Houses need to have an indoor/outdoor exchange of air to replenish oxygen used by the occupants and to remove moisture and pollutants generated by:
- cooking, and
- emissions from building materials and furnishings.
The homes of bygone years were constructed with ventilation systems built right into them, in a funny way. The lack of proper sealing, air or vapour barriers, minimal insulation and open fireplace chimneys allowed for the installed natural draft appliances or fireplaces which consume large quantities of combustion air from inside the house to pull fresh air in through all the leaks. As such, the air volume in the home would be turned over many times an hour, thus removing unwanted airborne contaminates, particulate and moisture.
As building technologies improved in the late 1960s and fuel prices rose in the 1970s, a focus was put on energy efficiency and a trend began towards making houses air tight. Openings were sealed, better windows and doors were installed, air and vapour retardant barriers were installed and better fitting and performing insulation was installed. As a result, homes, especially those with electric heat and thus very low air circulation, begin to suffer from poor indoor air quality, high humidity levels and mould growth. The health of the tenants, and the houses, suffered.
The building code responded to this issue by mandating the installation of a mechanical ventilation fan in every new home, a practice which is still in effect today. This system is often embodied by a three way switch which controls one of the bathroom fans, known as the principal exhaust, located beside the thermostat controls. The problems with this system are that:
- it is not operated by the furnace/air conditioner controls,
- it has to fight against well sealed homes to pull in fresh air,
- does not deliver the fresh air evenly around the home, and
- most importantly, in most homes, it is never used.
The main reasons for this system never being used are inconvenience and cost. From a cost perspective, all the air which is pulled out by the exhaust fan had money spent on it, to cool it, or to heat it and to moisture control it to a comfortable condition. From a convenience perspective, the home owner also has to remember to turn the fan on and then off again and these installed bathroom fans can be quite noisy while they are running.
A viable solution to this problem is a device called a Heat Recovery Ventilator or HRV. An HRV has an exhaust fan which is operated by the furnace and air conditioner controls. It collects air from inside the building envelope and exhausts it outdoors while inducing fresh air into the building. The energy efficiency comes via an air-to-air heat exchanger which recovers the heat from the incoming and outgoing air as defined by the season and moves it outside in the summer and inside in the winter. Simpler units take and replace the air right at the furnace and more expensive units have duct work to exhaust and supply fresh air to specific rooms of the home.An HRV is a great way to provide mechanical ventilation to your home to improve indoor air quality and is an asset and worthwhile upgrade in a home of any age.