PV & Minisplit Heat Pumps—A Case Study

Intermediate

Inside this Article

Mary and Ken Bakers’ home in central Maine is heated by the sun. A combination of passive solar gain and a PV-powered minisplit heat pump keeps the super-insulated home cozy through the northern winters.
To reduce heat loss, the east, west, and north walls have very few windows.
Large triple-pane windows on the south face admit available solar gain, while limiting heat loss.
The traditional post-and-beam frame was wrapped in structural insulated panels, giving the walls an insulative value of approximately R-40.
The Fujitsu minisplit indoor unit is mounted high on a wall in the home’s main room. Heat convects to second-story rooms up the open stairwell.
The outdoor unit is mounted on the east wall, above the snow line.
Reducing electricity usage by using energy-efficient appliances and lighting allowed the PV system to be smaller.
The 7.83 kW PV array.
The SolarEdge grid-tied inverter.
Ken & Mary.

In 2016, Mary and Ken Baker of Norridgewock, Maine, constructed a new home intending annual net-zero energy use. The two-story, 26-by-30-foot timber-framed home was built with structural insulated panels (SIPs) on an insulated concrete slab.

This tight, well-insulated envelope included triple-glazed windows and high-effiency doors, and a heat recovery ventilator (HRV) to preheat fresh incoming air for good indoor air quality. In addition, other good efficiency choices such as LED lighting and Energy Star appliances were selected. Their previous home was an old farmhouse that required significant amounts of oil and wood to heat. After years of handling large quantities of cordwood, the Bakers’ initial goal was to use no more than one cord per year to heat the home as they transitioned to a less labor-intensive lifestyle.

In the design process, several options were explored to generate the heat necessary to maintain comfort in the cold Maine winters. The south wall of the home was designed to collect passive solar heat. As the Bakers’ understanding of indoor air quality and heating demands increased, it became clear that there might be alternatives to using their woodstove that would still accomplish their goal of carbon neutrality.

The home’s floor plan allowed using a single-zone heat pump on the first floor. An open stairwell to the second floor distributes heat upstairs. Minisplit heat pumps (MSHPs) were not considered an acceptable primary heat source by their lending institution for home loan purposes, so electric baseboard heaters were placed in bedrooms, bathrooms, and in the kitchen to satisfy the lender. For homes in cold climates, smaller, dedicated heating appliances help maintain comfort in areas of the home where heat distribution from the MSHP may be challenging or in regions where the MSHP’s capacity is lower than the winter heating demand.

Design Heat Loss

The design heat loss—how much heat a home requires given a specific outdoor temperature—was calculated via Manual J, which accounts for insulation; air infiltration; the surface areas of the windows, doors, walls, floors, and ceilings; and design temperatures. For this home, the design heat loss was estimated to be 16,800 Btu/hour at an outdoor temperature of -5°F. Design temperatures can be obtained for many locations from the “Degree Day and Design Temperature” table from the International Plumbing Code (see “Web Extras”).

Comments (3)

David Vera 2's picture

How does this home behave in the summer? Does it not have enough cooling needs to warrant an air conditioner? The south facing windows with no awning would heat up the space quite a bit in summer days I would think

Frank Heller's picture

Efficiency Maine has long pioneered tight, low heat loss, houses. But there is a dark side to these houses when the air exchange fails to refresh interior air---often contaminated in Kitchens with toxic fumes; with outside air.

Even worse are open floor plans, which work against having rooms with specific environments...a cold bedroom with ample fresh air; a kitchen with even greater air exchanges.

In some cases, a tightly sealed house can exceed 2,000 ppm of CO2; enough to be a health hazard.

If you want to reduce CO2, there are some Japanese manufacturers of 'wet' air filters which greatly reduce the CO2 in outside air.

The result is a high CO2 buildup inside the house.

Donald Proven's picture

A properly designed HRV or ERV would deal with your CO2 issue. Heat Recovery Ventilator or Energy Recovery Ventilator. We have been installing HRVs up here in Manitoba in our energy efficient houses for 30 years. And with a properly designed HRV/ERV ventilation system installed, every "habitable space" is ventilated - bedrooms, living rooms, dens and play areas get fresh air and bathrooms, kitchens and laundry/utility spaces are exhausted to the outside. So "open concept" makes no difference to the air quality.

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