Pie chart showing that 56% of installed heat pump capacity is in the USA, 39 percent is in Europe, and 5% is in Asia.
Virtually all the heating systems people use in buildings work by generating heat energy where it's needed. Electric radiators convert electrical energy into heat by passing electric currents through thin wires called filaments or elements (similar to those in an electric toaster). Gas and oil central heating boilers burn energy-rich fuels with oxygen from the air in a process called combustion. The energy released in this way is used to heat hot water that circulates through the radiators in our homes.
Heat pumps work in a totally different way. Instead of creating heat, they simply pick it up and move it from one place to another—usually from the ground under your home into the building itself.
Chart: You might think heat pumps would be used only in really cold countries; Sweden and Switzerland account for a quarter of the world's heat-pump capacity between them. But the technology can be used pretty much anywhere. Half of worldwide installed heat pump capacity is in the United States (where there are over a million separate units), just over a third is in Europe (where Sweden dominates), and the remainder is in Asia. Statistics from Geothermal Heat Pumps: Overview of Market Status: February 2009 (Executive Summary), US Department Energy (see references below).
In a central heating system, there's a continuous circuit of water pipes, starting at the boiler, flowing through all the radiators in turn, and going back to the boiler again.
When cold water flows into the boiler, it gets heated up very rapidly. A pump circulates it through the radiator pipes, where it gives off its heat and cools, before flowing back to the boiler to pick up more heat and go round the loop again.
A heat pump system is very similar but, instead of the boiler, the source of heat is the ground underneath your home. Water (usually mixed with antifreeze) is forced through a long string of plastic pipes buried in the ground outside or directly underneath the building.
The water picks up some of Earth's heat and flows into the building, where the energy is removed by a heat exchanger and delivered through air ducts, cooling the water in the process. The cooled water then passes back outside to pick up some more heat from the ground.
You can see then that a heat pump system really consists of three separate parts:
- Ground loop: The network of plastic pipes, filled with water, that collects heat from Earth.
- Heat pump: A simple water pump that moves the water around a loop.
- Air duct: Also known as the air handler, this extracts the heat from the water and uses a fan to blow it through your home.
Artwork comparing three different kinds of closed-loop heat pumps work: horizontal loop, vertical loop, and horizontal loop using a lake or pond
Ground-loop pipes need to extract as much heat as possible and there are four different ways to achieve this. Different methods work best for different types of buildings depending on where they're located. For ordinary-sized homes, the simplest and commonest installation involves running the ground-loop pipes at a relatively shallow depth (about 1.5–2m or 4–6 ft) through a relatively large horizontal area next to your house. Office buildings and schools, which need to collect much more heat, typically use a smaller horizontal area of pipes but run them vertically much deeper (about 30–120m or 100–400ft). The heat doesn't necessarily have to come from soil or rock. If you live near a large pond or lake, you can use that as your heat source instead of the ground. The pipes run horizontally about 2.5m (8ft) below the water surface to prevent them from freezing up.
Artwork: Three types of closed-loop heat pump system:
1. A horizontal loop, where the pipes run near the ground surface but over a large area; 2. A vertical loop has much deeper pipes recovering more heat from deeper, warmer ground; 3. A horizontal loop using a lake or pond as its heat source.
These three types of system all use closed loops of pipes (so the same fluid is contained within and flows through the heating system at all times, like the coolant in a refrigerator). There's also an alternative design, known as an open-loop system, where you pump the water out from your house into an open, underground well and then pump it back out again. The water has to be very clean for you to be able to do this, however.
Heat pumps can warm your home in winter, but they have another huge advantage too: run them in reverse in summer and they can extract heat from your home and sink it back into the ground—working just like air conditioners, only much more cheaply. They can also supply you with free hot water in summer and roughly half-price hot water in winter. Because the pipes are buried underground, the ground-loops are virtually maintenance free and (unlike air conditioners) practically impossible to damage or vandalize. The manufacturers are so confident in their products that heat pumps typically carry warranties of 25–50 years. Customers like them too: according to the US Department of Energy, user satisfaction ratings are about 90 percent!
How good are heat pumps?
In a word: superb! According to the US Department of Energy, they give annual energy savings of 30–70 percent in heating costs in winter and 20–50 percent in cooling costs in summer. That means they pay for themselves in 2–10 years. They're about 50 percent more efficient than gas furnaces (gas central-heating boilers) and 75 percent more efficient than oil furnaces (oil boilers). All round, they're good for your pocket—and great for the planet. (Having said that, they do need to be properly installed. One UK study published in 2010 found that 80 percent of heat pumps were badly installed and massively underperforming—sometimes consuming more energy than they actually produced.)