A heat pump is a machine or device that moves heat from one location to another via work. Most often heat pump technology is applied to moving heat from a low temperature heat source to a higher temperature heat sink.
The last 20 years have seen massive worldwide investment in heat pump technology. The countries shown in green on the map below contain either manufacturers or suppliers of heat pumps.
In some regions of the world, heat pumps already play an important role in energy systems. This is generally as a result of government incentives and advertising. The heat pump market in the UK is drastically different to that of the world at large as can be seen in the 2 pie charts below. The UK market is dominated by the non-domestic sector whereas the global picture is the reverse.
As a result of society's increasing concern for ecological and environmental issues, the demand for more efficient ways to utilize heat and energy is rising. The heat pump industry uses technological advances such as year-round space heating to displace heat energy to a more useful location and purpose. This concept is accomplished by providing localized or redirected heat, while exchanging cool air with heated air.
The principles of heat pumps are actually the reverse of the technological and thermodynamic principles of an air conditioner unit. The majority of heat pumps give the added benefit of providing both heating in the winter and cooling in the summer. This can be accomplished simply by reversing the flow of the working fluid circulating through the coils. The heat pump is an entire thermodynamic system whereby a liquid and/or gas medium is pumped through an assembly where it changes phases as a result of altering pressure. Although relatively costly to setup, the heat pump system provides a more economical and efficient way to control temperatures and reuse existing heat energy.
· Heat Pumps and Energy Saving
This section gives a brief introduction to heat pumps. Based on six basic facts about heat supply the value of heat pumps is discussed. It is argued that heat pumps are very energy efficient, and therefore environmentally benign.
· An Efficient Technology
Heat pumps offer the most energy-efficient way to provide heating and cooling in many applications, as they can use renewable heat sources in our surroundings. Even at temperatures we consider to be cold, air, ground and water contain useful heat that's continuously replenished by the sun. By applying a little more energy, a heat pump can raise the temperature of this heat energy to the level needed. Similarly, heat pumps can also use waste heat sources, such as from industrial processes, cooling equipment or ventilation air extracted from buildings. A typical electrical heat pump will just need 100 kWh of power to turn 200 kWh of freely available environmental or waste heat into 300 kWh of useful heat. For more information about the technical aspects of these energy savings are achieved, see the section Heat pump technology.
· Six basic facts about heating
Through this unique ability, heat pumps can radically improve the energy efficiency and environmental value of any heating system that is driven by primary energy resources such as fuel or power. The following six facts should be considered when any heat supply system is designed:
- Direct combustion to generate heat is never the most efficient use of fuel;
- Heat pumps are more efficient because they use renewable energy in the form of low-temperature heat;
- If the fuel used by conventional boilers were redirected to supply power for electric heat pumps, about 35-50% less fuel would be needed, resulting in 35-50% less emissions;
- Around 50% savings are made when electric heat pumps are driven by CHP (combined heat and power or cogeneration) systems;
- Whether fossil fuels, nuclear energy, or renewable power is used to generate electricity, electric heat pumps make far better use of these resources than do resistance heaters;
- The fuel consumption, and consequently the emissions rate, of an absorption or gas-engine heat pump is about 35-50% less than that of a conventional boiler.
· Global Potential
Of the global CO2 emissions that amounted to 22 billion tonnes in 1997, heating in building causes 30% and industrial activities cause 35%. The potential CO2 emissions reduction with heat pumps is calculated as follows:
- 6.6 billion tonnes CO2 come from heating buildings (30% of total emissions).
- billion tonnes can be saved by residential and commercial heat pumps, assuming that they can provide 30% of the heating for buildings, with an emission reduction of 50%.
- A minimum of 0.2 billion tonnes can be saved by industrial heat pumps (estimation based on a study by Annex 21).
- The total CO2 reduction potential of 1.2 billion tonnes is about 6% of the global emissions! This is one of the largest that a single technology can offer, and this technology is already available in the marketplace. And with higher efficiencies in power plants as well as for the heat pump itself, the future global emissions saving potential is even 16%.
In some regions of the world, heat pumps already play an important role in energy systems. But if this technology is to achieve more widespread use, a decisive effort is needed to stimulate heat pump markets and to further optimize the technology. It is encouraging that a number of governments and utilities are strongly supporting heat pumps. In all cases it is important to ensure that both heat pump applications and policies are based on a careful assessment of the facts, drawn from as wide an experience base as possible. The IEA Heat Pump Centre sees it as one of its key roles to ensure that these facts are available to a wide audience, including policy makers, utilities, market parties and heat pump users.
