UPDATED on March 13, 2015
By designing a tight envelope with thick insulation, Passivhaus designers work hard to whittle a home’s space heating load to a bare minimum. Many European designers strive to get the heating load so low that all space heat can be provided by raising the temperature of the ventilation air.
In a home with an area of 1,600 square feet and a ventilation rate of 0.3 ac/h, ventilation air flow is only 64 cfm. Since Passivhaus designers try to keep the temperature of the ventilation air below 122°F (or, according to some sources, 131°F), it’s hard to pack much heat into the small volume of air that flows through typical ventilation ducts. That’s why it’s such a challenge to insulate a building’s shell well enough to deliver all of a home’s heat through its ventilation system.
In central Europe, where winter temperatures are much milder than they are in Minnesota or Maine, some designers have succeeded in supplying all of the space heat needed for a Passivhaus through ventilation ducts. In most cases, these homes are equipped with a “magic box” — the nickname for a combination appliance that includes a heat-recovery ventilator (HRV) and an air-source heat pump. Typically, the heat pump’s evaporator coil is located in the ventilation exhaust duct, downstream from the HRV, where the coil can scavenge heat from the exhaust air before it leaves the building.
Many (but not all) of these magic-box appliances also include a hot water tank. Such appliances use the air-source heat pump to heat domestic hot water; in most cases, the tank includes electric-resistance backup. Some manufacturers (including Zehnder) design their magic-box appliance around a ground-source heat pump instead of an air-source heat pump.
Magic boxes in North America
I’ve heard rumors that magic-box…