There was a touch of anxiety in the home of Jon and Rachel Rea as a bomb cyclone with plummeting subzero temperatures approached Colorado in December, for the couple no longer had a gas-fired furnace in their basement.
They had replaced the furnace three months earlier with a heat pump and now that new gizmo was going to face a major test. The heat pump gathers the slivers of heat lingering in the air even on a cold day, but with temperatures plunging toward 20 below zero would there be any heat left to capture?
The Reas “preheated” their Boulder town house, raising the thermostat in advance of the storm, and had the advantage of having neighboring homes on each side and new insulation in the basement.
Still, they were uncertain how their Mitsubishi heat pump would fare. The unit was rated to still work at 13 below zero. The temperature in Boulder on the night of Dec. 22 fell to 18 below.
“Overall, it was just another day,” Jon Rea said. “It was not much of an event.”
The heat pump was working nearly full-time during the cold snap, but the house remained comfortable.
“You wouldn’t have noticed anything if you stayed in the house,” he said.
Powered by electricity, heat pumps are having their Colorado moment, thanks to a combination of improved technology making the units more efficient in cold weather, and financial incentives from utilities and local, state and federal governments.
“Heat pumps are ready to take off,” said Neil Kolwey, industrial program director at the Southwest Energy Efficiency Project. “A lot of things are in place.”
Between 2021 and 2022, the number of heat pump rebates issued by Xcel Energy, the state’s largest electricity provider, more than tripled to 1,220. Tri-State Generation and Transmission Association, the second biggest electricity supplier, saw the number of rebates rise 40% to 942.
There are two kinds of heat pump systems: Those that take heat out of the air and geothermal systems, which rely on tapping the steady heat below ground.
An air-source system – including a compressor, ductwork or individual wall and ceiling units linked to the compressor – can run from $15,000 to $30,000. The ground source heat pumps, which rely on wells drilled as deep as 300 feet below home, are more efficient but are twice as expensive.
While not as visible as solar panels or electric vehicles, heat pumps are a key element in the transition to cleaner, more energy efficient homes – as heating and cooling make up a little less than half all home energy consumption, according to the U.S. Environmental Protection Agency.
“The fact that we burn natural gas in our homes is pretty inefficient,” said Lacey Tan, a manager in the carbon-free buildings program at energy consultant RMI. “We have to think about how we make our homes more energy efficient.”
Heat pumps are one answer.
“The heat pump, like cauliflower, is having its time,” Tan said.
The heat pump was devised in 1857 by Czech inventor Peter von Rittinger and was first used to dry salt out of salt marshes. They’ve been widely used in European homes since the 1950s.
The technology, which can both heat and cool a home, gained traction in the U.S. with 50,000 to 60,000 units being installed annually by 2008 – mainly in the South where they were a good fit for hot summers and mild winters.
In colder climes the efficiency of the heat pumps plummeted along with the temperature – until recently.
“Are heat pumps ready for prime time in colder climates has been a question all along,” said Dave Lis, director of technology and market solutions for the nonprofit Northeast Energy Partnership, NEEP, which promotes efficient clean energy strategies. “There is now a growing body of evidence that they operate at low temperatures.”
The challenge has been in the air itself for heat pumps don’t burn anything to make heat, they draw it out of the air. The trick has been being able to grab it on the coldest of days.
The workings of an “air source heat pump” go like this: A refrigerant, with a low boiling point, something like 55 degrees below zero, runs through a pipe exposed to the outside air and even a little bit of heat vaporizes the refrigerant.
The vaporized refrigerant, carrying the heat, goes through a compressor – which compresses it, raising the heat to about 105 degrees and moving it into the house.
The refrigerant, once again a liquid, is sent back outside to pick up some more heat. In the summer the process can be reversed, moving heat from inside to outside, cooling the house.
Most homes already have some heat pumps.
“Your air conditioning is a heat pump, your refrigerator is a heat pump,” said Josh Lake, a co-founder of Elephant Energy, a Boulder-based company designing home retrofits with heat pumps and other electric technologies. “They are all moving heat using electricity.”
Since the heat pump is just moving heat rather than generating heat, it can produce two to four times as much heat energy as it consumes in electricity, according to the U.S. Department of Energy.
