Window Heat Pumps Could Change the Game

By now there’s plenty of evidence showing why heat pumps are such a promising solution for getting buildings off fossil fuels. But most of that research has focused on single-family homes. Larger apartment buildings with steam or hot water heating systems — i.e. most of the apartment buildings in the Northeast — are more difficult and expensive to retrofit.

A new report from the nonprofit American Council for an Energy-Efficient Economy, however, assesses a handful of new technologies designed to make that transition easier and finds they have the potential to significantly lower the cost of decarbonizing large buildings.

“Several new options make decarbonizing existing commercial and multifamily buildings much more feasible than a few years ago,” Steven Nadel, ACEEE’s executive director and one of the authors, told me. “The best option may vary from building to building, but there are some exciting new options.”

To date, big, multifamily buildings have generally had two flavors of heat pumps to consider. They can install a large central heat pump system that delivers heating and cooling throughout the structure, or they can go with a series of “mini-split” systems designed to serve each apartment individually. (Yes, there are geothermal heat pumps, too, but those are often even more expensive and complicated to install, especially in urban areas.)

While these options have proven to work, they often require a fair amount of construction work, including upgrading electrical systems, mounting equipment on interior and exterior walls, and running new refrigerant lines throughout the building. That means they cost a lot more than a simple boiler replacement, and that the retrofit process can be disruptive to residents.

In 2022, the New York City Housing Authority launched a contest to try and solve these problems by challenging manufacturers to develop heat pumps that can sit in a window just like an air conditioner. New designs from the two winners, Gradient Comfort and Midea, are just starting to come to market. But another emerging solution, central air-to-water heat pumps, also presents an appealing alternative. These systems avoid major construction because they can integrate with existing radiators or baseboard heaters in buildings that currently use hot water boilers. Instead of burning natural gas or oil to produce hot water, the heat pump warms the water using electricity.

The ACEEE report takes the cost and performance data for these emerging solutions and compares it to results from mini-splits, central heat pumps, geothermal heat pumps, packaged terminal heat pumps — all-in-one devices that sit inside a sleeve in the wall, commonly used in hotels — and traditional boilers fed by biogas or biodiesel.

While data on the newer technologies is limited, so far the results are extremely promising. The report found that window heat pumps are the most cost-effective of the bunch to fully decarbonize large apartment buildings, with an average installation cost of $9,300 per apartment. That’s significantly higher than the estimated $1,200 per apartment cost of a new boiler, but much lower than the $14,000 to $20,000 per apartment price tag of the other heat pump variations, although air-to-water heat pumps came in second. The report also found that window heat pumps could turn out to be the cheapest to operate, with a life cycle cost of about $14,500, compared to $22,000 to $30,000 for boilers using biodiesel or biogas or other heat pump options.

As someone who has followed this industry for several years with a keen interest in new solutions for boiler-heated buildings in the Northeast — where I grew up and currently reside — I was especially wowed by how well the new window heat pumps have performed. New York City installed units from both Midea and Gradient in 24 public housing apartments, placing one in each bedroom and living room, and monitored the results for a full heating season.

Preliminary data shows the units performed swimmingly on every metric.

On ease of installation: It took a total of eight days for maintenance workers to install the units in all 24 apartments, compared to about 10 days per apartment when the Housing Authority put split heat pump systems in another building.

On performance: During the winter, while other apartments in the building were baking in 90-degree Fahrenheit heat from the steam system, the window unit-heated apartments maintained a comfortable 75 to 80 degree range, even as outdoor temperatures dropped to as low as 20 degrees.

On energy and cost: The window unit-heated apartments used a whopping 87% less energy than the rest of the building’s steam-heated apartments did, cutting energy costs per household in half.

On customer satisfaction: A survey of 72 residents returned overwhelmingly positive feedback, with 93% reporting that the temperature was “just right” and 100% reporting they were either “neutral” or “satisfied” with the new units.

The Housing Authority found that the units also lowered energy used for cooling in peak summer since they were more efficient than the older window ACs residents had been using. Next, the agency plans to expand the pilot to two full buildings before deploying the units across its portfolio. The pilot was so successful that utilities in Massachusetts, Vermont, and elsewhere are purchasing units to do their own testing.

The ACEEE report looked at a handful of air-to-water heat pump projects in New York and Massachusetts, as well, only two of which have been completed. The average installation cost per apartment was around $13,500, with each of the buildings retaining a natural gas boiler as a backup, but none had published performance data yet.

Air-to-water heat pumps have only recently come to market in the U.S. after having taken off in Europe, and they don’t yet fit seamlessly into the housing stock here. Existing technology can only heat water to 130 to 140 degrees, which is hot enough for the more efficient hot water radiators common in Europe but too cold for the U.S. market, where hot water systems are designed to carry 160- to 180-degree water, or even steam.

These heat pumps can still work in U.S. buildings, but they require either new radiators to be installed or supplemental heat from a conventional boiler or electric resistance unit. The other downside to an air-to-water system is that it can’t provide cooling unless the building is already equipped with compatible air conditioning units.

One strength of these systems over the window units, however, is that they don’t push costs onto tenants in buildings where the landlord has historically paid for heat. They also may be cheaper to operate than more traditional heat pump options, although data is still extremely limited and depends on the use of supplemental heat.

It’s probably too soon to draw any major conclusions about air-to-water systems, anyway, because new, potentially more effective options are on the way. In 2023, New York State launched a contest challenging manufacturers to develop new decarbonized heating solutions for large buildings. Among the finalists announced last year, six companies were developing heat pumps that could generate higher-temperature hot water and/or steam. One of them is now installing its first demonstration system in an apartment building in Harlem, and two others have similar demonstrations in the works.

The ACEEE report also mentions a few other promising new heat pump formats, such as an all-in-one wall-mounted heat pump from Italian company Ephoca. It’s similar to the window heat pump in that it’s contained in a single device rather than split into an indoor and outdoor unit, so it doesn’t require mounting anything to the outside of the building or worrying about refrigerant lines, although it does require drilling two six-inch holes in the wall for vents. These may be a good option for those whose windows won’t accommodate a window heat pump or who don’t like the aesthetics. New York State is also funding product development for better packaged terminal heat pumps that could slot into wall cavities occupied by less-efficient packaged terminal air conditioners and heat pumps today.

Gradient and Midea are not yet selling their cold-climate window heat pumps directly to consumers. Gradient brought a version of its technology for more moderate climates to market in 2023, which was only suitable for heating at outdoor temperatures of 40 degrees and higher. But the company has discontinued that model and is focusing on an “all-weather” version designed for cold climates, which is the one that has been installed in the New York City apartments. Gradient told me it is currently selling that model in bulk to multi-family building owners, utilities, and schools. Midea did not respond to my inquiry.

One big takeaway is that even the new school heat pumps designed to be easier and cheaper to install have higher capital costs than buying a boiler and air conditioners — a stubborn facet of many climate solutions, even when they save money in the long run. Canary Media previously reported that the Gradient product would start at $3,800 per unit and the Midea at $3,000. Experts expect the cost to come down as adoption and demand pick up, but the ACEEE report recommends that states develop incentives and financing to help with up-front costs.

“These are not just going to happen on their own. We do need some policy support for them,” Nadel said. In addition to incentives and building decarbonization standards, Nadel raised the idea of discounted electric rates for heat pump users, an idea that has started to gain traction among climate advocates that a few utilities have piloted.

“To oversimplify,” Nadel said, “in many jurisdictions, heat pumps subsidize other customers, and that probably needs to change if this is going to be viable.”

Editor’s note: This story has been updated to include comment from Gradient.