The mechanical system at the modular Passive House

Yesterday Michael Bonsby of Michael Bonsby Heating and Air Conditioning and his crew drove over to Greenwood to rough in the mechanical system. Similar to our Rockville Passive House, we are using a Mitsubishi mini-split heat pump system and a Zehnder ERV system.

Installing parts of the systems in the factory and parts on-site make the logistics a bit more complicated, and took some planning, but the overall design is quite simple. We were fortunate that Michael was willing to take his crew over to do all the rough-in work for the Mitsubishis. This eliminates the chance things falling through the cracks had we had a Greenwood mechanical subcontractor to do this work.

Because of the open plan of the first floor, heating and cooling of that level will be handled by a single minisplit cassette mounted in the ceiling of the kitchen. This central location allows it to blow air to the living room in the front and the dining room in the rear. Because the second floor is divided up into bedrooms, a centrally located cassette was not an option. We are therefore installing a ducted minisplit in a closet upstairs, with short duct runs to each of the spaces.

Ventilation will be handled by the Zehnder Comfoair 200, located in the mechanical closet on the first floor. Instead of a standard metal duct system we have used in the past, we are using the Zehnder Comfotube system. This allows us to sneak the 3” flexible ducts throughout the first and second floor. To facilitate this even more we designed a large chase running between the floors adjacent to the mechanical room and we are framing the second floor with open web wood truss joists. This allows running pipes and ducts “across the grain” without the need for dropped ceilings for anything running below the joists. An added benefit of the Comfotubes for modular construction is that anyone can install them — you don’t need sheet metal workers– so Beracah can do all the duct insulation work onsite with their regular crew.

Because the house is built over a crawl space, the first floor framing system is sealed and insulated and no ducts can run through that framing. We are therefore placing all the ERV supply and return ducts for both floors in the second floor framing cavity. For a more in-depth look at the installation of a Comfotube system, you might want to check out Zehnder’s video of the process for installing their Comfotube system.

Below are some photos of the rough-in work going on yesterday. Today they uncrated the windows and may try to install the first one tomorrow.

Bonsby 5The ceiling mounted Mitsubishi mini-split roughed into first floor ceiling. No ducts to worry about, just freon lines.

Bonsby 2A ceiling mounted diffuser box for the ERV system with Comfotubes already hooked up.

Bonsby 4Comfotubing about to go in.

Bonsby 3Comfotubing hanging in the mechanical room. All the tubing is run to the mechanical room ceiling and left loose like this. When the first and second floor modules are married at the site, the first and second floor portions of tubing will be field connected, the Zehnder unit will be installed, and all these tubes will be hooked up to it.