Given West Virginia’s hot and humid summers, keeping the house cool was a priority. While the operable windows provide some passive cooling, a 22-inch-diameter whole-house fan mounted in the attic boosts ventilation, helping move air throughout the house. Most effective whenever outdoor temperatures drop below indoor temperatures, the AirScape fan draws fresh, cooler air into the living space through open windows, the warmer air exiting through the roof vents. The fan can move 725 to 2,590 cfm, drawing 21 to 210 watts— far less power than a central air-conditioning system. At about $1,300, it is also significantly less expensive.
While the couple says that the fan works well for cooling, the window-opening strategy moves enough air so they rarely need it. However, in late July and August, when the humidity peaks and night temperatures remain in the 80s, they use a minisplit heat pump to keep the indoor temperatures cool and comfortable.
In the winter, they typically keep the house at about 69°F with one firing per day in their masonry heater. When outside temperatures drop below zero, they burn two or three fires. Popular in the wood-sparse tundra of Scandinavia and Russia, this massive heater works by directing the heated gases from a small, hot fire through baffled chambers, where nearly all the heat is absorbed by the masonry. Usually, one or two fires will provide enough heating for a 24-hour period. Because the fires burn at very high temperatures, very little ash or smoke is produced. The couple purchased a kit from Empire Masonry Heaters, and hired a mason to assemble the core. A subcontractor hired by the builder finished the exterior with river stones collected from the property. The Phoenix kit, roughly $4,800, included the core, doors, cleanouts, damper, and a pizza/bread oven.
For backup cooling and heating, they consulted with building systems engineer David Butler of Arizona-based Optimal Building Systems. This service is part of Coleman’s design package. Based on information provided on the plans and specifications, as well as phone calls and correspondence with the couple, Butler developed the home’s heating, ventilation, and air-conditioning (HVAC) system specifications. “More often than not, passive solar and other high-performance homes end up with grossly oversized HVAC equipment. This not only undercuts potential energy savings, but can lead to comfort and moisture-related problems,” Butler says.
Butler used Manual J protocol (often called “heat load calculation” or “cooling load calculation”) to estimate how much heating and cooling the home might require. However, based on his experience, the Manual J “significantly overstates” heating loads, even for conventional homes, and ignores internal and solar gains. Knowing that, he says, the numbers are merely a starting point. Given the low electricity rates in the area (about 9.5 cents per kWh), Butler calculated that a ground-source heat pump would be less expensive to operate than most other HVAC options. However, with the home’s small heating and cooling loads, it would have taken the couple many years to recover the system’s high installation costs. Instead, he recommended an electric Mitsubishi Mr. Slim multizone minisplit heat pump, with two ductless wall-mounted units (for loft and living areas) and a small ceiling-mounted ducted air handler for the bedrooms—all supplied by a variable-capacity outdoor unit. The system has a SEER rating of 17.5.
Minisplit systems work like a standard air-source heat pump, with an outdoor condenser/compressor, but without the expense or space required for ducts. The systems are more efficient, with fewer conditioning losses than with a conventional furnace. However, without ducts, there was the problem of getting conditioned air to the bedrooms. With each distribution unit costing about $3,500 installed, it would have been too expensive to put individual wall units in each bedroom. Even the smallest minisplit unit, he says, would have had several times more capacity than a bedroom’s peak load need—thus leading to large temperature swings as the unit cycles.
Instead Butler recommended ducted air distribution. The 8-inch-tall Mitsubishi ducted air handler, concealed in the hallway’s dropped ceiling, preserves the thermal integrity of the primary ceiling. The duct runs down the hall, feeding the bedrooms, where ducts are routed in the attic above the primary ceilings and buried under 16 inches of blown-in fiberglass insulation.