What is a VRF system?
Variable Refrigerant Flow (VRF) systems are typically installed with an air conditioner inverter which adds a DC inverter to the compressor to support variable motor speed. By operating at varying speeds, VRF units work only at the needed rate allowing for substantial energy savings at part load conditions. Heat recovery VRF technology allows individual units to heat or cool as required, while the compressor load benefits from the internal heat recovery.
The two main system types of VRF's are two pipe and three pipe systems. In two pipe VRF system all of the zones should be in cooling or heating. And in case of three pipe system with heat recovery, it is possible to heat certain zones while cooling others. While the VRF systems have greater initial cost, it allows for better zonal thermal control and overall greater efficiencies.
When considering the efficiency and effectiveness of an air-source VRF system, the coefficient of performance (COP) is typically the best metric – the higher the COP, the better the performance – and it's dependent on modulation level and outdoor temperature; the colder it is outside, the less efficient the system is at extracting heat from the air while operating at maximum capacity.
Heat pump VRF systems are famous for having some of the highest COPs in the market and have demonstrated radically-improved heating performance under low-temperature conditions (near or below 5°F), while continuing to offer highly efficient cooling at high temperatures. In order to address the myth that these systems do not work properly in colder climates, Northeast Efficiency Coop has created an ASHP product list with COP and other data about the performance of many heat pumps at lower temperatures. The lowest-performing system on the list has a COP of 1.75 at maximum capacity when the outdoor temp is 5°F. At warmer outside temperatures the heating COP can be as high as 4, however to ensure simulation results are conservative, a default of 1.75 is used on the platform.
How to model VRF systems
VRF systems can be defined and assigned to thermal zones by:
1 - Creating an Air system
Most commonly used approach to add VRF systems is to install them with Dedicated Outdoor Air Systems. DOAS systems complement VRF systems to meet building comfort demands and provide complete, high-performance HVAC solutions. Pairing both these systems provide: Simplified system design and Increased efficiencies. Thus, each VRF system in cove.tool operates with DOAS.
2 - Creating a VRF plant
To create a VRF plant, simply navigate to mechanical system page and select one of the two following options:
VRF Heat Pump: this plant can be selected to model the performance of a 2 pipe VRF system that can be used in cooling or heating mode
VRF Heat recovery: this option can be selected for 3 pipe VRF systems that can be used for simultaneous cooling and heating.
To get more information on how to modify the inputs for VRF terminal unit and plant, make sure to check out following resources:
VRF systems analysis.tool supports
a. DOAS with VRF
b. DOAS w/ VRF, with Electric Boiler and Cooling Tower
c. DOAS w/ VRF, with Gas Boiler and Cooling Tower
d. DOAS w/ VRF, with Ground Source Heat Pump