A heat pump that can be installed in minutes is allowing the city of Boston to quickly decarbonize a public housing complex for the elderly and provide air conditioning as climate change-driven heat waves intensify.

The Boston Housing Authority on Wednesday will announce a pilot project putting small heat pumps made by San Francisco-based startup Gradient in a 50-year-old, 100-apartment building. The units fit in a window and can provide heating and cooling for a space of about 500 square feet. That replaces the complex’s inefficient electric-resistance heating and doesn’t require the expensive and disruptive renovations that accompany the installation of a centralized heat pump system.

The housing agency manages about 10,000 units for more than 17,000 residents, and window heat pumps’ fast installation time will produce immediate emissions reductions, according to Kenzie Bok, the agency’s administrator. “Most importantly, it allows us to deliver in-unit cooling to our residents for the first time as soon as possible,” she said. “We can’t treat heat as this one-off emergency.”

Boston is the latest city to turn to window heat pumps to replace aging and polluting gas boilers and other fossil fuel systems in public housing. The New York City Housing Authority has installed 36 Gradient units and 36 window heat pumps made by Midea America, the US subsidiary of China’s Midea Group, as part of a pilot project. The agency plans to put in place 10,000 Gradient heat pumps and 20,000 Midea units in the coming years.

A state program that finances energy efficiency improvements through a surcharge on utility bills is paying for the Boston initiative.

A study released in July by the American Council for an Energy-Efficient Economy estimated that in a large multifamily building, installation and operating costs for window heat pumps would total $14,500 for each apartment over 24 years, compared with $25,000 per unit for a centralized heat pump system. There are about 23 million apartments in multifamily buildings in the US, according to the report. Nearly 1 million of those units are government-owned, making public housing a large potential market to scale decarbonization technologies.

The Boston Housing Authority expects even bigger savings, projecting it’ll cost $5,450 to retrofit units like those in the Hassan Apartments with window heat pumps compared with the $40,000 per unit it paid to install centralized systems at some smaller housing complexes.

Mayor Michelle Wu issued a directive in 2023 to decarbonize Boston public housing by 2030. The housing authority has also partnered with utility National Grid on a pilot project to replace gas heating with a geothermal heat pump system in a 129-apartment complex.

But window heat pumps offer a faster, cheaper way to decarbonize, according to Shilpi Tewari, a research fellow at RMIT University in Melbourne, Australia, and lead author of a 2025 study on the use of heat pumps in public housing.

She noted that older people, residents with chronic conditions and young children are particularly vulnerable to more frequent and deadly heat waves that keep temperatures elevated at night, giving residents no opportunity to cool down. Window heat pumps, she said, “are well suited for rapid deployment to mitigate immediate health risks.”

Bok said that heat waves have hit Boston the past two Junes, normally a relatively temperate month for the city. Most public housing apartments in the city don’t have air conditioning and residents struggle to afford window units, which are typically far less efficient than a heat pump.

Most window air conditioners also occupy a large part of a window and obstruct views. Gradient’s 140-pound unit is shaped like an inverted letter U and rests on a bracket, allowing it to take up only a few inches of window space. Two people can install the heat pump and have it up and running in about 45 minutes. It also plugs into a standard outlet, avoiding the need for electrical upgrades.

Founder and Chief Technology Officer Vince Romanin estimates that about 60% of US housing stock is compatible with the company’s window unit. He said Gradient didn’t set out to serve public housing but that market has become its focus.

“It lets us scale quicker,” Romanin said. “We didn’t want to build like the Tesla Roadster for rich people and then wait for the technology to trickle down.”