· Raw Materials
The manufacturing of heat pumps involves the use of large iron castings with stainless steel components and aluminum tubing. The castings, used in the pump and motor, will often have small amounts of nickel, molybdenum, and magnesium to improve the mechanical and corrosion-resisting characteristics of the casting. In smaller heat pumps, some components require the use of alloy steel to reduce weight. Depending on what type of working fluid is used (ammonia, water, or chlorofluorocarbons), the piping in the heat pump system may require corrosion resistant stainless steel or aluminum. In systems where consistency of thermodynamic properties is more critical, copper tubing may improve efficiency. Housing most of the components of the heat pump, the encasements are fabricated out of mild carbon sheet steel. The rest of the piping, fittings, valves, and couplings are stainless steel.
All heat pumps require a working fluid to transfer excess energy from one heat source to another. Traditionally, chlorofluorocarbons (CFCs) have been used as working fluids because of their superior thermodynamic properties. Because of the harmful effects CFCs are now known to have on the environment, they have been gradually phased out of production. Instead, water, hydrocarbons, and ammonia are frequently utilized in heat pump systems despite their lack of efficiency in some heat pump designs.
Heat pumps all have the same basic components. These components consist of a pump, a condenser, an evaporator, and an expansion valve. Despite the relative similarities of these components, heat pump designs vary greatly depending on the specific application of the pump. The two major designs, vapor compression and absorption, utilize different thermodynamic principles, yet both include similar components and provide similar system efficiencies.
Heat pumps demonstrate remarkable versatility in providing both air conditioning and heating in the same system by simply reversing the direction of flow of the working fluid. In this regard, heat pumps eliminate the need for dual systems in order to maintain a desired temperature. However, this will be costly as it requires a system that is able to pump in both directions. In extremely adverse climates, heat pumps lose some of their effectiveness and may require an additional heat source. This supplemental heat can come from geothermal heated water or electric heaters.
The typical heat pump operation uses the working fluid to receive heat from a source positioned close to the evaporator. At the evaporator, the fluid vaporizes into a low pressure vapor. Upon entering the pump, the vapor is compressed to high pressure and enters a condenser which returns the vapor to a liquid and ultimately gives off its stored heat to the desired source. An expansion valve then allows the system to return to its low pressure liquid state, and the cycle begins again.
· The Manufacturing
The pump is usually procured as a finished unit and installed into the system by integrating it with coupling and piping components. Designed for the specific size and fluid requirements of the system, the pump may be shipped, depending upon its size, directly to the installation site. This usually occurs with large commercial heat pumps supplying heat and/or refrigeration to office buildings. Smaller residential models may have the pump installed into an assembly that includes the condenser, evaporator, and various piping. These units, encased in a sheet metal box, will be comprised of various subassemblies for the condenser and evaporator in order to bolt every component to the box or to one another. Some of the brackets used will form the base of the unit where the pump will be bolted down to a metal pan and connected to an AC motor.
Assembled from several different sheets of metal, encasement units are sheared to size in a shear press. After they are cut to the proper dimensions, small assembly holes are punched in the metal using a Computer Numerically Controlled (CNC) punch press. These punch presses have either a moveable table to move the sheet metal or a moveable die which is able to punch holes in different spots of the metal. Punch presses are often directed where to punch by a computer-aided design (CAD) program. Different shaped punching tools are stored within the machine, providing it with the ability to punch all of the necessary holes by simply changing the computer program.
After punching, the sheet will move to a Numerically Controlled (NC) press brake, where it will be bent in different shapes and configurations. The press brake bends the metal into many different shapes by using dies or tooling. Unlike the CNC punch press, the press brake will require a manual change in tooling to perform a different bend. The sheet is then ready to be welded, riveted, or bolted to the other sheets and brackets. Once assembled, these sheets provide most of the stability of stand-alone units.
· Condenser and evaporator
The condenser and evaporator are made of many small, thin copper or aluminum tubes, which are bent around curved dies by tube bending machines. NC tube bending machines will be programmed to provide the same exact bend on each of the tubes, allowing them to be stacked one on top of the other. These tubes will then be attached to plates or fins through which the tubes will pass and be joined through tube expansion or joint welding. This creates a tightly sealed system. The tube and plate assembly will act as a heat exchanger by allowing the working fluid to pass through the system inside the tubes, while giving off the heat in the condenser to another fluid medium passing between the plates and acquiring the heat given off through the tubes.
