What Is a Heat Pump, and How Does It Work?

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 Brunswick^Footnote1.

• 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.

Important Terminology for Heat Pump Systems

Here are some common terms you may come across while investigating heat pumps.

Heat Pump System Components

The refrigerant is the fluid that circulates through the heat pump, alternately absorbing, transporting and releasing heat. Depending on its location, the fluid may be liquid, gaseous, or a gas/vapour mixture

The reversing valve controls the direction of flow of the refrigerant in the heat pump and changes the heat pump from heating to cooling mode or vice versa.

A coil is a loop, or loops, of tubing where heat transfer between the source/sink and refrigerant takes place. The tubing may have fins to increase the surface area available for heat exchange.

The evaporator is a coil in which the refrigerant absorbs heat from its surroundings and boils to become a low-temperature vapour. As the refrigerant passes from the reversing valve to the compressor, the accumulator collects any excess liquid that did not vaporize into a gas. Not all heat pumps, however, have an accumulator.

The compressor squeezes the molecules of the refrigerant gas together, increasing the temperature of the refrigerant. This device helps to transfer thermal energy between the source and sink.

The condenser is a coil in which the refrigerant gives off heat to its surroundings and becomes a liquid.

The expansion device lowers the pressure created by the compressor. This causes the temperature to drop, and the refrigerant becomes a low-temperature vapour/liquid mixture.

The outdoor unit is where heat is transferred to/from the outdoor air in an air-source heat pump. This unit generally contains a heat exchanger coil, the compressor, and the expansion valve. It looks and operates in the same manner as the outdoor portion of an air-conditioner.

The indoor coil is where heat is transferred to/from indoor air in certain types of air-source heat pumps. Generally, the indoor unit contains a heat exchanger coil, and may also include an additional fan to circulate heated or cooled air to the occupied space.

The plenum , only seen in ducted installations, is part of the air distribution network. The plenum is an air compartment that forms part of the system for distributing heated or cooled air through the house. It is generally a large compartment immediately above or around the heat exchanger.

Other Terms

Units of measurement for capacity, or power use:

• A Btu/h, or British thermal unit per hour, is a unit used to measure the heat output of a heating system. One Btu is the amount of heat energy given off by a typical birthday candle. If this heat energy were released over the course of one hour, it would be the equivalent of one Btu/h.
• A kW, or kilowatt, is equal to 1000 watts. This is the amount of power required by ten 100-watt light bulbs.
• A ton is a measure of heat pump capacity. It is equivalent to 3.5 kW or 12 000 Btu/h.

Air-Source Heat Pumps

Air-source heat pumps use the outdoor air as a source of thermal energy in heating mode, and as a sink to reject energy when in cooling mode. These types of systems can generally be classified into two categories:

Air-Air Heat Pumps. These units heat or cool the air inside your home, and represent the vast majority of air-source heat pump integrations in Canada. They can be further classified according to the type of installation:

• Ducted: The indoor coil of the heat pump is located in a duct. Air is heated or cooled by passing over the coil, before being distributed via the ductwork to different locations in the home.
• Ductless: The indoor coil of the heat pump is located in an indoor unit. These indoor units are generally located on the floor or wall of an occupied space, and heat or cool the air in that space directly. Among these units, you may see the terms mini- and multi-split:
  • Mini-Split: A single indoor unit is located inside the home, served by a single outdoor unit.
  • Multi-Split: Multiple indoor units are located in the home, and are served by a single outdoor unit.

Air-air systems are more efficient when the temperature difference between inside and outside is smaller. Because of this, air-air heat pumps generally try to optimize their efficiency by providing a higher volume of warm air, and heating that air to a lower temperature (normally between 25 and 45°C). This contrasts with furnace systems, which deliver a smaller volume of air, but heat that air to higher temperatures (between 55°C and 60°C). If you are switching to a heat pump from a furnace, you may notice this when you begin using your new heat pump.

Air-Water Heat Pumps: Less common in Canada, air-water heat pumps heat or cool water, and are used in homes with hydronic (water-based) distribution systems such as low temperature radiators, radiant floors, or fan coil units. In heating mode, the heat pump provides thermal energy to the hydronic system. This process is reversed in cooling mode, and thermal energy is extracted from the hydronic system and rejected to the outdoor air.

Operating temperatures in the hydronic system are critical when evaluating air-water heat pumps. Air-water heat pumps operate more efficiently when heating the water to lower temperatures, i.e., below 45 to 50°C, and as such are a better match for radiant floors or fan coil systems. Care should be taken if considering their use with high temperature radiators that require water temperatures above 60°C, as these temperatures generally exceed the limits of most residential heat pumps.

Major Benefits of Air-Source Heat Pumps

Installing an air-source heat pump can offer you a number of benefits. This section explores how air-source heat pumps can benefit your household energy footprint.

Efficiency

The major benefit of using an air-source heat pump is the high efficiency it can provide in heating compared to typical systems like furnaces, boilers and electric baseboards. At 8°C, the coefficient of performance (COP) of air-source heat pumps typically ranges from between 2.0 and 5.4. This means that, for units with a COP of 5, 5 kilowatt hours (kWh) of heat are transferred for every kWh of electricity supplied to the heat pump. As the outdoor air temperature drops, COPs are lower, as the heat pump must work across a greater temperature difference between the indoor and outdoor space. At –8°C, COPs can range from 1.1 to 3.7.

On a seasonal basis, the heating seasonal performance factor (HSPF) of market available units can vary from 7.1 to 13.2 (Region V). It is important to note that these HSPF estimates are for an area with a climate similar to Ottawa. Actual savings are highly dependant on the location of your heat pump installation.

Energy Savings

The higher efficiency of the heat pump can translate into significant energy use reductions. Actual savings in your house will depend on a number of factors, including your local climate, efficiency of your current system, size and type of heat pump, and the control strategy. Many online calculators are available to provide a quick estimation of how much energy savings you can expect for your particular application. NRCan’s ASHP-Eval tool is freely available and could be used by installers and mechanical designers to help advise on your situation.

For More Information please Visit :

Heating and Cooling With a Heat Pump (nrcan.gc.ca)