Mechanical Ventilation Options
After reducing moisture sources, mechanical ventilation can be used to further control moisture conditions and improve indoor air quality. Ideally, all mechanical ventilation systems should be “balanced” — the amount of air drawn into the home should equal the amount of air exhausted out.
Types of Ventilation
Passive (or natural) ventilation
Many older houses have to rely on passive ventilation. While windows can be comfortably opened in the summer months, in the winter, it usually only possible to open one for a short time. This can provide temporary ventilation, but is not always effective or economical. Passive ventilation cannot be controlled and it is difficult to ensure ventilation is provided where needed. Passive ventilation can also cause comfort problems due to drafts and adds to space conditioning bills. In some areas, opening a window for ventilation can be a security concern.
The minimum mechanical ventilation system for many existing houses consists of exhaust fans in bathrooms and a range hood in the kitchen. Exhaust-only ventilation works well in the room where it is installed but other areas of the house may not get the ventilation needed. Caution: Exhaust-only systems can depressurize a dwelling and this can cause dangerous combustion venting problems with some fuel-fired appliances, such as furnaces, water heaters and fireplaces. Exhaust-only systems may encourage dangerous soil gases to enter the house as well. The National Building Code of Canada contains measures to protect against house depressurization and prevent combustion spillage and soil gas concerns. If an exhaust-only system is considered, a qualified ventilation contractor should be consulted.
Balanced ventilation systems
Properly installed and operated, balanced ventilation systems don’t have the same worries as exhaust-only systems. A balanced ventilation system can be as simple as installing an exhaust fan, or fans, to run in conjunction with a fresh air intake duct that is connected to the furnace return air duct. The exhaust fans draw air out of the kitchen and bathrooms while the furnace draws air into the home through the fresh air intake duct and delivers it to all the rooms via the forced air duct system. The problem with this approach is that the furnace fan has to be wired to run when the exhaust fans run. Additionally, the introduction of cold outside air into the furnace ductwork also represents an added heating cost and the furnace fan electricity consumption can be significant over the course of a year. As well, it can be a complicated system to set up and properly to run in a balanced model.
Heat recovery ventilation
Heat recovery ventilators (HRVs) offer a well-engineered and simpler approach to achieving balanced ventilation but with the added benefits of recovering the energy in the outgoing exhaust air and using it to warm the incoming outdoor air. HRVs save on ventilation-related energy costs and help prevent occupant discomfort associated with the introduction of cool outdoor air into the home. HRVs can be installed in a number of configurations depending on space heating system type.