Read more: How It Works: Heat Recovery Ventilator – Popular Mechanics

While necessity may be the mother of invention, it’s increasing costs that spawn efficiency. Before the ’70s, we happily cranked up the thermostat when the house felt chilly. Once

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The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) sets the standard for residential ventilation at a minimum of .35 air changes per hour, and not less than 15 cubic feet per minute (cfm) per person. An old home may very well exceed these values—especially on a windy day. However, on a calm winter day, even a drafty house may fall below the recommended minimum ventilation standard.

There are partial solutions to the indoor air-quality problem. For example, an electrostatic filter installed in a forced-air heating system will reduce airborne contaminants, but it won’t help with moisture, stale air or gaseous pollutants. And, local

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