Exhaust ventilation systems aren't designed to bring fresh air into the house; but by exhausting stale air to the outside they create a negative pressure inside the building, causing outdoor air to enter the building, either through leakage pathways, windows or dedicated openings… On the other hand, supply ventilation systems are not designed to exhaust stale air; but by bringing in outside fresh air they create positive pressures within the house, forcing inside air to leak out.
Both systems can cause significant energy loss; both systems will introduce directly or indirectly non-tempered outside air, too cold in the winter, too hot in the summer…
And that's where balanced ventilation systems enter. Balanced ventilation systems are designed to overcome the disadvantages of exhaust and supply ventilation systems, by balancing the amount of air moving into and out of the house and by transfering part of the heat and humidity in the air streams (the fresh air stream and the stale air stream)..
The more simple balanced ventilation systems combine an exhaust system (involving the bathrooms and the laundry and kitchen) with a supply ventilation system for the bedrooms and the living room; they are a two-in-one system. This system doesn't provide the heat and humidity transfer mentioned above.
Other more complex systems are designed to limit energy loss (Heat Recovery Ventilation systems) and also, in addition, to control indoor moisture (Energy Recovery Ventilation systems).
Simple balanced ventilation systems
A balanced ventilation system can be – in its simplest version – a combination of two separated sub-systems: an exhaust and a supply ventilation system.
The supply ventilation system may use the existing furnace/HAVC blower and ductwork, or an independent fan and an independent ductwork.
The first solution is relatively inexpensive and easy to implement, but the fans and the ductwork of the heating and cooling systems are not optimized for ventilation - a reason why these systems may prove to be ineffective.
Obviously, the two-subsystems should work in tandem, and match each other in capacity.
The image at right, from EnergySavers.gov, shows a simple balanced ventilation system.
HRV/Heat recovery ventilation systems
Heat Recovery Ventilation (HRV) and Energy Recovery Systems (ERV) are sophisticated versions of the simple balanced ventilation systems mentioned in the previous point.
HRV systems involve a fan, two sets of ducts (one for air exhausting and other for fresh air supplying) an a heat exchanger system.
When the supply and the exhaust air streams pass through the heat exchanger (without mixing) the heat contained in the exhaust steam is transferred to the incoming supply airstream (in heating mode), or transferred from the supply air stream to the outgoing exhaust airstream (cooling mode). And by doing it, the system recaptures (or exhausts) at least 70-80% of the heat, which is a significant saving.
In other words: they can reduce the energy penalty associated with ventilation systems.
ERV/Energy recovery ventilation systems
Energy Recovery Ventilation (ERV) systems operate like HRV systems. The difference is that they involve a special heat exchanger able to both transfer heat and moisture from the airstreams.
ERV systems can reduce the costs of air conditioning, by keeping the indoor air drier, which is an important feature in humid climates.
The costs of balanced ventilation systems
Expect prices between $2.000 and $4.000 for a balanced ventilation system.
Prices depend on the size of the ductwork, its type (prefer steel ducts properly insulated) and on factors involving the fans and the heat-exchangers... Aprilaire, Lennox, Honeywell, Fantech and Stirling are well-known brands. See, for a more complete list: Toolbase Manufacturers of HRV systems.
Prices and customer reviews on Amazon: HRV ERV ventilation.
Advantages and disadvantages of balanced ventilation systems
Balanced ventilation systems have obvious advantages; they control pollutants and moisture and provide fresh air. They are critical in airtight homes in both cold and harsh-hot climates.
But they are a lot more expensive than simpler ventilation systems, and involve higher operating costs (electricity costs) and more maintenance.
On the other hand, though they can provide significant energy savings, it's easy to compromise the efficiency of these systems at the design and installation stages of the project. They require a well-designed ductwork (straight, non-corrugated ducts, properly sealed and sized) and a very well-calibrated multi-speed fan system.