This article is republished from The Conversation.

People who rent their homes, or don't have enough money to make major upgrades to their homes, have for many years been left out of a major shift in heating and cooling technology that can improve efficiency, save money and be better for the global climate: heat pumps.

Heating and cooling buildings consumes 35% of all the energy used in the United States each year. Many homes and businesses are converting their fossil fuel-powered heating and cooling systems to electric-powered heat pumps, which use electricity not to generate hot or cold air but to move heat into spaces needing warmth and out of spaces needing cooling.

Until recently, that process has required a significant amount of sizable and expensive equipment to be permanently installed in a building, which needs a professional contractor and can cost as much as US$10,000 just for the installation – in addition to the actual equipment. Often called mini-splits, these systems usually have a condenser outside the building that exchanges heat with the outdoor air and an evaporator inside that exchanges heat with the indoor air.

But now window heat pumps are becoming available in the U.S. Much like a window air conditioner, these self-contained devices can be installed without professional help and plugged into a wall outlet. Unlike window air conditioners, though, they can provide heat as well as cooling. They cost much less than a permanent system – between $3,000 and $4,000 – and can be moved to a new property if the owner relocates.

There aren't many options commercially available yet, and those on the market can't heat or cool very large spaces on their own. And they work less efficiently when heating homes in places with extremely cold outdoor temperatures. A few models are available on the market that are even cheaper, but they don't have efficiency ratings, don't work when outdoor temperatures are very cold, and are louder when running.

I have designed and evaluated a wide range of building energy efficiency technologies; here's how these window heat pumps work, and why they may allow apartment dwellers and residents of older houses to easily and relatively inexpensively make significant improvements to their homes' heating and cooling systems. Federal subsidies for this type of equipment expired in 2025, but some utility companies, states and local governments may still offer money to help pay the costs.

Moving heat from one place to another

Heat pumps use a reversible refrigeration cycle and can provide similar heating and cooling as electric-powered space heaters, furnaces and baseboard heaters, while using less than half the electricity.

The most common heat pumps transfer heat between air indoors and outdoors, but other systems can exchange heat with the ground or with bodies of water, such as lakes.

Heat pumps' capacities are defined by the amount of heat they can transfer in a particular period of time. A heat pump serving an entire home may need a capacity of 12,000 to 60,000 British thermal units (about 12,660 to 63,300 kilojoules) – but the window units' capacities are much lower, getting up to only about 9,000 Btu (9,500 kJ).

Performance varies based on the conditions outdoors, where the unit is either sending excess heat to cool the indoors or gathering heat to warm the indoors. In cooling mode, heat pumps are rated by their seasonal energy efficiency ratio, a figure that indicates how much cooling is achieved per unit of electricity used. The corresponding measurement for heating is called heating seasonal performance factor. In general, the larger these numbers are, the better they will perform. The U.S. Department of Energy has established minimum standards for those figures.

While these units operate even when outdoor temperatures are -13 degrees Fahrenheit (-25 degrees Celsius), their heating output is reduced to almost half of its rated capacity, and their energy efficiency falls to one-third of its rated performance at that temperature.

In addition to their low cost compared to conventional split heat pumps, packaged window heat pumps meet heating and cooling needs with lower energy demands and costs. But each window unit serves just one room, while a more common split unit can serve multiple rooms.

Packaged window heat pumps are easy and inexpensive to install and offer all-in-one heating and cooling options for apartments and older homes, with higher energy efficiency performance than traditional systems. Their main limitations include their low capacities and reduced energy efficiency in extremely cold climates or conditions.