Field diagnosis and supervisory-control evaluation of mixed cooling–heating operation in a heat-recovery VRF system: A single-building case study

Heat-recovery variable refrigerant flow (HR-VRF) systems can serve simultaneous cooling and heating loads, but persistent mixed operation may indicate insufficient zone coordination. This study screened mixed cooling-heating operation and evaluated a combined supervisory-control intervention in a multi-tenant office building. Data recorded at 60-min intervals from 40 outdoor-unit (OU) systems were used to classify Cooling, Heating, Mixed mode, and Fan only and to calculate estimated VRF electricity consumption from manufacturer-estimated OU and estimated indoor-unit (IU) fan electricity consumption. Mixed-mode intervals accounted for about 16% of the annual classified total. Thermostat-off fan-stop and very-low-load controls were implemented in the selected 10F S area and evaluated by a non-randomized 2024–2025 comparison. After outdoor-temperature adjustment, daytime estimated VRF electricity consumption on ordinary weekdays in four winter months was about 46% lower in 2025. On an unadjusted basis, annual Mixed-mode consumption was about 82% lower, while the outdoor-temperature-adjusted annual total was about 26% lower. The energy-weighted estimated heating coefficient of performance increased from 1.61 to 1.94. IU inlet-air temperature distributions remained broadly comparable. Unadjusted comparisons of non-intervention areas showed both increases and decreases, with no uniform building-wide reduction. Zone-level thermal loads and refrigerant-side recovered heat were not measured, and no formal uncertainty or sensitivity analysis was conducted. The results are observational point estimates and do not establish an isolated causal effect. This study demonstrates a field approach to screening and commissioning HR-VRF operation, but controlled multi-building validation is required.