Air source heat pumps
Encyclopedia
An air source heat pump
(ASHP) is a heating and cooling system that uses outside air as its heat source and heat sink. Under the principles of vapor compression refrigeration, an ASHP uses a refrigerant
system involving a compressor and a condenser to absorb heat at one place and release it at another.
In domestic heating use, an ASHP absorbs heat from outside air and releases it inside during winter, and can often do the converse in summer. When correctly specified, an ASHP can offer a full central heating solution and domestic hot water up to 80°C.
, contains some heat. An air-source heat pump moves ('pumps') some of this heat to provide hot water or space heating. This can be done in either direction, to cool or heat the interior of a building.
The main components of an air-source heat pump are:
Air source heat pumps can provide fairly low cost space heating. A high efficiency heat pump can provide up to four times as much heat as an electric heater using the same energy. In comparison to gas as a primary heat source, however, the lifetime cost of an air source heat pump may be affected by the high price of electricity versus gas (where available). Gas may cause higher carbon emissions, depending upon how the electricity is generated.
A "standard" air sourced heat pump found in most homes can extract useful heat down to about -5F or 0F (-15c). At colder outdoor temperatures the heat pump is inefficient; it could be switched off to run only on supplemental heat (or emergency heat) if the supplemental heat is sized large enough. There are specially designed heat pumps that, while giving up some performance in a/c mode, will provide useful heat extraction to even lower outdoor temperatures. An air source heat pump designed specifically for very cold climates can extract useful heat from ambient air as cold as -20F or even -25F (-30c), but these are uncommon in most homes.
Air source heat pumps can last for over 20 years with low maintenance requirements. There are numerous heat pumps from the 1970s and 1980s that are still in service as of 2011, even in places where winters are extremely cold. Few moving parts reduce maintenance requirements, however, the outdoor heat exchanger and fan must be kept free from leaves and debris. Heat pumps have significantly more moving parts than an equivalent electric resistance heater or fuel burning heater.
of installation is generally high, it is less than the cost of a ground source heat pump, because a ground source heat pump requires excavation to install its ground loop.
ASHP's are often paired with auxiliary or emergency heat systems to provide backup heat when outside temperatures are too low for the pump to work efficiently, or in the event the pump malfunctions. Propane, natural gas, or oil furnaces can provide this supplementary heat. All-electric heat pump systems have an electric furnace or electric resistance heat, or strip heat, which typically consists of rows of electric coils that heat up. A fan blows over the heated coils and circulates warm air throughout the home. This serves as an adequate heating source, but as temperatures go down, electricity costs rise, and power outages pose an even greater threat.
The outdoor section on some units may 'frost up' when outdoor temperatures are between 0°C and 5°C (between 32°F and 41°F) and there is sufficient moisture in the air which restricts air flow across the outdoor coil. These units employ a defrost cycle where the system switches to "A/C" mode to move heat from the home to the condenser to melt the ice. This requires the supplementary heater (resistance electric or gas) in the indoor section to activate, to temper the cold air being distributed. The defrost cycle reduces the efficiency of the heat pump significantly, although the newer (demand) systems are more intelligent and need to defrost less. As temperatures drop below freezing the tendency for frosting of the outdoor section decreases due to reduced humidity in the air.
It is difficult to retrofit conventional heating systems that use radiators/radiant panels, hot water baseboard heaters, or even smaller diameter ducting, with ASHP-sourced heat. The lower heat pump output temperatures would mean radiators would have to be increased in size or a low temperature underfloor heating
system be installed instead.
through the heat pump's indoor and outdoor coil
s. Like in a refrigerator, a compressor
, condenser
, expansion valve
and evaporator
are used to change states of the refrigerant between a cold liquid
and a hot gas
.
When the liquid refrigerant at a low temperature
passes through the outdoor evaporator heat exchanger coils, ambient heat is used to cause the liquid to boil. This boiling or change of state process amasses energy as latent heat
. The vapor
is then drawn into a compressor which further boosts the temperature of the vapor.
Passing into the building, the vapor enters the condenser heat exchanger coils where it transfers heat to indoor air, which is drawn across the coils by a fan
. As the vapor cools, it condenses back into a liquid, and releases its latent heat
to the air passing over the condenser unit.
Exiting the condenser, the cold liquid refrigerant is under high pressure. The refrigerant passes through an expansion valve
which reduces the pressure, draws in heat and allows the refrigerant to re-enter the evaporator to begin a new cycle.
Most heat pumps can also operate in a cooling mode where the cold refrigerant is moved through the indoor coils to cool the room air.