In order to provide strength or connectivity to the components, small brackets are punched out of mild carbon steel. The brackets are usually punched out of steel coil that is continuously fed first through a decoiler. Once it is decoiled, it is sheared, bent, and formed in one continuous process. This is done with a progressive die configuration, where the bracket remains attached to the coil as it moves from station to station. Each station adds something to the bracket, either a hole or notch, and sends it to the next station, until finally it is sheared from the coil. This process may be outsourced to vendors who specialize in progressive die or transfer press operations and can provide better cost control.
More tubing is fabricated and bent to provide the rest of the piping needed to connect the pump with the condenser and evaporator. Various fittings and connection components are utilized. The expansion valve, which is contained within some of the piping lines, is another component purchased as a whole unit. The expansion valve is a designed fitting that provides for the expansion of the working fluid and connection of smaller diameter tubing with larger diameter tubing. In small residential units, the valve is contained within the main box, while in larger commercial units, it may be installed on site in the piping system.
Components, subassemblies, brackets, and/or plates are painted or powder coated for corrosion resistance. Before painting, however, some parts are treated with a special solvent to remove any grease or oil left from the manufacturing operations. This is usually done by submersing the parts in large tanks filled with solvent and then drying them in a special oven. Some parts, which are specially coated with zinc, nickel, or chrome, will be fed through an acid bath before being dipped into tanks of coating solution. Once cleaned, the parts are manually loaded onto trays or hung on specially designed racks and fed into a paint booth. The paint is applied with a pressurized paint dispenser that will spray paint into each crevice.
After passing vigorous inspections, the heat pump is sent to packaging, where the system will be boxed and shipped to the installation site.
Generally, heat pumps will be installed at the construction site. The compressor and evaporator will be constructed of massive 3 in (7.5 cm) diameter tubing and have larger chambers, where the working fluid will change phases. The pump itself will be bolted to a concrete pad and connected with a large DC motor or natural gas generator. The fittings and valves will be shipped and installed into the piping system, while supported by brackets and braces anchored to existing walls. These installations exhibit significant engineering challenges and often require cooperation between the contractor and heat pump manufacturer.
· Quality Control
Each component that is procured from an outside supplier will usually be inspected for dimensional compliance before being assembled. Other components will be checked during their fabrication to ensure quality. The final assembly will then be tested by filling it with the appropriate working fluid and connecting the system to a power source to turn the pump. By measuring, with transducers or switches, the temperature and pressure levels of the fluid in different stages, the final system can be checked against predetermined criteria.
Until the 1990s, the common refrigerant was often chlorofluorocarbons such as R-12 (dichlorodifluoromethane), one in a class of several refrigerants using the brand name Freon, a trademark of DuPont. Its manufacture was discontinued in 1995 because of the damage that CFC's cause to the ozone layer if released into the atmosphere. One widely-adopted replacement refrigerant is the hydro fluorocarbon (HFC) known as R-134a (1, 1, 1, 2-tetrafluoroethane). Interestingly, R-134a is not as efficient as the R-12 it replaced (in automotive applications) and therefore, more energy is required to operate systems utilizing R-134a than those using R-12. Other substances such as liquid ammonia, or occasionally the less corrosive but flammable propane or butane, can also be used. Since 2001, carbon dioxide, R-744, has increasingly been used, utilizing the transcritical cycle. In residential and commercial applications, the hydro chlorofluorocarbon (HCFC) R-22 is still widely used, however, HFC R-410a is considered to be more environmentally friendly, and thus is increasingly being used.
The term 'heat pump' is a slight misnomer; heat is not 'pumped', but instead is 'moved' by these devices. According to the second law of thermodynamics heat cannot spontaneously flow from a colder location to a hotter area; work is required to achieve this. Heat pumps differ in how they apply this work to move heat, but they can essentially be thought of as heat engines operating in reverse. A heat engine allows energy to flow from a hot 'source' to a cold heat 'sink', extracting a fraction of it as work in the process. Conversely, a heat pump requires work to move thermal energy from a cold source to a warmer heat sink. Since the heat pump uses a certain amount of work to move the heat, the amount of energy deposited at the hot side is greater than the energy taken from the cold side by an amount equal to the work required. Conversely, for a heat engine, the amount of energy taken from the hot side is greater than the amount of energy deposited in the cold heat sink since some of the heat has been converted to work.
One common type of heat pump works by exploiting the physical properties of an evaporating and condensing fluid known as a refrigerant.
A simple heat pump's vapor-compression refrigeration cycle:
1) Condenser,2) Expansion Valve,3) Evaporator,4) Compressor.
