Q: I'm considering a house ventilation system that pulls fresh air from outdoors into the air handler of my heating and cooling system. Do I have to include some kind of powered exhaust vent also, to compensate for the incoming airflow?
A: Modern tight houses do benefit from a reliable, controlled supply of fresh air. There are three ways to provide this: an “exhaust-only” or “negative pressure” setup that blows air out of the house, pulling makeup air in through holes or cracks in the building envelope; a “supply-only” or positive pressure system that draws outdoor air in through a duct, forcing the indoor air to escape through cracks or holes; or a “balanced” system with ducts and fans for both intake and exhaust.
The system you describe, a duct from outdoors to the return side of your air handler, is one of the simplest supply-only setups. An add-on control for the heating and cooling system, the AirCycler (www.aircycler.com), will open and shut the intake damper as required and run the air handler fan on a user-specified schedule (typically, for 10 minutes every ½ hour to 1 hour).
Building science consultant Terry Brennan explains: “When the air handler comes on and runs 10 minutes in every hour, it greatly increases the effectiveness of your ventilation, by continually mixing the fresh air into the house air. That way, the bedrooms are not under-ventilated, and temperatures are evened-out throughout the building.”
But won't positive air pressure inside the house create a risk of condensation and moisture damage within the cold walls? That question is often raised, says Brennan, but decades of building research have convinced him that over-pressurizing the house is a negligible concern—at least in most of the U.S.
Outside of extremely cold climates, says Brennan, “the simple story is that until you get to such high indoor humidity that you would have other moisture-related problems such as window condensation anyway, you can bring in something like 25 to 75 cfm (cubic feet per minute) of outdoor air and pressurize the building with it, without worrying about forcing moisture into the walls.”
In practice, notes Brennan, a 25-cfm to 75-cfm intake airflow does not create problematically high pressures across the wall: “You would have to build very airtight to have 50 cfm make a significant pressure difference in your house.” Spread out over a typical leakage area of some 100 square inches, the fan-produced pressure would amount to only ¼-pascal or less—minor compared to the pressures resulting from natural weather effects. “If it's at all windy, the wind's the boss,” Brennan observes, “and as soon as it's 40 degrees or cooler outside, the stack effect is the boss.”
Brennan offers field data to back up his point. In one experiment, Brennan and his colleagues set up a Pennsylvania house's HVAC system to create overpressure conditions in the basement. “This was a research project to figure out whether it would be reasonable to pressurize basements to keep radon out. We ran the air handler continuously on low speed, and we put a supply register in the basement with no return.” The net effect was to depressurize the upstairs living area, while over-pressurizing the basement.
“Sure enough,” says Brennan, “if you pressurize the basement, the radon disappears.” But the team worried about water vapor: “If we were pushing warm air out through the enclosure, would we get condensation and maybe have a moisture problem? So we kept track of the moisture content of the rim joists. And lo and behold, the rim joists didn't gain any moisture at all. They started out dry, and they remained dry over the course of the winter.”