”
  3. If you haven’t created an account, click “Sign up”
    • Enter your information and click “Register”
    • You will receive an email (may take several days) when your registration has been processed
  4. Log in using your username and password
  5. Click on “Library”
  6. Select “08 VRV/LC Sales Partners”
  7. Select “Sales Tools & Applications Resources”
  8. Select “Sales and Applications Tools”
  9. Select “Cold Climate VRF Analysis Tool”
  10. Download .zip file

For additional assistance, contact

screen capture of the tool download page

Key observations

VRF chartIdeal system design is highly dependent on the shape of the heating performance curves for the VRF units. The optimal mechanical room setpoint occurs at a temperature close to, but higher than, the temperature at which the system’s heating efficiency and capacity drop off significantly.

The configuration with the lowest utility cost and EUI for buildings in Madison, WI is a mechanical room with operable louvers and a supplemental gas unit heater. The operable louvers also result in lower supplemental heater size than the fixed louver case.

For base conditions in Madison, the optimal mechanical room temperature setpoint is somewhere between -12 degrees F and 5 degrees F, and that this range held true until gas price dropped below $0.25/therm.

Significant thought should go into defrost and condensate handling at sub-freezing temperatures: equipment should be built, configured, and placed such that defrost and condensate water can be collected and flow to a drain with limited exposure to the cold mechanical room along the way.

The resulting energy savings in using a demand defrost approach as opposed to a timed approach is about 4% of VRF energy.

Sizing of equipment is critical for VRF systems, not just due to impacts in first cost, but also because sizing heavily affects efficiency and comfort.

This project was possible with significant funding from Daikin North America, LLC. Masters Building Solutions also provided technical input. The project would not have been possible without these two organizations’ assistance. Former Seventhwave staffer Alisa Petersen also contributed to this project.

For more information

Scott Hackel

Scott Hackel

PE, LEED AP, Director of Engineering
email me

Scott Schuetter

Scott Schuetter

PE, LEED AP, Senior Research Engineer
email me