The Fairfax Zero Energy House: Mechanical system overview
The mechanical system for the Fairfax house differs from our earlier houses in several ways. We have zoned the house differently; we have taken a different approach to the heating and cooling equipment; and we have changed the ERV equipment.
This house is zoned from side to side rather than up and down. This decision had to do with the very different window configurations on the north and south sides as well as the room arrangement in the house.
The Rockville and Bethesda houses have a multi-zone heat pump with two ducted mini-split units: one for the top two floors and one for the bottom two floors. This house has three independent and very small (9000 btu) ducted mini split heat pumps. One unit handles the north half of the house; a second unit handles the south half of the house; and a third unit handles the basement space. We could have gotten away without the third basement unit, but there will be a computer room in the basement with multiple work stations, as well as large groups of people from time to time. The basement unit won’t run much, but when it does it will definitely be needed. This approach was a little more expensive than the multi-zone unit approach but it made sense in light of the fact that this will be a zero energy home. The efficiency of the independent 1:1 minisplits (23 SEER) is far superior to the multi-zone approach (16 SEER). The lower energy demand translates directly to fewer solar panels on the roof.
In the Rockville and Bethesda houses we used Zehnder energy recovery ventilator (ERV) units. In this house we are using the more economical CERV unit by BuildEquinox. A CERV (conditioned energy recovery ventilator) is basically an ERV with a small heat pump inside it. In addition to exchanging heat and humidity from the incoming and outgoing air streams, it conditions the incoming air. While a standard ERV may have 90% efficiency in the temperature exchange, the fresh air coming into the room will still be colder than the room air in the winter and hotter in the summer. With the CERV, the incoming air is at exactly the right room temperature. The other advantage the CERV offers is that it doesn’t run all the time the way an ERV does. BuildEquinox likes to call the CERV “an ERV with a brain.” Built-in software that can be accessed from an IPad monitors VOC’s, CO2 and CO, and turns on the fans when needed, saving further energy. Peter Schneider has had success with this equipment in his VerMod homes and we are following his lead here.