Air Leakage -vs- Controlled Ventilation
Until fairly recently the majority of homes were built without any consideration given to proper air sealing, as a result, uncontrolled air leakage tended to make the houses very inefficient and expensive to heat, not to mention uncomfortable for the occupants. However, this type of construction does provide a form of ventilation, although accidental, which is usually able to help reduce the build up of air pollutants. Uncontrolled air leakage is not a recommended way of providing a home with ventilation, it is inconsistant and the benefits it offers in relation to air quality are more than outweighed by the problems that it can cause:
- Increased heat loss making the home much more expensive to heat (uncontrolled air leakage can account for over one-third of a homes total heat loss)
- Cold drafts/cold rooms
- Structural damage caused by warm, moist air leaking into the building shell (which can lead to condensation, mold and severe structural damage)
A much more reliable and effective approach to use in todays world of well insulated and air sealed homes is some type of controlled 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.
Mechanical Ventilation Systems
There are a variety of systems available, from exhaust only types to continuous, balanced mechanical ventilation systems. Balanced systems are recommended because they exhaust stale indoor air and replace it with an equal amount of fresh outside air, on a continuous basis all over the home.
The main types of residential mechanical ventilation systems are listed, along with brief descriptions and the known advantages and disadvantages of each type.
Passive Ventilation Systems
Passive Ventilation Systems are systems which don't use mechanical equipment in order to operate. Examples are an open window or the use of a fresh air (make-up air) duct through which, fresh air is drawn into the home to replace exhausted house air.
Advantages
- Provides some ventilation
- Inexpensive
Disadvantages
- Open windows are impractical in cold weather.
- No means to control the rate of ventilation.
- Uneven distribution of fresh air.
- Cold air entering the house must be heated.
- Cold drafts.
Exhaust Only Systems
Exhaust Only Systems, as the name implies, simply exhaust air from the house, examples are bathroom and kitchen exhaust fans and central exhaust ventilators. While this can help in the removal of some pollutants, if provision is not made to replace the exhausted air, a negative pressure can be created in the home. Replacement air is then drawn into the house through any cracks and openings. This can lead to drafts, backdrafting appliances and in houses which experience problems with soil gases, such as Radon, will increase the rate at which the gas is drawn into the house. These types of problems can be controlled by connecting a passive make-up air duct. As house air is exhausted a negative pressure is created, which draws replacement air in through the make-up air duct.
Advantages
- Direct exhaust
- Relatively inexpensive
Disadvantages
- Doesn't provide for direct make-up air
- Can create a negative pressure in the home
- Frosting of exterior vents during winter
- Ongoing operational cost
- Poor distribution of fresh air
Balanced Mechanical Ventilation Systems
Balanced mechanical ventilation systems are designed to exhaust warm, stale air from the house, while at the same time bringing in an equal amount of fresh outside air, thereby preventing any pressure differences from occurring. A properly designed system should exhaust 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.
Advantages
- Balanced and controllable ventilation rates
- Even distribution of fresh air
- Allows for continuous air filtration
Disadvantages
- Installation cost
- Ongoing operational and servicing costs
- Must be designed and installed properly
Non-Heat Recovery Ventilation Systems
Non-Heat Recovery Systems are one example of a balanced mechanical ventilation system. These systems exhaust stale house air and supply an equal amount of fresh outside air. It should be a system which exhausts and supplies air all over the home.
Advantages
- Balanced and controllable ventilation rates
- Even distribution of fresh air
- Allows for continuous air filtration
Disadvantages
- Installation costs
- Ongoing operational and servicing costs
- Cold incoming air must be pre-heated
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.
Advantages
- Balanced and controllable ventilation rates
- Pre-heats cold incoming air
- Even distribution of fresh air
- Most effective in cold climates
- Allows for continuous air filtration
Disadvantages
- Installation costs
- Ongoing operational and servicing costs
- Must be designed and installed properly
- Can be difficult to install in existing homes
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Information Technology Specialists Inc.