The problem in colder regions is there is less and less heat in the air as temperatures get lower and lower, so heat pumps must work harder and harder and become less and less efficient.
“Five years ago, heat pumps weren’t particularly viable in Colorado,” Lake said.
There have been a host of technological tweaks that have made heat pumps workable in Colorado and in Maine. These include a better heat exchanger, better designed fans, a variable speed compressor and sophisticated electronic controls.
“In the dead cold of winter there is still heat to move, but it is not very warm so the compressor has to do more work,” said David Petroy, whose company NTS Energy designs heat pump systems for homes and businesses.
At 50 degrees, a heat pump can move four units of heat for every unit of electricity it uses, but at zero, that drops to an efficiency of two units of heat for every unit of electricity, Petroy said.
The variable speed compressor can be more efficient over a range of temperatures and can really ramp up at low temperatures.
One of the key differences between putting in a heat pump or a furnace is that a heat pump system requires slightly more design in retrofitting an existing home or in a new build. Some homeowners also want heating backup, such as an electric heating unit, a small propane burner or a pellet stove.
Though those backups, Petroy said, are “things you may need four days out of the year.”
In installing a furnace, a heating and air conditioning contractor relies on a Manual J calculation, a formula which includes such variables as the size of the house, insulation and the number of people living in it.
Get the calculation, size the furnace, and install it.
“You need to be a little more thoughtful in designing a heat pump system, particularly in an all-electric house, which may also need a larger electrical panel installed,” Petroy said.
Instead of just installing one large unit, say in the basement, the vaporized refrigerant can be sent from a compressor to individual wall and ceiling units, known as mini-split system, enabling a homeowner to control temperature floor by floor, room by room.
“Right sizing your heating and cooling is pretty important,” Lake said.
While the heat pump market is just getting started in Colorado, it has been building for a decade in the Northeast, with an average growth of almost 30% a year since 2017 and more than 250,000 units installed in 2023.
NEEP has worked on programs to encourage state policies – its region stretches from New Jersey to Maine – promoting heat pumps, through education and engagement, developing a network of knowledgeable heat and air conditioning, or HVAC, contractors, and the creation of a cold climate heat pump product list.
And some of the coldest parts of the region were the early adopters.
“When we saw initial growth in heat pumps, we were seeing in places like Maine and Vermont where there was a lot of delivered fuels,” Lis said. “That makes the economics of heating with heat pumps a lot better.”
About 61% of the 600,000 dwellings in Maine are heated with fuel oil and another 11% with propane. A hearty 9% of homes are heated by wood, the rest by electricity and natural gas.
The average annual cost of heating a Maine home with a heat pump ($2,300) is almost half what it is for oil or propane, according to the Efficiency Maine Trust, which administers the state’s energy efficiency programs, including heat pump rebates.
“When we started our rebate program in 2012, there were two groups of Mainers: one group that didn’t know what a heat pump was and one that knew it didn’t work in cold weather,” said Andy Meyer, Efficiency Maine’s senior program manager for residential programs.
Since then, the program has issued rebates of $1,200 to $2,400 for about 100,000 heat pumps and the demand hasn’t waned.
“We are issuing a rebate for a heat pump every six minutes,” Meyer said. “It’s been a fundamental revolution.”
Some Maine households will buy one or two of the smaller wall or window-mounted heat pumps, which cost a few thousand dollars and look like an air conditioner.
“These will heat a room and maybe the next room with the third room cold,” Meyer said. “It’s a lot like heating your home with wood.”
Like Maine, a large portion of rural Colorado depends upon expensive propane for home heating, according to Peter Rusin, Tri-State’s member relations manager.
Tri-State, a power wholesaler, and its 18 rural Colorado electric cooperatives have promoting heat pumps and offering rebates up to $2,400.
In some mountain towns propane is selling for $4 a gallon, almost double the state average, Rusin said.
“Switching to a heat pump really reduces the energy burden,” he said. “There is no other technology, no other opportunity like this to reduce rural energy poverty.”
To meet the demand requires a corps of adept installers and when Efficiency Maine started its program it had to scour the state to find 20 contractors, Meyer said. Today, there are 800 vetted by the trust.
“The contractors are really important,” Rusin said, “because they are the ones selling the system, really, they are the ones educating the public.”
