Variable Refrigerant Flow (EnergyPlus HVACTemplate)

The Variable Refrigerant Flow (VRF) system in EnergyPlus HVACTemplate is a sophisticated solution for climate control. It efficiently adjusts refrigerant flow based on varying heating or cooling needs in different building zones. This technology enables precise temperature control and energy savings, making VRF an advanced and flexible option for maintaining comfortable indoor environments.

A VRF comprises one Plant HVAC component and one Space HVAC component (per selected Space).

• Plant HVAC component - VRF Condenser Unit

• Space HVAC component - Variable Refrigerant Flow

The 'Select HVAC Template' form allows for multiple Spaces to be served. Clicking into the 'Spaces' field shows a list of available Spaces. Each selected Space will be served with one Variable Refrigerant Flow Space HVAC component.

Variable Refrigerant Flow (VRF) (EnergyPlus HVACTemplate) Metered Outputs

Zone Sizing

Zone sizing determines peak heating and cooling loads for each zone by simulating extreme design days, the hottest day for cooling and coldest day for heating, rather than full annual periods. The outputs include design load in kW and required supply airflow in L/s for each zone. Peak occurrence timing varies depending on orientation and internal gains. A sizing factor, typically between 1.15 and 1.25, is applied as a safety margin.

For VRF systems, these results reveal load diversity since different zones peak at different times. This allows the outdoor unit to be downsized using coincidence factors, which is essential for heat recovery VRF design that enables simultaneous heating and cooling.

Zone HVAC Terminal Unit Variable Refrigerant Flow

The terminal unit is the indoor unit serving individual zones, containing a coil, fan, and controls. It modulates refrigerant flow to meet zone loads. Key design parameters include the Design Supply Air Flow Rate in L/s (airflow during cooling, heating, and ventilation-only modes) and the Design Outdoor Air Flow Rate in L/s (minimum ventilation airflow for air quality).

The unit connects to the outdoor VRF system through refrigerant piping, receiving variable refrigerant flow based on zone demand. Each terminal unit operates independently, providing heating or cooling as required by the zone thermostat.

During operation, the unit modulates refrigerant flow via expansion valves, adjusts fan speed, introduces specified outdoor air, and cycles between modes as needed. Multiple indoor units can operate simultaneously in different modes when connected to heat recovery VRF systems.

Coil Cooling DX Variable Refrigerant Flow

The cooling coil provides modulating capacity through variable refrigerant flow. Design parameters from zone sizing include the Design Size Rated Air Flow Rate in L/s (volumetric airflow across the coil), Design Size Gross Rated Total Cooling Capacity in kW (total heat removal including both sensible and latent), and Design Size Gross Rated Sensible Heat Ratio or SHR (the fraction devoted to sensible versus latent cooling, typically around 0.80, meaning 80% sensible and 20% latent).

The key characteristic is that the coil modulates refrigerant flow to match loads at part-load conditions, avoiding the on/off cycling seen with fixed-capacity systems. Capacity modulation is achieved through electronic expansion valve control and variable compressor speed at the outdoor unit.

During operation, the coil continuously adjusts refrigerant flow and evaporating temperature to meet zone cooling demands efficiently across a wide range of operating conditions, from minimum capacity (typically 10-20%) to maximum rated capacity.

Coil Heating DX Variable Refrigerant Flow

The heating coil provides modulating capacity using heat pump technology, extracting heat from outdoor air. Design parameters include the Design Size Rated Air Flow Rate in L/s (airflow across the coil during heating) and Design Size Gross Rated Heating Capacity in kW (total heat delivery capacity).

These values typically mirror the cooling coil values, indicating balanced design. Performance decreases as outdoor temperature drops, with capacity degradation below 0°C and potential defrost cycling below -5°C depending on system design and outdoor humidity conditions.

During operation, the coil modulates refrigerant flow and condensing temperature to meet zone heating demands. The system maintains efficiency across varying loads through variable refrigerant flow control. Defrost cycles activate periodically during heating mode in cold, humid conditions to maintain performance.

Fan On Off

The fan system model simulates fan performance in VRF terminal units, calculating airflow delivery and power consumption. During zone sizing, it establishes the Design Maximum Air Flow Rate in L/s (the capacity needed for peak cooling load) and the Design Electric Power Consumption in kW (power required at maximum airflow, which scales with zone size).

The fan operates in on/off mode, running when the terminal unit is active and stopping during idle periods. Some VRF systems offer continuous fan operation for improved air circulation and ventilation, though this increases energy consumption.

During simulation, fans cycle with terminal unit operation. When the unit is active for heating or cooling, the fan delivers the required airflow. Power consumption is proportional to operating time rather than modulating with load, unlike variable speed fans which reduce power at part-load conditions.

Air Conditioner Variable Refrigerant Flow

This is the complete outdoor unit system serving all indoor units, including variable-speed compressors, outdoor coil, fans, and controls. Design parameters include the Design Size Rated Total Cooling Capacity in kW (maximum heat rejection capacity), Design Size Rated Total Heating Capacity in kW (maximum heat delivery capacity), Resistive Defrost Heater Capacity (melts frost on the outdoor coil during heating in cold conditions), and Design Size Evaporative Condenser Air Flow Rate in L/s (airflow across the outdoor coil).

An important concept is diversity sizing. The outdoor unit may be sized smaller than the total indoor capacity since zones don't peak simultaneously, which optimises cost and efficiency. Combination ratios (total indoor capacity divided by outdoor capacity) typically range from 100% to 150%.

During operation, the system continuously modulates compressor speed, adjusts refrigerant distribution, cycles fans, and executes defrost cycles as needed. The outdoor unit communicates with all indoor units, balancing refrigerant flow to meet varying zone demands whilst maximising system efficiency.