Why window units could be a better way to heat apartments

Compact heat pumps that slot into windows could be a planet-friendly way to efficiently heat and cool apartment buildings without breaking the bank, experts say.

While many single-family residences have switched to heat pump technology, which is one of the primary ways to curb emissions from buildings, making the change in large multifamily housing has proved to be more challenging. Retrofitting existing buildings that typically use central heating systems can be complicated and costly, said Steven Nadel, executive director of the American Council for an Energy-Efficient Economy, a nonprofit.

“Multifamily has been much more of a problem, but we have a lot of people who live in multifamily,” Nadel said. “We need to do something about it.”

A newer type of heat pump that hangs over a windowsill and plugs into a standard 120-volt outlet could be the answer, Nadel and other experts said. An ACEEE analysis of different residential heating systems that don’t use fossil fuels found that window heat pumps are the cheapest option. Over a 24-year period, buying, installing and running these heat pumps in a standard apartment cost an average of about $14,500, according to the study.

“From a solutions standpoint, it’s a huge opportunity” for heat pumps in large existing apartment buildings, said Ryan Shea, a buildings program manager at the energy think tank RMI.

How do window heat pumps work?

Window heat pumps function similarly to standard air-source technology, Shea said, drawing heat from the outdoor air and bringing it inside. But unlike other kinds of heat pumps, all the components are combined into a single unit.

Units that go into windows are typically U-shaped and slide over a windowsill. This design means windows that slide up and down can still be closed even if a heat pump is installed, unlike traditional window air conditioners.

Window heat pumps usually weigh about 130 pounds and could require two people to install, Nadel said.

What are the benefits?

Cost is one major advantage, experts said.

The ACEEE analysis was based on an average of three window heat pumps per apartment. In comparison, over a 24-year time frame central air-to-water heat pumps cost an average of just under $22,000 per apartment, while mini-split heat pumps cost about $25,000 to operate.

Window heat pumps are also easier to install, Shea said.

“You can just replace the window air conditioner with a window heat pump that does both cooling and heating,” he said. Many large older apartment buildings in the United States don’t have central air conditioning.

The technology does require a dedicated outlet, Nadel said. In some apartments, that might mean needing to create additional outlets.

Transitioning existing large apartment buildings to heat pump technology could also have benefits for the planet.

Keeping homes comfortable in frigid winters and blistering summers can account for more than half of an average American household’s energy usage, collectively generating about 441 million tons of carbon dioxide each year. But because heat pumps move heat instead of creating it, they can be three to four times more efficient than traditional heaters and slash a home’s carbon dioxide emissions by as much as eight tons per year.

Should you get one?

At least two companies are selling window heat pumps for cold climates, but only directly to building owners and energy efficiency programs, according to ACEEE. Gradient, a San Francisco-based startup, has a suggested list price of $3,800. Midea, a Chinese company, said their units will cost $2,800. These prices are for small quantities, and bulk orders are likely to be cheaper, the ACEEE report notes.

Current models of window heat pumps are suitable only for single or double hung windows that slide up and down, Nadel said. They won’t fit in apartment buildings with windows that open and close by sliding horizontally.

Transitioning to window units in apartments could also change who pays for heat. In apartment buildings with centralized heat, landlords typically cover that cost, Nadel said. But if you switch to window heat pumps, the cost of electricity to run the units would probably go on the tenant meter.

“Hopefully the landlords would cut the rent proportionally, so that this extra cost doesn’t get moved to the tenant,” Nadel said.

The cost of running heat pumps compared to technology that uses fossil fuels can also depend on local electric and gas rates. And while cold-climate models can provide heat when outdoor temperatures are as low as zero degrees Fahrenheit without backup heating, the technology does tend to work more efficiently in warmer climates, Nadel said.

Still, he encouraged people who are interested in getting a window heat pump to check with their local utility, noting that a growing number of utilities are running pilot programs.

“They are generally the lowest-cost option for adding cooling and providing a heat pump for these multifamily buildings,” Nadel said.