Running the Ductwork
After locating and hanging the HRV, we cut holes in the exterior walls for the weather hoods that let air into and out of the building. The AHFC requires that the hoods be at least 6 feet away from each other horizontally. And the IRC requires that the intake be located a minimum of 10 feet from any sources of contamination, such as chimneys, plumbing vents, and areas where car exhaust is present. The manufacturers recommend placing hoods at least 18 inches above the ground.
Locating registers. Exhaust registers have to go near the ceiling, because this is where the warm, humid air we want to get rid of naturally accumulates. Supply registers also work best if they can be placed in or near the ceiling, because that allows the relatively cool supply air to mix with the warmer room air as it sinks. Another good practice is to site both supply and exhaust registers across the room from the door, so air can mix as it travels into or out of an area.
Retrofitting ducts and registers into an existing home can be difficult. The drywall is up, the framing is where it is, and the budget may not allow you to open up walls. In a house with baseboard heat where we can’t put the supply duct in the wall, we often install supply registers in the floor next to the heat strip. As fresh air comes out of the register, convective currents from the heating system disperse it throughout the room. While not ideal, this approach does allow us to put the register across from the door and next to an exterior wall in which we couldn’t or wouldn’t want to install a duct. The floor registers themselves are easy to install — they fit into a supply boot connected to a main duct run in or under the joist bay below.
Through the plate and into the wall. For exhaust ducts, we like to go from the main underfloor duct run to a wall-mounted register by cutting an opening through the subfloor and bottom plate of a partition wall and sliding an oval duct riser into the stud cavity above. Once the duct is in position, we cut a hole through the drywall — aligned with a precut hole in the side of the riser — and snap in the register. If the register has to be in the wall and there’s no way to get the duct into the framing cavity, we’ll cut a hole in the floor and bring a round exhaust duct through it. With luck, we can go through a closet to a wall register, but in some cases we have to box it into a corner or run it along the ceiling and build a soffit around it.
Each joint in the metal ductwork is connected with three self-tapping sheet-metal screws and carefully sealed with duct mastic or butyl tape. We use canned spray foam to seal around ducts where they penetrate floors and rim joists.
The ventilation system can be equipped with a variety of control mechanisms. We recommend using a top-of-the-line programmable controller from the HRV manufacturer. These controllers can be set to run intermittently or continuously at varied speeds, or programmed to respond to indoor and outdoor humidity. In the recirculation mode, they can move air from place to place within the home to equalize variations in temperature. An example of this would be a house with a wood stove, where the room with the stove is too warm and the back bedrooms too cold.
Spot ventilation. In addition to the master control in the main room of the house, there should be booster controls in bathrooms and kitchens. These units — which work by either turning on the HRV or increasing the exhaust speed — can be used to provide extra ventilation in those areas. Different types are available, from simple push-button timers to more sophisticated, programmable units that respond automatically to changes in humidity.