In mild weather, the COP of an air source heat pump can be up to 4. However, on a very cold winter day, it takes more work to move the same amount of heat indoors than on a mild day. The heat pump's performance is limited by the Carnot cycle
and will approach 1.0 as the outdoor-to-indoor temperature difference increases, which for most air source heat pumps happens as outdoor temperatures approach −18 °C / 0 °F. Heat pump construction that enables carbon dioxide as a refrigerant may have a COP of greater than 2 even down to -20°C, pushing the break-even figure downward to -30 °C (-22 °F). A ground source heat pump has comparatively less of a change in COP as outdoor temperatures change, because the ground from which they extract heat has a more constant temperature than outdoor air.
The specific design of a heat pump has a considerable impact on its efficiency. Many air source heat pumps are designed primarily as air conditioning units, mainly for use in summer temperatures. Designing a heat pump specifically for the purpose of heat exchange can attain greater COP ratings and an extended life cycle. The principal changes are in the scale and type of compressor and evaporator.
Seasonally adjusted heating and cooling efficiencies are given by the heating seasonal performance factor (HSPF) and seasonal energy efficiency ratio
(SEER) respectively.
In units charged with HFC refrigerants, the COP rating is reduced when heat pumps are used to heat domestic water to over 60°C or to heat conventional central heating
systems that use radiators to distribute heat (instead of an underfloor heating array).
(GWP) and ozone depletion potential
(ODP). Recent government mandates have seen the phase-out of R-22 refrigerant and its replacement with more environmentally sound R410a refrigerant.
Heat pump
A heat pump is a machine or device that effectively "moves" thermal energy from one location called the "source," which is at a lower temperature, to another location called the "sink" or "heat sink", which is at a higher temperature. An air conditioner is a particular type of heat pump, but the...
(ASHP) is a heating and cooling system that uses outside air as its heat source and heat sink. Under the principles of vapor compression refrigeration, an ASHP uses a refrigerant
Refrigerant
A refrigerant is a substance used in a heat cycle usually including, for enhanced efficiency, a reversible phase change from a liquid to a gas. Traditionally, fluorocarbons, especially chlorofluorocarbons, were used as refrigerants, but they are being phased out because of their ozone depletion...
system involving a compressor and a condenser to absorb heat at one place and release it at another.
In domestic heating use, an ASHP absorbs heat from outside air and releases it inside during winter, and can often do the converse in summer. When correctly specified, an ASHP can offer a full central heating solution and domestic hot water up to 80°C.
Description
Outside air, at any temperature above absolute zeroAbsolute zero
Absolute zero is the theoretical temperature at which entropy reaches its minimum value. The laws of thermodynamics state that absolute zero cannot be reached using only thermodynamic means....
, contains some heat. An air-source heat pump moves ('pumps') some of this heat to provide hot water or space heating. This can be done in either direction, to cool or heat the interior of a building.
The main components of an air-source heat pump are:
- An outdoor heat exchanger coil, which extracts heat from ambient air
- An indoor heat exchanger coil, which transfers the heat into a water tank or indoor heating system, such as radiators or under-floor circuits
Air source heat pumps can provide fairly low cost space heating. A high efficiency heat pump can provide up to four times as much heat as an electric heater using the same energy. In comparison to gas as a primary heat source, however, the lifetime cost of an air source heat pump may be affected by the high price of electricity versus gas (where available). Gas may cause higher carbon emissions, depending upon how the electricity is generated.
A "standard" air sourced heat pump found in most homes can extract useful heat down to about -5F or 0F (-15c). At colder outdoor temperatures the heat pump is inefficient; it could be switched off to run only on supplemental heat (or emergency heat) if the supplemental heat is sized large enough. There are specially designed heat pumps that, while giving up some performance in a/c mode, will provide useful heat extraction to even lower outdoor temperatures. An air source heat pump designed specifically for very cold climates can extract useful heat from ambient air as cold as -20F or even -25F (-30c), but these are uncommon in most homes.
Air source heat pumps can last for over 20 years with low maintenance requirements. There are numerous heat pumps from the 1970s and 1980s that are still in service as of 2011, even in places where winters are extremely cold. Few moving parts reduce maintenance requirements, however, the outdoor heat exchanger and fan must be kept free from leaves and debris. Heat pumps have significantly more moving parts than an equivalent electric resistance heater or fuel burning heater.
Usage
Air source heat pumps are used to provide interior space heating and cooling even in colder climates, and can be used efficiently for water heating in milder climates. A major advantage of ASHP's is that the same system may be used for air conditioning in summer and heating in winter. Though the costCost
In production, research, retail, and accounting, a cost is the value of money that has been used up to produce something, and hence is not available for use anymore. In business, the cost may be one of acquisition, in which case the amount of money expended to acquire it is counted as cost. In this...
of installation is generally high, it is less than the cost of a ground source heat pump, because a ground source heat pump requires excavation to install its ground loop.
