Heat pumps are commonly cited as the most cost-efficient way to heat a home. In certain cases this is true, a properly designed system can provide a very comfortable indoor environment while using less energy than other types of heating such as natural gas or baseboard electric. While the machines are by their very nature efficient, an improperly designed or installed system can provide headaches and grief for a homeowner. This primer will explain some heat pump basics before we delve deeper into more complex heat pump systems in future columns.
Heat pumps operate using a refrigeration cycle, similar to a refrigerator or an air-conditioner. The heat pump compresses refrigerant into a hot gas. This hot gas is passed through a coil where it loses heat to the ambient space. As it loses heat, the gas condenses into a warm liquid. The coil where this takes place is called the condenser. The warm liquid exiting the condenser then moves through a valve that causes rapid loss of pressure. This valve is called an expansion valve. The refrigerant also cools significantly while losing pressure. The now-cold refrigerant enters another coil termed the evaporator, where it heat and boils into a cool gas. The cool gas enters the compressor, and the cycle repeats. This is a basic refrigeration cycle.
Heat pumps use the refrigeration cycle to transport heat from either the air or the ground outside your house into your home. Heat pumps that absorb heat from outside air are called “air-source heat pumps”. Central air-source heat pumps use condensing units located outside to absorb heat into refrigerant. Two pipes connect the condensing unit to an evaporator coil inside a furnace where the hot refrigerant heat air that is then used to heat the home. A ductless mini-split, also an air-source system, eliminates the need for the furnace coil, and delivers heat directly to the room in which it is installed.
Another form of heat pump is the “water-source heat pump”, also known as a ground-source or geothermal heat pump. These types of system circulate water through pipes in the ground, in a well, or even a lake or river. The refrigerant in this case absorbs heat from the water and delivers it to a coil in a furnace in a similar manner to the central air-source heat pump.
The heat absorbed from the air or ground outside your home isn’t free. The refrigeration cycle requires a compressor to operate. These compressors use electric motors to compress the refrigerant gas. A heat pump will use a certain amount of electricity to produce a given amount of heat. We call this ratio of heat energy to electric energy the coefficient of performance, or COP. If a heat pump has a COP of 3.0, it can produce 3.0 kWh of heat per 1.0 kWh of electricity. A heat pump with a lower COP will produce less heat per kWh, while a heat pump with a high COP produces more heat.
The COP will vary with the temperature of the air or ground. Soil and groundwater temperatures do not vary as much as air temperature. This provides a ground-source heat pump with a relatively constant COP. An air-source heat pump is expected to provide heat at outside temperatures ranging from –15°C to 15°C. When the air temperature is much lower, the compressors are required to provide much higher pressure refrigerant to produce sufficient heat. As the compressors work harder, they consume more electricity. Therefore, as temperatures drop, so will an air-source heat pump’s COP.
There will come a point where the compressors in a heat pump can no longer keep up with plunging temperatures. The unit then will lock-up and cease operation. This can occur at temperatures as high as 0°C or as low as -20°C. Modern residential units tend to lock out around -15°C. People still tend to prefer having heat inside when temperatures outside drop this low, so back-up heat must be provided. This is usually accomplished with an electric coil located in the furnace, or even with baseboard heaters throughout the home.
Ground-source heat pumps are generally considered more energy efficient than air-source heat pumps. They tend to have higher COPs, and are not forced to rely on backup electric heat when temperatures plunge. The major drawback of a geothermal style system is the initial cost. The cost of burying pipes in large trenches or deep wells greatly exceeds the cost of placing an air-source condensing unit on a 4 foot square concrete pad outside your home. There will come a point using a geothermal system where your energy savings will exceed the initial cost. If you plan on being in a home on a medium-to-long term time frame, the ground-source heat pump will save more money over the lifetime of the system than an air-source system.
There are many other factors that influence heat pump operation and efficiency. Multi-storey homes must have heat pump systems designed carefully to avoid over-heating or over-cooling certain areas of the house. Installing too large or too small a heat pump system can erode energy efficiency and shorten the life of a system. These pitfalls aside, heat pumps can provide significant savings over standard types of residential heat.
