Older homes rarely suffered from a lack of fresh air. Air leaking in through cracks and holes in older homes and poorly built new homes can allow the entire house air volume to change more than once every hour. Air also leaks in to replacing air which is used by the chimneys and exhaust appliances or through upper storey windows (because hot air rises). Energy efficient homes do not have such air leakage problems. Homes today can be built so airtight that the entire volume of the home would take many days to be replaced. This would, however, lead to poor indoor air quality causing stuffiness, indoor pollution, odour buildup and high humidity problems.
Although energy efficient homes stay cooler in summer because of high insulation levels, natural ventilation should be provided with opening windows or screened doors. On one and one-half or two-storey homes, windows opening on different levels will promote natural ventilation by convection on warm summer days and nights. Openings on different sides of one-level homes will permit cross-ventilation. There are times ventilation may be required in the winter as well. Sunny warm winter days (with a low sun angle) may cause short overheating problems in a well-insulated, properly oriented home - a fast and easy solution is to simply open a window or two for a short period of time.
Mechanical Ventilation Systems
A much more reliable and effective approach to use in today's world of well insulated and air sealed homes is some type of controlled mechanical ventilation system. With a mechanical ventilation system occupants are able to control the ventilation rate, and have the ability to keep air pollutant levels as low as possible while increasing oxygen levels and avoiding the problems associated with uncontrolled air leakage. There are a variety of systems available, from exhaust only types to continuous, balanced mechanical ventilation systems.
Balanced ventilation systems are recommended because they exhaust stale indoor air and replace it with an equal amount of fresh outside air, thereby preventing any pressure differences from occurring. These systems should be designed to exhaust warm, stale air from major pollutant sources, such as bathrooms, kitchens, hallways and laundry rooms, while distributing fresh incoming air equally throughout the rest of the house.
Balanced Mechanical Ventilation Systems
Non-Heat Recovery Systems are one type of balanced mechanical ventilation system. These systems use separate fans to exhaust stale house air and supply an equal amount of fresh outside air. This maintains the pressure balance within the house. It should be a system which exhausts and supplies air all over the home with separate ductwork or through a forced air system. Ventilation rates should be maintained between one quarter to one third air changes per hour (ACH).
Heat Recovery Ventilation Systems
Heat Recovery Systems are another example of a balanced mechanical ventilation system. They exhaust stale air and supply an equal amount of fresh air. The two streams of air are passed through the core of the heat exchanger, where heat from the exhaust air is passed to the cooler incoming air. Fresh air supplied to the rooms of the house has already been pre-heated, reducing the problems with cold drafts and the extra expense of pre-heating cold incoming air.
Since the stale, humid air that has to be exhausted contains heat, reclaiming some of that heat can reduce the energy loss while pre-heating cold incoming air. An air to air heat exchanger (also called a Heat Recovery Ventilator or HRV) is commonly used in energy efficient housing to extract heat from the outgoing air.
Currently available units are capable of extracting 70% to 80% of the heat from the exhausting air. Tying a heat exchanger into the return air duct of a forced-air heating system works well. The incoming fresh air is distributed evenly to all living spaces by the heating system duct work. An alternative for housing using non-forced air heating systems is to have the air-to-air heat exchanger separately ducted into each room. Either way the fresh air will mix well and, if a ventilation rate of one quarter to one third the total house volume is maintained each hour, humidity, odours and indoor air pollution will not be problems.
Drawing air from bathrooms through a heat exchanger instead of exhausting it outdoors also saves heating energy in winter months. As mentioned in the construction section 'Roofs and Ceilings', exhaust ducts should be vented down interior walls to the floor joist space where they could easily be attached to a heat exchanger. Excessively humid air can cause an ice buildup in a heat exchanger but most commercially made models have a defrost cycle to control ice buildup.
Because of potential grease and lint problems, range hoods or clothes dryers should not be exhausted through an air-to-air heat exchanger. Recirculating range hoods with good quality filters will eliminate having to exhaust air from that source.