Exhaust only ventilation systems are prevalent in the portfolio.  Most new construction projects have a balanced system, when the exhaust & supply systems are working in unison.  Exhaust only systems are problematic for a few reasons.  They create a negative pressure inside the building which allows air from the exterior and neighboring units to enter other units.  The air from the outside is likely colder, warmer and/or more humid so the heating and cooling systems have to work harder to temper that air.  Air from neighboring units is likely to have odors that are unpleasant to residents.  There is also the opportunity for contaminants to enter the building (i.e. Radon) if an exhaust only system in used. Exhaust systems are typically installed in bathrooms and kitchens.  See below for more information and preferred products on both.