Window-mounted heat pumps could make apartments more sustainable. Here’s how New York is leading the way

"Nineteenth-century technology, incompatible with twenty-first-century needs."

That’s how the New York City Housing Authority (NYCHA) describes the old, inefficient steam-heat boiler and radiator networks used by the 528,000 people occupying the 2,400 buildings it controls.

With the aim of reducing both energy costs and CO2 emissions of these buildings, the authority is pilot testing window-mounted heat pumps for use in apartment blocks, reports Euronews.

Buildings generate 26% of global energy-related CO2 emissions, according to the International Energy Agency: 8% directly and 18% indirectly from electricity and heat use.

Heat pumps are an efficient, sustainable way to address building-related emissions – but until now, size and space requirements have made them impractical for apartment buildings.

The technology works by moving heat from one place to another, either drawing heat from outside to warm a building or pushing heat from inside out to cool it. Heat can be drawn from or pushed to a number of different sources, including external air, water sources, underground, or residual heat from industry, says the International Renewable Energy Agency (IRENA).

Each unit of electricity consumed by a heat pump can produce four units of thermal energy, says the IEA, compared to a one-to-one ratio for fossil-fuelled systems. As less electricity is needed to heat or cool a building, heat pumps are more efficient, cheaper to run and produce fewer CO2 emissions than traditional systems.

Reducing heat pumps to the size of a household air conditioner unit enables apartment dwellers, including renters, to embrace more sustainable heating and cooling.

Most heat pumps require ducts to be fitted or extensive wiring that includes installing a cumbersome external compressor, giving apartment renters little incentive to buy one.

However, the heat pumps being tested by NYCHA resemble a slim air conditioning unit that sits neatly astride a window ledge, with half inside and half outside to remove the need for ducts, wiring or building work. The unit can be installed in a matter of minutes and plugged into an electricity socket for power.

Renters have complete control of their apartment’s heating and cooling, unlike municipal flats with centralized boiler systems where users have little or no control, and they can reduce both costs and carbon footprint. If the apartment’s occupant moves, the unit can simply be unplugged and taken to their new home.

While heat pumps are good for the environment and can reduce energy use, which in turn reduces energy bills, they are expensive to buy compared to existing combustion-based heating systems.

However, heat pumps can combine with existing gas infrastructure in buildings to minimize efficiency losses with in-place gas-fired boilers, IRENA notes.

In Europe, the race to electrify heating and cooling has slowed, with strong sales growth over the past decade or so tailing off in recent years. Year-on-year heat pump sales in 14 European Union states dropped by 5% in 2023, from almost 2.77 million units to around 2.64 million.

Sales in the bloc were impacted by the postponement of the EU’s Heat Pump Action Plan, which was intended to support the nascent market.

This slowdown in heat pump sales is reflected in the US market, although sales still exceeded gas-fired furnaces – that pump air through ducts to heat homes – for the second consecutive year.

As in Europe, the slowdown followed around a decade of robust growth. US gas furnace sales fell sharper than air-source heat pumps in 2023, demonstrating comparative growth in market-share of heat pumps in 2022.

The World Economic Forum’s Fostering Effective Energy Transition 2023 report points to heat pumps as one of a number of solutions needed to advance the energy transition.

Replacing existing fossil fuel systems with heat pumps could potentially reduce global CO2 emissions by at least 500 million tonnes in 2030 – the equivalent of the annual CO2 emissions of all cars in Europe right now, says the IEA.

The energy transition will be fuelled by innovation. In New York’s case, successful trials of smaller, more portable heat pumps could see 4,000 slimmed-down units deployed in the next two years, giving renters in apartments an incentive to invest in more sustainable energy.