"With new homes becoming increasingly airtight, building codes have been revised to require the supply and distribution of fresh air and the removal of stale air to prevent indoor air quality problems. This has lead to a steady increase over the past 10 years in the use of packaged heat recovery ventilators (HRVs). HRVs are relatively complicated devices that require frequent maintenance and adjustment. Concerns existed that most homeowners do not understand the problems that can occur if their HRV is not maintained and adjusted, such as over-or under-ventilation, backdrafting of combustion appliances or building envelope deterioration due to excess air exfiltration or poorly controlled humidity" -- CMHC TS 96-215