How do window-unit heat pumps work?

From the outside, the sleek white boxes poking out of the windows of 24 apart­ments at Woodside Houses in Queens could easily be mistaken for any other air condi­tioner. But inside, the heat pumps that make up the heating and cooling pilot project have dras­ti­cally improved resi­dents’ comfort — and helped NYCHA to meet its green­house gas reduc­tion goals.

The heat pumps installed in apart­ments throughout this devel­op­ment were the result of a 2022 city and state-funded industry compe­ti­tion to create a heating and cooling product that didn’t run on fossil fuels, was easy to install, and could be rela­tively afford­able on a large scale. The winning manu­fac­turers, Midea and Gradient, created versions of a heat pump unit that could be installed through a stan­dard window, without the need for construc­tion, drain piping, or system-wide elec­trical upgrades.

These smaller window units still work according to the same prin­ci­ples as other heat pumps: They extract heat from the outside air and move it into an apart­ment during the winter, and do the reverse in the summer, pulling heat from warm inside air and leaving the apart­ment cooled and comfortable.

Most heat pump tech­nolo­gies, like duct­less mini-split heat pumps, are composed of a condenser that sits outside and an evap­o­rator coil that sits inside, and refrig­erant piping to connect them. Installing this equip­ment often requires cutting through walls and making major elec­trical upgrades, changes that would have been unwork­able for Woodside Houses. 

The smaller units created by Midea and Gradient, by contrast, shorten the gap between the ​“inside” and ​“outside” compo­nents of a heat pump — condenser, compressor evap­o­rator, etc. — by putting all of them into one machine that sits on either side of a window sill. And they do it all on stan­dard 120V power, so no elec­trical amperage upgrades were necessary.

Preliminary results from moni­toring the perfor­mance of the elec­trical heat pumps show that they consume signif­i­cantly less energy than a steam boiler. During the summer when the pumps were used to cool apart­ments, they slightly increased the build­ings’ peak energy demand, but as time went on, the heat pumps saw a drop in daily elec­tricity use compared to the old air condi­tioners they replaced. 

In addi­tion to energy use, the mate­rials used in these heat pumps generate fewer green­house gases compared to tradi­tional air condi­tioners. These new units use a much more eco-friendly refrig­erant, compared to typical HVAC systems: R32, which has about one-third of the Global Warming Potential (GWP) than the more commonly used R410A, so should there be a leak in the heat pumps, the atmos­pheric damage is less­ened. ​“If heat pumps start leaking refrig­erant, you’ve negated a lot of the GHG reduc­tion advan­tage,” Sahagian said. “[But] because these units are self-contained, the like­li­hood of a refrig­erant leak during their oper­ating life­time is rela­tively small.”

While mini-split systems typi­cally rely on one or two central units to serve an entire apart­ment, window heat pumps are room-specific — allowing for indi­vidual control, energy savings, and redun­dancy in the event of a malfunction. 

A year following the initial instal­la­tion in 2023, resi­dents reported quieter, more consis­tent comfort and easier temper­a­ture control. Based on its popu­larity, and the emis­sions reduc­tions that have come along with the new tech, NYCHA has committed to installing 30,000 more units over the next three years, starting with full deploy­ment of Midea units at Woodside, to be completed prior to the 2025 – 2026 heating season. The program will then expand to cover the remaining build­ings in the development.

And interest is growing beyond NYCHA. Energy consul­tants and the private build­ings they work with are eyeing the units as a more scal­able, less intru­sive way to elec­trify older build­ings. What started as a public housing exper­i­ment could end up trans­forming the retrofit market citywide.