How do Heat Pumps Work?

Most people who live in cold or four-season weather are familiar with the concept of central heat. For decades, nearly every home that offered this essential feature used one of two methods for getting heat to every room in the house; either a furnace with pressurized steam and hot water, or a wood-burning stove that operated along similar principles with hot gas. Today there is a third option, however, one that involves no “fuel” whatsoever: the heat pump.

Heat pumps have been around for some time, but the reversible mechanics behind them remain essential unchanged. Most of us who live with refrigeration and air conditioners only see one side of this equation which is cooling. Flip the process around, however, and you get a fairly efficient system that radiates heat and exhausts cold air out into the atmosphere. If you have plenty of power but do not relish the thought of using gas, coal or wood, an efficient heat pump may represent a nice trade-off for your needs.

How does a heat pump work? The underlying physics are actually quite intuitive. If you have ever used rubbing alcohol or perspired on a breezy day, you probably noticed the immediate cooling sensation that resulted. This is the effect of evaporation on your skin. As liquid molecules gain enough energy to turn into gas and fly away, they take away some heat with them. What is left behind in this case loses energy in the process, which presents as cold.

Evaporation and condensation are flip sides of the same coin. One cools, the other heats. Compressing a gas back into a liquid requires a mechanical crushing force, but the result is that the gas turns hot. Countless industrial and domestic products make use of this cycle, often by adding a gas such as Freon that can be quickly phase-shifted from liquid to gas and back again. Dramatic temperature swings can be attained in this way with a minimal outlay of power.

Heat pumps make use of the same principle by circulating compressed gases through two sets of coils to achieve the desired effect. What distinguishes a heat pump from an air conditioner is that a heat pump is by definition reversible, meaning the process can move in either direction. Although the underlying physics are the same, you get mirror image effects depending on which way the valve is thrown.

Like air conditioners, every heat pump has four basic parts: the compressor, the condenser, the expansion valve and the evaporator. The process begins when the compressor crushes the gas in a piston-like device, turning it into a very hot liquid. This liquid circulates through the condenser, which is usually a set of metal coils whose job is to absorb and radiate the heat. Check the coils on the back of your refrigerator, or the hot air exhausted by a window A/C unit, and you get an idea of just how much heat is generated here.

The expansion valve releases the pressure on this hot liquid, causing the liquid to evaporate instantly. Think of the first pop when you open a soda can even if it’s been at room temperature, the escaping gas feels cold. This is the nature of gas when it is suddenly exposed to lower pressure, and it’s the driving property behind the refrigeration side of any heat pump. Evaporation coils follow, turning cold in the process and giving you consistently cool air.

The beauty of this system is that the only energy source is electricity itself, meaning you can operate a heat pump without needing to replenish a nonrenewable fuel source. The downside, of course, is that most of the heat generated by the process comes from the atmosphere to begin with. In frigid temperatures below around 15 degrees Fahrenheit, a heat pump will no longer do an adequate job of heating your home.

Look into the market for heat pumps and you will notice just how many designs are available. From ground source heat pumps to newer technologies that make use of magnetism, the market for heat pumps is forever making strides in efficiency. A basic working knowledge of the underlying science can help you make a more informed choice as you invest for your home.

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