Updated February 10, 2026 at 1:06 PM EST
Boston, like many cities in the Northeast, has a building problem. More specifically, its buildings have a problem: They emit tons of climate pollution every year.
In Boston, buildings account for 70% of the city’s annual carbon emissions, with most of it coming from oil and natural gas heating systems. The city’s main strategy to reduce those emissions is to install heat pumps, which use electricity to provide heating and cooling.
But putting in a heat pump usually requires a lot of complicated, expensive and disruptive work. Contractors have to drill through walls and run refrigerant lines to an outside unit. And they may have to install ductwork or upgrade the building’s electrical panel.
The upfront costs can be especially high in older multi-family buildings, where workers may encounter outdated electrical wiring and asbestos, and where special equipment might be needed.
Now, imagine a heat pump that’s both relatively cheap to purchase and as easy to install as a window air conditioner. As it turns out, this technology exists.
Meet the window heat pump.
It’s a new type of heat pump that’s compact, hangs over a windowsill and plugs into a standard 120 volt outlet — no HVAC technician or electrician required.
Efficiency in 30 minutes
The units aren’t widely available to the public yet, but the Boston Housing Authority is piloting them in the Hassan Apartments, a 100-unit complex in Mattapan for seniors and disabled adults. The 50-year-old building is one of many older structures across the Northeast with an energy-gobbling heating system and no air conditioning — factors that make it a prime candidate for a climate retrofit.
“It’s really exciting for us because we feel like it could be the tip of an iceberg of converting many thousands of units,” said Kenzie Bok, administrator of the Boston Housing Authority.
In one of the building’s apartments, a pair of workers began the installation by screwing a large metal bracket on the windowsill, which will keep the saddle shaped heat pump secure. Next, they lifted the 140-pound unit onto the bracket and slid it into place. Then they added a bit of weather stripping to prevent air from leaking in — and voilà.
“This is a simple in and out process,” said Erik Nerstheimer with Rise Engineering, the contractor doing the work for the housing authority. “And it comes in the box mostly assembled, so it’s just a couple of adjustments.”
The whole process takes less than half an hour, a totally different experience than installing mini-splits or other heat pump systems currently on the market. Those can take days or weeks to get up and running.
And in this case, an easier installation translates to lower costs. The pilot project will cost about $5,400 per apartment — and that covers the heat pump, the installation, and a new smart thermostat. A study the agency did a few years ago, by contrast, found it would cost seven to eight times more to install a mini-split or ducted heat pump in one of its apartments.
So, how do residents rate their new window heat pumps?
Wenda Dotton, who got her unit in August, said she loves it.
“You ought to see it,” she said. “It’s cool looking, kind of futuristic. And it doesn’t make noise. I like that.”
It also works well, she said. During the summer, the unit in her living room kept her entire one-bedroom apartment cool. And so far this winter, it’s done a fairly good job keeping her warm.
“I’m quite comfortable,” she said. “I just keep it at 67 — kind of not too hot, not too cool. I just like to have a little bit of a breeze, more of an airy feel.”
Dotton said her one complaint about the new system is that she wishes she had a second unit in her bedroom.
“It takes a while for the heat to drift over there when it’s real, real cold,” she said. As a result, on several particularly chilly nights this winter, she’s turned on the old baseboard heating system before going to bed.
Though heat pumps have a reputation for not heating well when temperatures drop below freezing, manufacturers say newer cold-climate models don’t have that problem, provided the system is appropriately sized for the living space.
“Generally, we find people putting one in every major room,” said Vince Romanin, the founder and CTO of Gradient, the company whose window units were purchased for the Boston pilot. “So, as an example, a two-bedroom apartment in a city is going to have, probably three Gradients: one in each bedroom, and one in the living room.”
Promising results
It’s still early days in the Boston Housing Authority’s test run with the technology, but a pilot project in a New York City apartment building that launched in 2023 showed encouraging results. Apartments with window heat pumps maintained more consistent temperatures during the winter than those relying on the building’s old gas-fired boiler system. They also used 86% less energy for heating, which cut resident’s winter heating bills in half.
The ongoing pilot in New York has been so successful that Mayor Zohran Mamdani recently announced his administration would purchase heat pumps for another 700 public housing units. The city aims to install a total of 10,000 heat pumps in public housing units by 2030.
Beyond public housing, window unit heat pumps may be among the most cost-effective ways to reduce emissions in many older multi-family buildings. A recent report from the the American Council for an Energy-Efficient Economy, a nonprofit research organization, found that outfitting a two-bedroom apartment with these units would likely cost about $9,300. Other heat pump options, by contrast, had a $14,000-$20,000 price tag.
For Bok of the Boston Housing Authority, the relatively low expense of the window heats pumps is just one of their benefits. She’s thrilled that many residents will have access to cooling for the first time; she’s excited about reducing the agency’s climate impact; and she’s happy about the money these heat pumps will save the agency in heating expenses.
Heat pumps are far more efficient than the building’s baseboard heating system, so operating them will use significantly less energy. The housing authority estimates savings of roughly $60,000 a year on utility costs through the pilot, money it can instead invest in other capital improvements.
While these projections are encouraging, this technology is not a panacea. One of the biggest challenges in making buildings more climate friendly is that every structure is unique and may require different solutions.
These window heat pumps, for example, only work in sash windows, meaning those that open up and down. And depending on the configuration of the apartment or the building’s existing heating system, the units may not be the best, or even cheapest, option for an upgrade.
What’s more, they haven’t been widely available — though that appears to be changing. San Francisco-based Gradient, which makes the model used in the Boston Housing Authority pilot, doesn’t sell directly to consumers. But at least one U.S. distributor recently started selling a model made by Midea, a Chinese company.
In a city like Boston — with all its older buildings, college dorms and renters — these window heat pumps could offer a simple, more affordable climate solution.
Editor’s note: This story has been updated to include the U.S. distributor now selling a window heat pump made by Midea.
Correction: Vince Romanin is Gradient’s founder and CTO. An earlier version of this story misstated his title.
This article was originally published on WBUR.org.
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