Exhaust Recirculation Percentage in cove.tool is the amount of heat energy recovered from the exhaust air stream. Recirculating air is common in buildings such as offices where outside air requirements are low and the internal environment is clean. The opposite is true for buildings where air may be contaminated from internal sources, such as labs and hospitals.

ISO 13790 Energy performance of buildings - Calculation of energy use for space heating and cooling defines the methodology cove.tool uses to calculation the energy recovered. The following 4 options to quantify the heat energy recovered from the exhaust air stream are applicable to this method:

For buildings where recirculation will not occur 'None' will correctly represent the operation. For buildings where recirculation is expected, simply estimate the percentage that will be allowed and select the closest option.

The Exhaust Recirculation Percentage input is located on the Baseline page, under the Building Systems tab.

Understanding HVAC Concepts
The control of air flow or ventilation in buildings is important for maintaining suitable indoor air quality (IAQ) and subsequently maintaining a comfortable and healthy interior environment. Outdoor air enters a building through its air intake to provide ventilation air to building occupants. Likewise, building exhaust systems remove air from a building and expel the contaminants to the atmosphere. If the intake or exhaust system is not well designed, contaminants from nearby outdoor sources (e.g., vehicle exhaust, emergency generators, exhaust stacks on nearby buildings) or from the building itself (e.g., laboratory fume hood exhaust, plumbing vents) can enter the building before dilution. Poorly diluted contaminants may cause odors, health impacts, and reduced indoor air quality.