Electric Heat Comes of Age: Installing Our Mini-Split Heat Pump
Installing a Mitsubishi air-source heat pump in our new house
Thirty-five years ago, when I first got involved with energy efficiency and renewable energy, the mere suggestion that one might heat with electricity would be scoffed at by those of us seeking alternatives to fossil fuels.
Amory Lovins, founder of the Rocky Mountain Institute, likened using electricity for heating to “cutting butter with a chainsaw.” Electricity is a high-grade form of energy; it doesn’t make sense to use it for a low-grade need like heating, he argued. It made much more sense, we all agreed, to produce that 75-degree warmth with solar collectors or passive-solar design.
So, it represents a bit shift that I’m now arguing that electricity can be the smartest way to heat a house. And that’s what we’re doing in the farmhouse we’re rebuilding in southern Vermont. I should note, here, that all of our electricity is being supplied by a solar array on our barn.
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Heating with electricity makes sense if instead of using that electricity directly to produce heat—through electric-resistance strip heaters—we use a device called a heat pump. For every one unit of energy consumed (as electricity), two to three units of energy (as heat) are delivered. This makes heat pumps significantly less expensive to operate than oil or propane heating systems in terms of dollars per delivered unit of heat.
Heat pumps use electricity in a seemingly magic way, to move heat from one place to another and upgrade the temperature of that heat in the process. Heat pumps seem like magic because they can extract heat from a place that’s cold—like Vermont’s outdoor air in January, or underground—and deliver it to a place that’s a lot warmer.
Very significantly, heat pumps can be switched from heating mode to cooling mode with a flip of a switch. In the cooling mode, they work just like standard air conditioners.
Ground-source heat pumps (often mistakenly referred to as geothermal heat pumps) rely on the ground (or groundwater) as the heat source in the heating mode (and heat sink for cooling), while air-source heat pumps use the outside air as the heat source and heat sink. Because temperatures underground are much warmer than the outside air in winter, the efficiency of ground-source heat pumps is typically higher than that of air-source heat pumps.
But the ground-source heat pumps are really expensive. Friends in southern Vermont have spent $35,000—or even more—to install residential-sized ground-source heat pumps. The cost is so high because of trenching or drilling wells.
By contrast, air-source heat pumps are much simpler and far less expensive. The most common types today—and what we installed at Leonard Farm—are referred to as ductless minisplit heat pumps (see Ductless Mini-Splits and Their Kin: The Revolution in Variable-Refrigerant-Flow Air Conditioning). There is an outdoor compressor (a box about three feet on a side and a foot deep), an indoor unit (evaporator with blower) that mounts on an interior wall, and copper tubing that carries refrigerant between the two.
The typical installed cost of a ductless minisplit is $3,000 to $5,000, though many variables affect the cost.
These air-source heat pumps are viable today, even in cold climates, because of dramatic improvements in the past few decades. Much of this innovation has been driven by Japanese companies, including Mitsubishi, Daikin, Fujitsu, and Sanyo (now part of Panasonic). Several decades ago, air-source heat pumps only made sense in climates that rarely dropped below 30°F in the winter; today some of these systems, including ours, will function well at temperatures below zero degrees F.
Point-source heating and cooling
Ductless minisplit heat pumps are ideally suited for compact, highly energy efficient homes. Our house has R-values greater than R-40 in the walls and R-50 in the roof, plus very tight construction with a heat-recovery ventilator for fresh air. In tight, superinsulated homes, a single space heater (point-source heating system) can work very well, because with all the insulation fairly uniform temperatures are maintained throughout the house.
With our 1,700 square-foot house, the two upstairs bedrooms may stay a little cooler than the downstairs, but we like a cooler bedroom. In a larger house or one that isn’t as well insulated, several ductless minisplit heat pumps or a ducted heat pump option might be required.
Our Mitsubishi heat pump
We installed a state-of-the-art Mitsubishi M-Series FE18NA heat pump that is rated at 21,600 Btu/hour for heating and 18,000 Btu/hour (1-1/2 tons) for cooling. Marc Rosenbaum, P.E. ran heat load calculations showing peak heating demand (assuming –5°F outside temperature) about 23,000 Btu/hour, assuming the air leakage we measured several months ago, before the house envelope was completed. If the air leakage ends up being cut in half from that measured level, the design heat load would drop to a little over 19,000 Btu/hour.
We think the FE18NA model will work fine for nearly all conditions, but we are also installing a small wood stove—the smallest model made by Jotül—for use on exceptionally cold nights.
The indoor unit of our heat pump is about 43” long by 13” tall by 9-3/8” deep and installed high on a wall extending in from the west wall of the house—next to an open stairway to the second floor; it is controlled with a hand-held remote. The outdoor unit, installed just off a screen porch on the west side of the house is 35” tall by 33” wide by 13” deep. It is under an overhang and held off the ground by granite blocking.
ARC Mechanical from Keene, New Hampshire did a great job with installation, and the system has now been turned on. We won’t move in until December, but it’s nice to know we have heat.
Alex is founder of BuildingGreen, Inc. and executive editor of Environmental Building News. In 2012 he founded the Resilient Design Institute. To keep up with Alex’s latest articles and musings, you can sign up for his Twitter feed.
(2013, October 22). Electric Heat Comes of Age: Installing Our Mini-Split Heat Pump. Retrieved from https://www.buildinggreen.com/blog/electric-heat-comes-age-installing-our-mini-split-heat-pump