One of the challenges in building a Passive House is finding mechanical equipment small enough to match the low energy load and efficient enough to meet the usage requirements. The Karuna House, with its unique custom design, posed additional quandaries that required Hammer & Hand to actually rethink how mechanical systems fit into the scope of a Passive House green home building project.
For one thing, the house is larger than the typical single-family Passive House, and has a higher glazing-to-opaque wall ratio. It also has three separate zones: the main downstairs living area, the upstairs bedrooms, and a private guest wing. Each of these zones has different exposure to the sun, so we needed a way to accommodate the diverse loads throughout the year. To further complicate the picture, the designer made use of every interior space, so there was nowhere to hide ductwork.
These complexities pretty much ruled out the more common ways of heating a Passive House — like adding a resistance element to the ventilation system or using a mini-split heat pump. Instead, a low-temperature hydronic radiant floor system was installed, with hot water provided by a Daiken Altherma air-to-water heat pump. The same unit also provides all the domestic hot water for the house. The all-electric Altherma has very high efficiency — over 300% when it’s heating low-temperature radiant water — which means it produces three times the energy required for its operation. It’s also variable speed, so it modulates with the load, which further reduces its electric consumption. Watch as mechanical contractor Jonathan Cohen of Imagine Energy explains how the Altherma system is configured, and describes the zone-control strategies that maximize its efficiency.
In a Passive House — or any high-performance home, for that matter — domestic hot water is typically a larger energy load than heating. To meet the Passive House standard, this means not only generating the hot water efficiently with the heat pump, but also reducing losses through the distribution system. We used a three-pronged approach in the Karuna project: First, the hot water supply pipes were carefully sized to reduce the amount of hot water left in the lines after each use. Second, every hot water line was fully insulated from the mechanical room all the way to the fixture. Finally, an on-demand hot water recirculation system was installed (rather than a conventional system that runs 24/7).
In the next video, Hammer & Hand’s Skylar Swinford describes the domestic hot water strategy, starting in the mechanical room and moving upstairs to the master bath, where he explains how motion-controlled hot water recirculation saves both water and energy.
A key feature of any Passive House is a flawless air barrier between inside and out. This prevents energy loss from air infiltration and exfiltration and allows the thermal insulation to perform at its rated R-value. But airtightness also means that the house requires mechanical ventilation. Unfortunately, many traditionalist builders still believe that tight houses have stuffy air, or that vent fans waste heat. In fact, with a properly designed Passive House, the exact opposite is true.
Here, Jonathan Cohen explains how the Karuna House’s heat recovery ventilator captures 90% of the heat from the exhaust air. The Zehnder unit delivers comfortably warm, fresh air throughout the house while using very little energy and making almost no sound. Jonathan also gives us a look at the Daiken heat pump’s outdoor unit, and explains the advantages of the air-source heat pump over a ground-source installation.
Beside being quiet, the Zehnder HRV is visually discreet. In this video, Skylar takes a walk through the house and shows us the small supply and exhaust ports. He also explains why controlled, balanced ventilation makes sense in any house: Besides saving energy, increasing comfort, and guaranteeing good indoor air quality all year round, it’s really the only way to know where the house’s makeup air is coming from:
One of the homeowner’s goals was to reach net-zero — that is, energy produced on site would equal the energy consumed annually. In an all-electric Passive House like the Karuna house, photovoltaic panels make great sense. Here, Jonathan gives us a look at the 9.9 Kw PV array, which in this case was locally sourced, right down to the cells manufactured by SolarWorld’s Oregon facility. We also get a stunning view of Karuna’s exterior.