One of the most important components of a Passive House is a HRV, a Heat Recover Ventilator.
“What is a Heat Recovery Ventilator?” you might ask. In short, it is the lungs of the house. Airtight houses such as these require means of providing fresh air, and, while of course windows can be opened or a whole-house fan could be used, these aren’t good methods when the temperature outside isn’t what you’d like it to be. The HRV brings in outside air and exhausts stale air at a controlled rate. It passes through filters and a heat exchanger that, when desired, transfers the heat from the warm outgoing air to the cold incoming air without cross-contamination. This way you get fresh air without heat loss- pretty good deal, huh?
With such an important task to perform, we wanted to get the best unit available. The most important characteristics are a high motor efficiency and good heat recovery abilities. To get the best, a building code appeal was needed. The Zehnder ComfoAir 350 from Germany is well tested and widely used in Europe but has not yet been UL-listed, so it isn’t normally allowed. However, with its track record and performance characteristics it was easily approved by the appeal committee and we set about installing the system.
The Zehnder HRV is particularly ingenious in its design. Each supply or return register is connected back to the main unit (located in the utility room) via a 3” flexible duct. The ducts’ size and flexibility is a big advantage since they fit in standard walls and eliminate the more common metal ducting, which is bulky, often leaks, and requires a sheet metal worker. The ducts are flexible yet smooth on the inside, so the air moves freely and the system stays clean. The whole system essentially snaps together. Of course, with 13 ducts converging in the same place at the unit, it looks a lot more complicated than it really is.
Normally a ventilation system is separate from the heating system. However, since we have such low heat loads, we can provide most of the heat for the house with a 1,000 watt in-line duct heater (see picture). Compare that to a typical hairdryer at 1,875 watts! Supplemental heat will be provided in the bathrooms for those extra cold mornings.
The next step is to get the HRV tested and balanced, which will be performed by Zehnder. But we’re looking forward to getting it fired up to see how it works!