A Combined system/Dilute Ventilation system using both exhaust and supply systems can be more effective than extract only in large offices, as controlled mechanical intake can be used to give positive gentle air movement in warm weather to create a feeling of freshness. It also reduces the number and size of openings required in the structure for replacement fresh air. Heat Recovery units can be incorporated in this type of system for energy saving during the Winter months and under certain conditions, a degree of cooling during spring and early summer. Where buildings are air conditioned, these savings will continue in the summer.