When does demand-controlled ventilation pay back its embodied carbon? An incremental life cycle assessment for office, lecture and retail spaces in Beirut, Lebanon

Demand-controlled ventilation (DCV) is widely prescribed as an energy-efficiency measure, yet its added sensors, actuators, controllers and drives carry embodied carbon, and its controls draw power around the clock. Studies that count only the embodied side can conclude that DCV raises a building's footprint; studies that count only operation cannot tell when, or whether, the hardware pays for itself. This study quantifies the carbon payback of CO2-based DCV against constant outdoor air (CAV) for three space types in Beirut, Lebanon — an office, a lecture hall and a retail floor — where a humid coastal climate imposes a large latent ventilation load and where roughly 60% of electricity comes from private diesel generators. An incremental, attributional life cycle assessment following ISO 14040/44 was built for a functional unit of ASHRAE 62.1-compliant outdoor-air provision to 1,000 m² over 20 years. Operational energy (fans, cooling, heating and control parasitics) was modelled on 288 representative hours derived from Beirut climate normals; embodied carbon of the incremental hardware was taken from manufacturer EPDs and PEPs, with EN 15978 replacement cycles. Under the Lebanese electricity mix (0.626 kg CO2e/kWh), DCV saved 530, 14,341 and 10,799 kWh/yr per 1,000 m² in the office, lecture hall and retail cases, and its incremental embodied carbon of 626, 488 and 269 kg CO2e was repaid in 22.7, 0.7 and 0.5 months respectively. Twenty-year net benefits were 5.5, 179 and 135 t CO2e per 1,000 m². Monte Carlo analysis (10,000 paired runs) confirmed a positive net benefit in every run for the lecture hall and retail cases, but only in 79% of runs for the office, where the floor-area component of the ventilation requirement limits airflow reduction to about 10% and control parasitics consume 39% of the gross saving. Break-even analysis placed the office threshold at 3.2 persons/100 m² of design density or 66% mean occupancy. DCV is a carbon win in densely and variably occupied spaces and should be mandated there; in low-density offices it needs commissioning requirements and low-standby controls to be justified.