This report examines the retrofit of French Quarters Shared Facilities, a 151-unit Toronto condominium complex, where an air-to-water heat pump system replaced a chiller at end-of-life. Based on two years of detailed performance monitoring, it was found that the heat pump system met all cooling needs and offset approximately 70% of the space heating energy previously provided by the existing boiler. Natural gas use was reduced by 21,000 m³/year, and carbon emissions were reduced by 30 tonnes CO₂e/year. The report documents numerous lessons learned related to system design, control strategies, operational costs, auxiliary energy use, operation and maintenance, as well as other factors. These findings offer practical guidance for future retrofits and support the broader decarbonization of multi-unit residential buildings.