Heat pumps are a proven technology that have been used for decades, both in Canada and globally, to efficiently provide heating, cooling, and in some cases, hot water to buildings. In fact, it is likely that you interact with heat pump technology on a daily basis: refrigerators and air conditioners operate using the same principles and technology. This section presents the basics of how a heat pump works, and introduces different system types.
Heat Pump Basic Concepts
A heat pump is an electrically driven device that extracts heat from a low temperature place (a source), and delivers it to a higher temperature place (a sink).
To understand this process, think about a bicycle ride over a hill: No effort is required to go from the top of the hill to the bottom, as the bike and rider will move naturally from a high place to a lower one. However, going up the hill requires a lot more work, as the bike is moving against the natural direction of motion.
In a similar manner, heat naturally flows from places with higher temperature to locations with lower temperatures (e.g., in the winter, heat from inside the building is lost to the outside). A heat pump uses additional electrical energy to counter the natural flow of heat, and pump the energy available in a colder place to a warmer one.
So how does a heat pump heat or cool your home? As energy is extracted from a source, the temperature of the source is reduced. If the home is used as the source, thermal energy will be removed, cooling this space. This is how a heat pump operates in cooling mode, and is the same principle used by air conditioners and refrigerators. Similarly, as energy is added to a sink, its temperature increases. If the home is used as a sink, thermal energy will be added, heating the space. A heat pump is fully reversible, meaning that it can both heat and cool your home, providing year-round comfort.
Sources and Sinks for Heat Pumps
Selecting the source and sink for your heat pump system goes a long way in determining the performance, capital costs and operating costs of your system. This section provides a brief overview of common sources and sinks for residential applications in Canada.
Sources: Two sources of thermal energy are most commonly used for heating homes with heat pumps in Canada:
Air-Source: The heat pump draws heat from the outside air during the heating season and rejects heat outside during the summer cooling season. It may be surprising to know that even when outdoor temperatures are cold, a good deal of energy is still available that can be extracted and delivered to the building. For example, the heat content of air at -18°C equates to 85% of the heat contained at 21°C. This allows the heat pump to provide a good deal of heating, even during colder weather. Air-source systems are the most common on the Canadian market, with over 700,000 installed units across Canada. This type of system is discussed in more detail in the Air-Source Heat Pumps section.
Ground-Source: A ground-source heat pump uses the earth, ground water, or both as the source of heat in the winter, and as a reservoir to reject heat removed from the home in the summer. These heat pumps are less common than air-source units, but are becoming more widely used in all provinces of Canada. Their primary advantage is that they are not subject to extreme temperature fluctuations, using the ground as a constant temperature source, resulting in the most energy efficient type of heat pump system. This type of system is discussed in more detail in the Ground-Source Heat Pumps section.
Sinks: Two sinks for thermal energy are most commonly used for heating homes with heat pumps in Canada:
Indoor air is heated by the heat pump. This can be done through:
A centrally ducted system or
A ductless indoor unit, such as a wall mounted unit.
Water inside the building is heated. This water can then be used to serve terminal systems like radiators, a radiant floor, or fan coil units via a hydronic system.
An Introduction to Heat Pump Efficiency
Furnaces and boilers provide space heating by adding heat to the air through the combustion of a fuel such as natural gas or heating oil. While efficiencies have continually improved, they still remain below 100%, meaning that not all the available energy from combustion is used to heat the air.
Heat pumps operate on a different principle. The electricity input into the heat pump is used to transfer thermal energy between two locations. This allows the heat pump to operate more efficiently, with typical efficiencies well over 100%, i.e. more thermal energy is produced than the amount of electric energy used to pump it.
It is important to note that the efficiency of the heat pump depends greatly on the temperatures of the source and sink. Just like a steeper hill requires more effort to climb on a bike, greater temperature differences between the source and sink of the heat pump require it to work harder, and can reduce efficiency. Determining the right size of heat pump to maximize seasonal efficiencies is critical. These aspects are discussed in more detail in the Air-Source Heat Pumps and Ground-Source Heat Pumps sections.
Efficiency Terminology
A variety of efficiency metrics are used in manufacturer catalogues, which can make understanding system performance somewhat confusing for a first time buyer. Below is a breakdown of some commonly used efficiency terms:
Steady-State Metrics: These measures describe heat pump efficiency in a ‘steady-state,’ i.e., without real-life fluctuations in season and temperature. As such, their value can change significantly as source and sink temperatures, and other operational parameters, change. Steady state metrics include:
Coefficient of Performance (COP): The COP is a ratio between the rate at which the heat pump transfers thermal energy (in kW), and the amount of electrical power required to do the pumping (in kW). For example, if a heat pump used 1kW of electrical energy to transfer 3 kW of heat, the COP would be 3.
Energy Efficiency Ratio (EER): The EER is similar to the COP, and describes the steady-state cooling efficiency of a heat pump. It is determined by dividing the cooling capacity of the heat pump in Btu/h by the electrical energy input in Watts (W) at a specific temperature. EER is strictly associated with describing the steady-state cooling efficiency, unlike COP which can be used to express the efficiency of a heat pump in heating as well as cooling.
Seasonal Performance Metrics: These measures are designed to give a better estimate of performance over a heating or cooling season, by incorporating “real life” variations in temperatures across the season.
Seasonal metrics include:
Heating Seasonal Performance Factor (HSPF): HSPF is a ratio of how much energy the heat pump delivers to the building over the full heating season (in Btu), to the total energy (in Watthours) it uses over the same period.
Weather data characteristics of long-term climate conditions are used to represent the heating season in calculating the HSPF. However, this calculation is typically limited to a single region, and may not fully represent performance across Canada. Some manufacturers can provide an HSPF for another climate region upon request; however typically HSPFs are reported for Region 4, representing climates similar to the Midwestern US. Region 5 would cover most of the southern half of the provinces in Canada, from the B.C interior through New BrunswickFootnote1.
Seasonal Energy Efficiency Ratio (SEER): SEER measures the cooling efficiency of the heat pump over the entire cooling season. It is determined by dividing the total cooling provided over the cooling season (in Btu) by the total energy used by the heat pump during that time (in Watt-hours). The SEER is based on a climate with an average summer temperature of 28°C.