HVAC-Integrated Direct Air Capture in Buildings: A Systematic Review and Normalization of Energy, Climate, Cost, and Energy-Saving Performance

Integrating direct air capture (DAC) with building heating, ventilation, and air conditioning (HVAC) systems could use existing air handling infrastructure for CO2 capture while reducing ventilation-related energy demand through increased recirculation. However, reported energy, climate, cost, and energy-saving outcomes are difficult to compare because studies use different system boundaries, heat supply assumptions, and baselines. This review systematically synthesizes 38 studies on HVAC-, air handling unit-, duct-, and ventilation-integrated DAC. A normalization framework separated electrical demand, thermal demand, heat delivery assumptions, cost boundaries, and energy-saving baselines. Across 15 studies in the harmonized energy comparison, the grid-drawn electricity equivalent ranged from 475 to 2774 kWhe/tCO2, strongly influenced by regeneration heat supply and system boundary choices. Break-even grid carbon intensities ranged from 360 to 2103 gCO2/kWh, with 13 of 15 cases maintaining a positive operational gross-capture balance at an illustrative 400 gCO2/kWh. These climate indicators were interpreted as carbon removal metrics only where the captured CO2 was associated with sufficiently durable retention; for vented or otherwise non-durable pathways, they were treated as diagnostic operational balances rather than evidence of carbon removal. Six comparable gross levelized cost estimates were normalized to 173–382 USD2024/tCO2, with a median of 240 USD2024/tCO2. Reported building energy savings could not be pooled because 18 studies used seven baseline types, with at least one system shifting from energy saving to penalty across climates. The evidence remains predominantly modelled and unevenly reported, highlighting the need for boundary-explicit field measurements and consistent reporting.