Colorado is facing the same need to build a network as Maine and the Northeast faced a decade ago.
When Sebastian Canaday, an electrical engineer, contacted HVAC contractors to put a heat pump in his Boulder County home, a couple of them tried to talk him out of it and one who took down the specification came back with a bid for a gas furnace.
A neighbor recommended the Save Home Heat Company, which installed a dual-fuel system, with a heat pump and a backup gas furnace for about $30,000. The compressor, heat pump and furnace are all integrated and can be controlled from Canaday’s mobile phone.
“They talk to each other,” he said.
Canaday’s system plugged into the existing ducts from his old gas furnace with the compressor sending the vaporized refrigerant to a coil atop the new furnace. While Rea’s mini-split heat pump is rated to operate at 75% capacity at 13 below zero, a duct heat pump, like Canaday’s is rated to 3 below zero.
So Canaday’s system is set to shift to the furnace at 6 degrees. In December, the furnace did about 30% of the heating, he said.
In addition to the heat pump, Canaday had solar panels added to his roof and during the day he raises the thermostat to 73 degrees to pre-warm the house for the night.
“The solar panels are basically powering the heat pump,” Canaday said. When the sun goes down, they lower the thermostat.
“Solar panels and heat pumps go together like peanut butter and jelly,” said Elephant Energy’s Lake.
One local HVAC contractor ready to get into the heat pump business is Grayson Goodwill, general manager of the Denver-based The Cooler Company.
“Xcel is pushing hard in the direction of heat pumps,” Goodwill said. “People might be skeptical about them being able to heat at low temperatures, but they’re the future.”
For the past few months, Goodwill has taken courses offered by Xcel and by heat pump manufacturers to get up to speed on installation practices.
“Heat pumps take specific training,” he said.
“There is a learning curve for installation and some manufacturers have changed the way these are installed to make it easier,” Tan said.
Goodwill said he hopes 2023 will mark his company’s first heat pump installation.
“Considering the cost of natural gas and particularly for a home with solar, heat pumps are the way to go,” he said.
If that’s the way to go, Rainer Gerbatsch is already there. The retired building contractor lives in the Geos Neighborhood, a net-zero energy housing development in Arvada.
His 2,700-square-foot town house has a heat pump, a heat-pump hot water heater, a heat-pump clothes dryer, as well as solar panels on the roof, a Tesla Powerwall battery in the garage and an electric vehicle charger.
Gerbatsch has a ground sourced, or geothermal, heat pump with a well 300 feet deep below his basement, bringing up a mixture of warm water and methanol at about 55 degrees that heats the air and is compressed to raise the temperature.
Opening the utility closet in his basement, Gerbatsch said “it just looks like a gas heater … except it is cycling water.”
The Geos homes are designed for passive solar heating, with triple-pane windows, thicker walls, ample insulation and large south-facing windows. So, even in the winter the temperature in the house can reach 70 degrees in the afternoon, without the heat pump.
Not everyone can live in a Geos house, but the Reas did try to come as close as he could adding not only a heat pump, but a heat pump hot water heater and a high-speed EV charging station. The house already had an electric stove.
The couple had wanted to add solar panels, but because of their flat roof, the cost was prohibitive.
All this, Rea, who is a manager in RMI’s electricity practice, said was motivated by a desire to cut his greenhouse gas emissions.
“The way to get to net-zero emissions, we have to stop burning fossil fuels,” he said.
Rea calculated that their old gas furnace emitted about 1.9 tons of carbon dioxide a year while the current emissions intensity of Xcel Energy’s electricity is 0.4 tons a year.
“The solution to getting to that future clean energy state is electrifying all buildings,” Rea said.
Again, if that’s the solution, Gerbatsch is there. Through a Span panel, Gerbatsch can monitor and control everything in his all-electric home.
In January, the system told him that 6% of the electricity was going to his appliances, 3% to lighting, 51% to heating and 40% to charge two EVs for a total of 1,615 kilowatt-hours.
While that is more than double the average Xcel Energy residential use, it doesn’t faze Gerbatsch because during the summer his solar array generates more electricity than his home needs and he sends it to the grid, building up credits with the utility company.
“Basically,” he said, “I don’t have any utility bills.”