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In many older office buildings, the constant hum of air conditioning masks a hidden cost. Cooling systems that run continuously, regardless of whether rooms are occupied, can account for a large share of a building's electricity use. A growing alternative is the use of occupancy sensors linked to the cooling system. These small devices detect movement and body heat, allowing the system to reduce cooling in empty meeting rooms and corridors. Trials in several commercial buildings have reported noticeable drops in energy bills, with comfort levels in occupied spaces left unchanged. The main barrier remains the upfront cost of installing the sensors throughout an existing building.