ASHP's are often paired with auxiliary or emergency heat systems to provide backup heat when outside temperatures are too low for the pump to work efficiently, or in the event the pump malfunctions. Propane, natural gas, or oil furnaces can provide this supplementary heat. All-electric heat pump systems have an electric furnace or electric resistance heat, or strip heat, which typically consists of rows of electric coils that heat up. A fan blows over the heated coils and circulates warm air throughout the home. This serves as an adequate heating source, but as temperatures go down, electricity costs rise, and power outages pose an even greater threat.
The outdoor section on some units may 'frost up' when outdoor temperatures are between 0°C and 5°C (between 32°F and 41°F) and there is sufficient moisture in the air which restricts air flow across the outdoor coil. These units employ a defrost cycle where the system switches to "A/C" mode to move heat from the home to the condenser to melt the ice. This requires the supplementary heater (resistance electric or gas) in the indoor section to activate, to temper the cold air being distributed. The defrost cycle reduces the efficiency of the heat pump significantly, although the newer (demand) systems are more intelligent and need to defrost less. As temperatures drop below freezing the tendency for frosting of the outdoor section decreases due to reduced humidity in the air.
It is difficult to retrofit conventional heating systems that use radiators/radiant panels, hot water baseboard heaters, or even smaller diameter ducting, with ASHP-sourced heat. The lower heat pump output temperatures would mean radiators would have to be increased in size or a low temperature underfloor heating
Underfloor heating
Underfloor heating and cooling is a form of central heating and cooling which achieves indoor climate control for thermal comfort using conduction, radiation and convection...
system be installed instead.
Technology
Heating and cooling is accomplished by moving a refrigerantRefrigerant
A refrigerant is a substance used in a heat cycle usually including, for enhanced efficiency, a reversible phase change from a liquid to a gas. Traditionally, fluorocarbons, especially chlorofluorocarbons, were used as refrigerants, but they are being phased out because of their ozone depletion...
through the heat pump's indoor and outdoor coil
Coil
A coil is a series of loops. A coiled coil is a structure in which the coil itself is in turn also looping.-Electromagnetic coils:An electromagnetic coil is formed when a conductor is wound around a core or form to create an inductor or electromagnet...
s. Like in a refrigerator, a compressor
Gas compressor
A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume.Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas...
, condenser
Condenser (heat transfer)
In systems involving heat transfer, a condenser is a device or unit used to condense a substance from its gaseous to its liquid state, typically by cooling it. In so doing, the latent heat is given up by the substance, and will transfer to the condenser coolant...
, expansion valve
Expansion valve
An expansion valve is a slide valve used in a steam engine to control the cut-off. It rides on the back of an adapted main slide valve and is driven by an additional eccentric that has more advance than the main eccentric. The cut-off is adjusted in one of two ways...
and evaporator
Evaporator
An evaporator is a device used to turn the liquid form of some chemical into its gaseous form. For example, an evaporator is used in an air conditioning system to allow the compressed cooling chemical to evaporate from liquid to gas, absorbing heat in the process.-Uses:As stated above, an...
are used to change states of the refrigerant between a cold liquid
Liquid
Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...
and a hot gas
Gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...
.
When the liquid refrigerant at a low temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
passes through the outdoor evaporator heat exchanger coils, ambient heat is used to cause the liquid to boil. This boiling or change of state process amasses energy as latent heat
Latent heat
Latent heat is the heat released or absorbed by a chemical substance or a thermodynamic system during a process that occurs without a change in temperature. A typical example is a change of state of matter, meaning a phase transition such as the melting of ice or the boiling of water. The term was...
. The vapor
Vapor
A vapor or vapour is a substance in the gas phase at a temperature lower than its critical point....
is then drawn into a compressor which further boosts the temperature of the vapor.
Passing into the building, the vapor enters the condenser heat exchanger coils where it transfers heat to indoor air, which is drawn across the coils by a fan
Fan (mechanical)
A mechanical fan is a machine used to create flow within a fluid, typically a gas such as air.A fan consists of a rotating arrangement of vanes or blades which act on the air. Usually, it is contained within some form of housing or case. This may direct the airflow or increase safety by preventing...
. As the vapor cools, it condenses back into a liquid, and releases its latent heat
Latent heat
Latent heat is the heat released or absorbed by a chemical substance or a thermodynamic system during a process that occurs without a change in temperature. A typical example is a change of state of matter, meaning a phase transition such as the melting of ice or the boiling of water. The term was...
to the air passing over the condenser unit.
Exiting the condenser, the cold liquid refrigerant is under high pressure. The refrigerant passes through an expansion valve
Valve
A valve is a device that regulates, directs or controls the flow of a fluid by opening, closing, or partially obstructing various passageways. Valves are technically pipe fittings, but are usually discussed as a separate category...
which reduces the pressure, draws in heat and allows the refrigerant to re-enter the evaporator to begin a new cycle.
