Variable Refrigerant Flow (incl. Outside Air)
Variable Refrigerant Flow (VRF) systems with Outside Air integration expand on traditional VRF setups. They introduce the capability to incorporate outside air for ventilation, enhancing indoor air quality. This comprehensive HVAC solution efficiently regulates refrigerant flow, providing both precise temperature control and fresh outdoor air for a healthier indoor environment.
It is composed of:
VRF Condenser Unit
Air Handling Unit
VRF Terminal Unit
The 'Select HVAC Template' form provides options, to allow you to configure the system with:
A single VRF Condenser Unit connected to multiple VRF Terminals Units or multiple VRF Condenser Units connected to VRF Terminal Units
Optional connection to a central Air Handling Unit (for outside air supply)
A single AHU connected to multiple VRF Terminal Units or multiple AHUs connected to multiple VRF Terminal Units
Choice of Spaces for VRF Terminal Units
VRF 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 Meters
Zone meters are virtual sub-meters that track energy flows within each zone throughout the annual simulation, providing monthly summaries. They track two categories: electricity meters record electrical demand in kWh for fans and terminal units (representing utility costs), while supplied energy meters track the thermal energy in kWh delivered for heating and cooling (the useful output).
Each meter records total monthly energy in kWh, peak instantaneous power in kW, and the timestamp when peak occurs. You can use this data to compare zones and identify consumption patterns, assess efficiency by comparing electrical input to thermal output, and validate that load diversity and part-load performance match expectations.
Plant Meters
Plant meters track the central equipment, which for VRF systems means the outdoor condensing units serving all zones. The system separates tracking for cooling and heating: VRF Condenser Cooling Electricity shows summer peaks occurring in mid-to-late afternoon, while VRF Condenser Heating Electricity shows winter peaks in early morning hours.
These meters provide monthly energy totals and peak demands, revealing the coefficient of performance under real conditions, total building HVAC electrical demand, and validation of outdoor unit sizing against rated capacity.
Fan System Model
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).
During the simulation, fans modulate to match actual loads, consuming less power at part-load conditions than at design maximum.
Zone HVAC Terminal Unit VRF
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).
During operation, the unit modulates refrigerant flow via expansion valves, adjusts fan speed, introduces specified outdoor air, and cycles between modes as needed.
Coil Cooling DX VRF
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 here is that the coil modulates refrigerant flow to match loads at part-load conditions, avoiding the on/off cycling you'd see with fixed-capacity systems.
Coil Heating DX VRF
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. Keep in mind that performance decreases as outdoor temperature drops.
Air Conditioner VRF
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. During operation, the system continuously modulates compressor speed, adjusts refrigerant distribution, cycles fans, and executes defrost cycles as needed.
Air Loop HVAC
The central air distribution system delivering conditioned outdoor ventilation air to zones served by VRF terminal units. In VRF systems with dedicated outdoor air systems (DOAS), the air loop handles ventilation separately from VRF heating and cooling. It consists of an air handling unit, ductwork, and air terminals introducing outdoor air to each zone.
Air handling units (AHU's) are the central equipment conditioning and distributing outdoor ventilation air. Design parameters include the Sum of Air Terminal Maximum Flow Rates in L/s (total ventilation required across all terminals), Adjusted Design Air Flow Rates in L/s (final design values after adjustments), and Design Supply Air Flow Rate in L/s (total AHU fan capacity). When heating and cooling flow ratios equal 1.000, airflow remains constant regardless of mode, typical for ventilation-only systems.
Multiple AHUs with different capacities indicate dedicated outdoor air systems for each zone or zone group, providing decoupled ventilation while VRF terminal units handle thermal loads.
Controller Outdoor Air
The control component regulating outdoor air intake based on ventilation requirements and schedules. Maximum Outdoor Air Flow Rate in L/s sets the highest permitted outdoor airflow, while Minimum Outdoor Air Flow Rate in L/s sets the baseline during occupied periods. When maximum and minimum rates are identical, the controller operates at fixed airflow without modulation, typical for dedicated outdoor air systems serving VRF installations where consistent ventilation is maintained regardless of zone thermal loads.
Fan System Model (Air Handling Unit)
The computational model simulating fan performance for AHUs delivering outdoor ventilation air. Design Size Design Maximum Air Flow Rate in L/s represents the fan's volumetric capacity, matching the outdoor air controller's maximum flow rate. Design Electric Power Consumption in kW represents power required at maximum airflow. Zero or near-zero fan power for smaller units may indicate passive outdoor air delivery or reliance on VRF terminal unit fans for circulation.
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