VRF Condenser Curves Matter!

Variable Refrigerant Flow (VRF) systems use one outdoor condenser unit to serve multiple indoor units throughout a building. Unlike traditional systems that operate at fixed capacity, VRF systems modulate refrigerant flow to match the precise heating or cooling demand in each zone.

This variable capacity operation makes them highly efficient and increasingly popular in commercial buildings, hotels, and residential developments. But this continuous modulation also makes them more complex to model accurately.

The condenser curve challenge

VRF condensers exhibit significantly different performance characteristics across their operating envelope. A unit with a nominal capacity of 35 kW at standard rating conditions (35°C outdoor, 27°C indoor wet-bulb) might only deliver 28 kW at 40°C outdoor temperature, or provide 40 kW at milder 20°C conditions.

Outdoor dry-bulb temperature, indoor wet-bulb temperature, part-load ratio, and combination ratio all influence both thermal capacity and compressor power draw. Yet simplified modelling approaches treat rated capacity as constant, ignoring performance degradation at temperature extremes or variations across the part-load operating range.

Without accounting for these variations, you might oversize equipment by 15-20%, underestimate annual energy costs by thousands of $$$$, or miss comfort issues that only manifest at peak outdoor conditions.

Performance curves address this directly

Condenser performance curves mathematically describe capacity and energy input ratios as functions of operating conditions. Capacity modifier curves adjust output based on temperatures, whilst energy input ratio curves account for compressor efficiency variations. Boundary curves define transition points where performance characteristics shift between operating regimes.

EnergyPlus includes default VRF performance curves that work well for generic modelling and early design studies. However, to accurately model your specific system, these defaults need replacing with technical performance data from the manufacturer.

Different VRF models from Daikin, Mitsubishi, Toshiba, or other suppliers have distinct performance characteristics. A high-efficiency unit will have different capacity degradation at extreme temperatures compared to a standard model, and part-load curves vary between inverter designs and control strategies.

This matters particularly for VRF because these systems rarely operate at full rated output. Understanding coefficient of performance (COP) at 30% part-load versus 80% part-load is fundamental to capturing actual energy consumption patterns.

What this means for your projects

Our platform now allows you to input manufacturer-specific performance curves, so your energy models reflect the actual VRF system you're specifying rather than generic assumptions. This yields more accurate load calculations, appropriate equipment sizing, and defensible energy predictions that directly impact project outcomes and operational performance.