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”).

Heating Capacity

We wanted to ensure the selected MSHP would maintain comfort throughout the heating season. A Fujitsu 15RLS3H unit, which can effectively move heat at outdoor temperatures as low as -15°F and can maintain comfort in this home at outdoor temperatures below -5°F.

Analyzing the regional heating data from NOAA (see “Web Extras”), which documents the incidence rate of outdoor temperatures for nearby weather stations, we calculated that the estimated heating demand would exceed the capacity of the MSHP less than 1% of the time. During these short spells, the home’s passive solar, plus thermal mass and internal heat gain, will likely provide a buffer.

Annual Heating Demand & Heat Loss

Most heat-loss calculations are based on an outdoor temperature of 65°F­, which represents the temperature below which the heating system will be turned on­­, and the design temperature. In this case, the temperature difference is 70°F (65°F minus -5°F). Dividing the design heat load of 16,800 Btu per hour by this temperature difference gives a heat loss per hour per degree of approximately 240 Btu/hr./°F. Multiplying this by 24 hours per day gives us the heat loss per degree over the course of a day. For this home, it’s 5,760 Btu/day/°F.

This calculation enables using heating degree-days (HDDs) to estimate the annual heating demand. HDDs are a measure of one full day at 1°F below the outdoor base temperature (typically 65°F). For example, if the average temperature for a day is 30°F, this equates to 35 HDDs (65°F - 30°F). By multiplying HDDs by the heat loss per degree over the course of a day, we can estimate the heating demand for the full day. By using this approach—considering the number of heating degrees over the winter—we can estimate a home’s annual heating demand.

In this particular example, the annual HDDs are 7,510. (This information can be obtained from the IPC or from degreedays.net.) To estimate a total annual heating demand, multiply the HDDs (7,510 day-°F/yr.) by the heat loss (5,760 Btu/day/°F). This results in an annual heating demand of approximately 43,260,000 Btu per year.

Passive solar gain and internal sources of heat also contribute to this home’s heating, and engineering models are typically used to determine these values. In this case, the estimated gain from these sources during the heating season was calculated (through F-Chart Software) to be 23,450,000 Btu per year, which reduces the estimated annual heating demand to 19,810,000 Btu per year.

Estimating MSHP Electrical Demand

Since electricity provides this heat, it is beneficial to convert the heating demand into kWh. Dividing the estimated annual heating demand (19,810,000 Btu/year) by the number of Btu in a kWh (3,412 Btu/kWh) equals about 5,800 kWh per year. This figure represents the energy consumption using electric resistance heat, which has an efficiency of 100%.

The average coefficient of performance (COP) for a Fujitsu 15RLS3 installed in a climate like central Maine is roughly 3. This means that the unit averages 3 kWh of heat for each kWh used. Since the MSHP is expected to offset close to 100% of the heating demand, we can divide the heating demand (5,800 kWh/yr.) by the MSHP’s annual COP to obtain an estimated electrical consumption of 1,935 kWh.

Sizing the PV Array

For this location, using PVWatts, and adjusting for shading from trees and snow, results in an estimated annual output of 1,245 kWh per kW of PV capacity. The PV capacity needed to offset the MSHP’s electrical consumption is roughly 1.6 kW (1,935 ÷ 1,245).

We estimated the other electrical consumption for the home to be approximately 7,800 kWh/year, which included lighting, water heating, appliances, cooling, and a future electric vehicle. An additional 6.3 kW of PV capacity is required to offset the non-MSHP consumption.

The system we installed has 27 REC Twin Peak 290 W modules and a SolarEdge SE7600A-US inverter. The total system capacity is 7.83 kW and should result in the home being net zero in an “average” year.

Livability

The Bakers have been very pleased with the new home’s performance. After decades of handling wood and coal, Ken reports, “The use of heat pumps and solar has made our home effortless to live in. When the kids come home, they no longer fight for the space near the heating vents and find home to be more relaxing.”

Comments (4)

tmccarthy821@gmail.com's picture

is this a total poach of the Go LOgic... design or what. I didnt see any credits.

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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