Window-mounted heat pumps offer the lowest cost solution to decarbonize large multifamily buildings using fossil fuel central heating systems, according to a new study from the American Council for an Energy-Efficient Economy (ACEEE).

The recently released report compares the 24-year life-cycle costs, which includes the initial system cost and the annual energy cost, of various space heating systems per apartment for multifamily buildings. Window air-to-air heat pumps cost the least at roughly $14,500 (€12,300), followed by central air-to-water heat pumps, which require electric or renewable fuel backup, at $22,300 (€18,900). Mini-split heat pumps cost $24,500 (€20,800) per apartment, and variable refrigerant flow (VRF) heat pumps cost $31,600 (€26,800).

“Each building will have unique opportunities, but our analysis indicates a potential order to investigate options,” said Steven Nadel, Executive Director of ACEEE and the study’s lead author. “In most cases, the three heat pump options with the lowest life-cycle costs (window, mini-split and central air-to-water heat pumps) should be considered, with the best choice often based on building-specific characteristics.”

The ACEEE report notes that about 18% of all U.S. housing units ‒ roughly 23 million apartments ‒ are contained in multifamily buildings with more than five units. The Energy Information Administration (EIA) estimates that 36% of these apartment buildings use natural gas central heating systems.

NYC study: In 2021, the New York City Housing Authority (NYCHA) contracted partnered with the California-based startup Gradient and Chinese manufacturer Midea to supply 120V window heat pumps for a pilot project. The pilot installed 74 units in 24 NYCHA-owned apartments.

• The roughly 25in (63.5cm) wide, 130‒140lb (59‒63.5kg) saddle-shaped units fit in a window, with the bulk hanging below the window frame.
• Both manufacturers’ heat pumps use R32 variable speed compressors, with the Gradient unit operating in ambient temperatures down to −7°F (−21.7°C) and the Midea down to −13°F (−25°C).
• “Preliminary results over the first winter [include] an 87% reduction in BTU per square foot for heating relative to steam heating in matching apartments and a 50% decrease in energy costs per apartment per heating degree day,” the ACEEE report said.

An opportunity for R290: Gradient said it is open to using propane (R290) refrigerant in future window heat pumps. However, U.S. safety standards limit the allowable charge size of R290 in residential HVAC to 114g of propane, which is well below the international IEC 60335-2-40 standard of up to 988g for split AC systems.

• The factory-charged and sealed window heat pump manufactured by Gradient contains 780g of R32, the company told NaturalRefrigerants.com, adding that the design keeps the refrigerant in the outdoor section of the saddle-shaped unit.
• “We’re optimistic that ASHRAE and UL standards will evolve to allow for broader use of R290, and we’re actively monitoring those developments,” said Gradient. “As standards progress, we’ll be exploring every opportunity to incorporate R290 into our future products, with it offering an excellent balance of efficiency and environmental sustainability.”
• Regarding safety concerns, a recent resource guide on propane split ACs published by the GIZ (the German Society for International Cooperation) noted that 600,000 single-split R290 AC units have been sold without any reported safety issues. This was attributed to the units, which were also declared to be the “most energy-efficient product on the local market,” being installed by certified professionals.

A3 standards: Hydrocarbon regulations in the U.S. are currently under review.

• In March 2024, an ASHRAE working group was authorized to study and test indirect systems to increase the propane charge to 4.9kg.
• ASHRAE Committee Chair Jim VerShaw said in March 2024 that the new R290 charge approval under ASHRAE-15 will not be out until 2025 or later. Once approved by ASHRAE, it must be endorsed by UL under standard 60335-2-40 and then by the Environmental Protection Agency (EPA).
• “Already 23 states have passed legislation that will enable [the uptake of] hydrocarbons once the A3 standards are updated and approved by the EPA,” said Avipsa Mahapatra, Climate Campaign Director at the EIA, in a presentation at ATMO America 2025. The event was organized by ATMOsphere, publisher of NaturalRefrigerants.com, and held June 11‒12 in Atlanta.

Quotable: “Existing multifamily buildings can be decarbonized, contributing to reduced emissions and efforts to slow climate change,” Nadel said. “There has been substantial progress in recent years improving system options, developing new approaches and learning what works and what does not. These efforts should be accelerated.”

“Existing multifamily buildings can be decarbonized, contributing to reduced emissions and efforts to slow climate change.”

Steven Nadel, Executive Director of ACEEE