A house built to modern standards of tight construction will allow in only a very small amount of outside air, which improves the home’s energy efficiency. The flip side of this is that fresh air is needed to dilute the products of human habitation (cooking, cleaning, breathing, combustion, etc.), as well as the chemicals that off-gas from building materials. According to the U.S. Environmental Protection Agency, indoor air pollution levels can be as much as five times higher than outdoor levels. So good ventilation is an important part of maintaining good indoor air quality.
Dilution and at-source (spot) exhausting are the main methods for ensuring good indoor air quality. Although houses can be designed to “self-ventilate,” this process depends upon local climate conditions and wind shading—protection by tall walls and trees—which are not controllable. Why not just open the windows? In some very mild climates, this might be a fine strategy. But in most of the United States, trying to ensure adequate air exchange defeats the energy-saving intent of a well-sealed building envelope.
Blower-door tests can reveal how “leaky” or “tight” a house is by measuring how difficult it is to pull air out of the house when it’s all closed up. The rater closes the windows, doors, fireplace dampers, and other openings that are normally closed when the HVAC system is running. The blower door is aluminum-framed canvas with a large fan. When placed in a central outside doorway, it can either pressurize or depressurize the house. As the fan pulls air out of the house, the inside air pressure drops below the outside air pressure. To quantify the process, the rater adjusts the fan until the pressure difference between inside and outside is 50 Pascals. A pressure gauge, connected separately to both outdoor and indoor air with small rubber hoses, determines the pressure difference, and with a conversion table, the rater can determine airflow.
The fan’s airflow, the climate, the size of the house, and the exposure to wind is used to calculate the normal air changes per hour (ACH) rating for the house. With the blower door test at a negative 50 Pascals, a typical home might leak at 15 air changes per hour (15 ACH50). Tightly constructed houses may measure 1 ACH50 or less. (The infiltration rate of a “passive house” has to be 0.7 ACH50 or less.) Estimating the natural infiltration rate of a building is an important step in evaluating indoor air quality and the possible need for mechanical ventilation.
Both heat and energy recovery ventilators use fans to bring in outside air and to exhaust air, balancing the amount of air in versus out. This air balance minimizes airflow through the building envelope—by reducing the pressure differential between inside and outside. They recover energy from the outgoing airstream via a heat exchanger, which allows heat to move between the two airstreams without mixing or cross-contaminating them.
There are various recommendations for sizing mechanical ventilation systems. An older one called for 0.35 air changes per hour (ACH). Alternate recommendations are based upon a recommended airflow per room. Figures vary by manufacturer, but the American Society of Heating, Refrigeration and Air Conditioning Engineers—which sets standards for good residential indoor air quality—recommends continuous ventilation of 0.01 cfm per square foot of living space, plus 7.5 cfm per person. So a 1,500-square-foot home with four residents would require 45 cfm.
—Adapted from “Heat & Energy Recovery Ventilators” by Neil Smith (HP145) and “How Efficient is Your House?” by Allison A. Bailes III (HP106)