Most heat pumps can also operate in a cooling mode where the cold refrigerant is moved through the indoor coils to cool the room air.
Efficiency Ratings
The 'Efficiency' of air source heat pumps is measured by the Coefficient of performance (COP). A COP of 3 means the heat pump produces 3 units of heat energy for every 1 unit of electricity it consumes. Within temperature ranges of -3°C to 10°C, the COP for many machines is fairly stable at 3-3.5.In mild weather, the COP of an air source heat pump can be up to 4. However, on a very cold winter day, it takes more work to move the same amount of heat indoors than on a mild day. The heat pump's performance is limited by the Carnot cycle
Carnot cycle
The Carnot cycle is a theoretical thermodynamic cycle proposed by Nicolas Léonard Sadi Carnot in 1824 and expanded by Benoit Paul Émile Clapeyron in the 1830s and 40s. It can be shown that it is the most efficient cycle for converting a given amount of thermal energy into work, or conversely,...
and will approach 1.0 as the outdoor-to-indoor temperature difference increases, which for most air source heat pumps happens as outdoor temperatures approach −18 °C / 0 °F. Heat pump construction that enables carbon dioxide as a refrigerant may have a COP of greater than 2 even down to -20°C, pushing the break-even figure downward to -30 °C (-22 °F). A ground source heat pump has comparatively less of a change in COP as outdoor temperatures change, because the ground from which they extract heat has a more constant temperature than outdoor air.
The specific design of a heat pump has a considerable impact on its efficiency. Many air source heat pumps are designed primarily as air conditioning units, mainly for use in summer temperatures. Designing a heat pump specifically for the purpose of heat exchange can attain greater COP ratings and an extended life cycle. The principal changes are in the scale and type of compressor and evaporator.
Seasonally adjusted heating and cooling efficiencies are given by the heating seasonal performance factor (HSPF) and seasonal energy efficiency ratio
Seasonal energy efficiency ratio
The efficiency of air conditioners is often rated by the Seasonal Energy Efficiency Ratio which is defined by the Air Conditioning, Heating and Refrigeration Institute in its standard ARI 210/240, Performance Rating of Unitary Air-Conditioning and Air-Source Heat Pump Equipment.The SEER rating of...
(SEER) respectively.
In units charged with HFC refrigerants, the COP rating is reduced when heat pumps are used to heat domestic water to over 60°C or to heat conventional central heating
Central heating
A central heating system provides warmth to the whole interior of a building from one point to multiple rooms. When combined with other systems in order to control the building climate, the whole system may be a HVAC system.Central heating differs from local heating in that the heat generation...
systems that use radiators to distribute heat (instead of an underfloor heating array).
Risks and Precautions
- Many air source heat pumps can heat water up to 70°C without difficulty when the air temperature is 5°C or greater. However, to minimize the risk from LegionellosisLegionellosisLegionellosis is a potentially fatal infectious disease caused by gram negative, aerobic bacteria belonging to the genus Legionella. Over 90% of legionellosis cases are caused by Legionella pneumophila, a ubiquitous aquatic organism that thrives in temperatures between , with an optimum temperature...
it is advised that hot water is heated to above 60°C, which may not be possible for some units if outside temperature dips.
- Most air source heat pumps lose their efficiency as the external temperatures fall below 5 degrees Celsius (about 41 degrees Fahrenheit). In colder climates, the system needs to be installed with an auxiliary source of heat to supplement the heat pump in the event of extremely cold temperatures or when it is simply too cold for the heat pump to work at all.
- An Auxiliary Heat/Emergency Heat system, for example a traditional furnace, is also important if the heat pump is malfunctioning or being repaired. In Northern climates, split-system heat pumps matched with gas or oil furnaces will work even in extremely cold temperatures.
Controversy
Units charged with HFC refrigerants are often marketed as low energy or a sustainable technology, however the HFCs have the potential to contribute to global warming, as measured in global warming potentialGlobal warming potential
Global-warming potential is a relative measure of how much heat a greenhouse gas traps in the atmosphere. It compares the amount of heat trapped by a certain mass of the gas in question to the amount of heat trapped by a similar mass of carbon dioxide. A GWP is calculated over a specific time...
(GWP) and ozone depletion potential
Ozone depletion potential
The ozone depletion potential of a chemical compound is the relative amount of degradation to the ozone layer it can cause, with trichlorofluoromethane being fixed at an ODP of 1.0. Chlorodifluoromethane , for example, has an ODP of 0.055...
(ODP). Recent government mandates have seen the phase-out of R-22 refrigerant and its replacement with more environmentally sound R410a refrigerant.