Air-tight houses need mechanical ventilation for fresh air. In homes built to pass the tough Passive House standard, this requirement is even more absolute — the standard requires mechanical ventilation for many reasons, but one good reason is the envelope air-tightness. The basic Passive House standard calls for a blower door test of 0.6 ACH50, far tighter than even the most stringent building code. In a building that tight, natural air flows won't supply adequate fresh air.
In practice, ventilation in a Passive House has to come from a heat recovery ventilator (HRV) or energy recovery ventilator (ERV). That's because the Passive House standard also includes very low allowable energy use limits. To meet the spec, designers must avoid the heating or cooling load that would accompany fresh air ventilation without heat recovery.
This month, JLC visited Placetailor's "Supply House" spec project in the Boston, Mass., neighborhood of Roxbury to see an advanced ERV being installed (see slideshow). The project is a two-family building with one upper and one lower apartment on a sloped, wooded infill site on Roxbury's Beech Glen Road, in an area that is seeing a building and remodeling boom as homebuyers and renters flock to the area in search of affordable housing.
Placetailor opted to install ERVs made by Air Pohoda, a newcomer to the U.S. market. The Air Pohoda units have an innovative feature: They come with an active pre-chiller for incoming air that serves to remove humidity from the incoming airstream in summer, when the humidity is concentrated. In theory, this offers the advantage of energy-efficient humidity control for the building along with energy recovery. Placetailor is one of the first U.S. builders to put Air Pohoda to the real-world test. (Watch this YouTube video of Air Pohoda technician Roman Salomoun introducing and explaining the Air Pohoda i ERV technology.)
The design for the three-story, two-family house has a downstairs unit occupying the entire first floor and half of the second floor, while the upstairs unit occupies half the second floor and all of the third floor. Heavy engineered beams in the floor structure supporting the second floor, which are necessary to achieve the open floor plan in the first story and to support the party wall dividing the two second-floor occupancies, would tend to obstruct the air supply and return lines for the ERV air distribution system. But the building's double-stud exterior walls, designed to provide a fat insulated wall free of thermal bridging, also create a wide-open void for the three-inch corrugated plastic air hoses. Staying one step ahead of the insulators, Placetailor project manager Travis Anderson and carpenter Diego Gutierrez ran the lines from the ceiling registers through the ceiling and wall to the Air Pohoda air handler.
