Bathroom and kitchen fans are two well-known examples of exhaust ventilation systems; they move air out of the house… Supply ventilation systems, on the contrary, move air into the house.
Exhaust ventilation work by depressurizing the building; fans create negative pressures into the living space, which brings outside air into the house through dedicated vents or, in many cases, through leaks in the building envelope.
Supply ventilation, on the contrary, work by pressurization; it sucks outside air into the house, creating positive pressures and causing inside air to leak out through holes, cracks and openings, or through ducts and vents, if any exist, or through open windows and outlets.
exhaust ventilation systems
Exhaust ventilation systems can be very simple, as shown by common bathroom and kitchen exhaust fans.
Most homes have them.
Just make sure that the exhaust fans are properly ducted to the outdoors; otherwise the system may be just moving moisture and stale air to elsewhere in the house.
But besides simple exhaust fans there are also central exhaust systems, with a fan or a multi-port ventilation unit installed at the basement or attic or other place outside the home’s shell, pulling air – through ducts - from the rooms, and exhausting it to the outdoors.
These systems are critical in airtight homes, and can work in conjunction with spot ventilation; they can run full-time at an adjustable rate, or be scheduled to run according to your needs.
Types of supply ventilation systems
As mentioned above, supply ventilation systems work by pressurization; they bring outside air into the house, causing an equal amount of inside air to exit the building.
Most supply ventilation systems use the existing ductwork of the central heating and cooling systems, and their blowers. They only require a small supply duct (and a small motorized damper) connecting the air handler to the outdoors, to bring in the outside air.
Though inexpensive and easy to install this solution doesn't provide ventilation when the heating/cooling system isn't running (though there are now modern fan controls to allow it) and can be very inefficient: heating and cooling systems are designed to move many times more air than that needed for ventilation. The ducts of the heating and cooling systems are not optimized for ventilation purposes; they usually waste too much energy.
The best supply ventilation system is a small stand-alone system, with small dedicated ducts. Like central exhaust systems, these systems can run full-time at a certain rate, or be scheduled to run automatically, at certain times.
supply ventilation advantages
Supply ventilation systems may have some advantages over exhaust systems. They allow better control of the air that moves into the house; more exactly, they allow outdoor air to be filtered (to remove dust and pollutants) or dehumidified, which is very important in high-humidity climates or in high-humidity periods.
Exhaust ventilation fans, on the contrary, may cause the entry of radon and molds from crawlspaces, or dust (from attics), fumes (garages), flue gases, and so on. It can be a serious problem when there aren't dedicated vents, and the outdoor air moves into the house through holes and cracks.
Supply Ventilation Disadvantages
But supply ventilation systems have also their own disadvantages, especially if they aren't properly designed.
Since they create positive pressures in the house, if the system isn't designed to filter and heat the air moving into the house, or if that air isn't previously dehumidified, that can cause moisture issues and higher energy costs.
On the other hand, indoor moist air will move across the building envelope, through cracks and holes in the walls and ceilings, causing mold, mildew and other damage.
The alternative is a well designed balanced heat recovery ventilation systems.
Images in this page: Energy.Gov Moisture Control