This study proposes a novel HVAC retrofit for the space heating systems of post-war multi-unit residential buildings (MURBs). Typically, these buildings do not have in-suite temperature control and each suite must be retrofit with a thermostatic valve to provide this control to residents. However, given the potentially high installed cost of these valves, this retrofit strategy can be cost-prohibitive. To alleviate this issue, a retrofit strategy that uses temperature sensors in each suite with a control system that combines the signals from these sensors is proposed. This approach allows for suite control to be grouped, thereby reducing the equipment quantity, and cost of the retrofit. Field data from a case study building in Toronto, Canada, was used to develop and validate an EnergyPlus simulation tool. Results show that with a financially optimal grouping strategy, a 14% reduction in space heating energy consumption, a 68% reduction in overheating, and a payback period 10 years can be achieved. These results compare favourably to the typical strategy of installing one valve per suite, which resulted in a payback period of 17.8 years. These findings support that using this novel control strategy can be an effective way of economically improving post-war MURB performance.