The working fluid, in its gaseous state, is pressurized and circulated through the system by a compressor. On the discharge side of the compressor, the now hot and highly pressurized gas is cooled in a heat exchanger called a condenser until it condenses into a high pressure, moderate temperature liquid. The condensed refrigerant then passes through a pressure-lowering device like an expansion valve, capillary tube, or possibly a work-extracting device such as a turbine. This device then passes the low pressure, barely liquid (saturated vapor) refrigerant to another heat exchanger, the evaporator where the refrigerant evaporates into a gas via heat absorption. The refrigerant then returns to the compressor and the cycle is repeated.
In such a system it is essential that the refrigerant reaches a sufficiently high temperature when compressed, since the second law of thermodynamics prevents heat from flowing from a cold fluid to a hot heat sink. Similarly, the fluid must reach a sufficiently low temperature when allowed to expand, or heat cannot flow from the cold region into the fluid. In particular, the pressure difference must be great enough for the fluid to condense at the hot side and still evaporate in the lower pressure region at the cold side. The greater the temperature difference, the greater the required pressure difference and consequently more energy is needed to compress the fluid. Thus as with all heat pumps, the energy efficiency (amount of heat moved per unit of input work required) decreases with increasing temperature difference. Thus a ground-source heat pump, which has a very small temperature differential, is relatively efficient. (Figures of 75% and above are quoted.)
Due to the variations required in temperatures and pressures, many different refrigerants are available. Refrigerators, air conditioners, and some heating systems are common applications that use this technology.
In HVAC applications, a heat pump normally refers to a vapor-compression refrigeration device that includes a reversing valve and optimized heat exchangers so that the direction of heat flow may be reversed. The reversing valve switches the direction of refrigerant through the cycle and therefore the heat pump may deliver either heating or cooling to a building. Because the two heat exchangers, the condenser and evaporator, must swap functions, they are optimized to perform adequately in both modes. As such, the efficiency of a reversible heat pump is typically slightly less than two separately-optimized machines.
In plumbing applications, a heat pump is sometimes used to heat or preheat water for swimming pools or domestic water heaters.
In somewhat rare applications, both the heat extraction and addition capabilities of a single heat pump can be useful, and typically results in very effective use of the input energy. For example, when an air cooling need can be matched to a water heating load, a single heat pump can serve two useful purposes. Unfortunately, these situations are rare because the demand profiles for heating and cooling are often significantly different.
Heat pumps for heating and cooling buildings can be divided into four main categories depending on their operational function:
· Heating-only heat pumps, providing space heating and/or water heating.
· Heating and cooling heat pumps, providing both space heating and cooling -The most common type is the reversible air-to-air heat pump, which either operates in heating or cooling mode. Large heat pumps in commercial/institutional buildings use water loops (hydronic) for heat and cold distribution, so they can provide heating and cooling simultaneously.
· Integrated heat pump systems, providing space heating, cooling, water heating and sometimes exhaust air heat recovery - Water heating can be by desuperheating only, or by desuperheating and condenser heating. The latter permits water heating when no space heating or cooling is required.
· Heat pump water heaters, fully dedicated to water heating - They often use air from the immediate surroundings as heat source, but can also be exhaust-air heat pumps, or desuperheaters on air-to-air and water-to-air heat pumps. Heat pumps can be both monovalent and bivalent, where monovalent heat pumps meet the annual heating and cooling demand alone, while bivalent heat pumps are sized for 20-60% of the maximum heat load and meet around 50-95% of the annual heating demand (in a European residence). The peak load is met by an auxiliary heating system, often a gas or oil boiler. In larger buildings the heat pump may be used in tandem with a cogeneration system (CHP).
In residential applications room heat pumps can be reversible air-to-air heat pumps (ductless packaged or split type units). The heat pump can also be integrated in a forced-air duct system or a hydronic heat distribution system with floor heating or radiators (central system).
In commercial/institutional buildings the heat pump system can be a central installation connected to an air duct or hydronic system, or a multi-zone system where multiple heat pump units are placed in different zones of the building to provide individual space conditioning. Efficient in large buildings is the water-loop heat pump system, which involves a closed water loop with multiple heat pumps linked to the loop to provide heating and cooling, with a cooling tower and auxiliary heat source as backup.
The different heat sources that can be used for heat pumps in residential and commercial buildings are described in the section Heat sources. The next paragraph describes the types of heat and cold distribution systems that can be used in buildings.
· Performance and Efficiency
Commonly used performance and efficient terminology in connection with cooling and heating systems:
· SEER - Seasonal Energy Efficiency Ratio
The term SEER is used to define the average annual cooling efficiency of an air-conditioning or heat pump system. The term SEER is similar to the term EER but is related to a typical (hypothetical) season rather than for a single rated condition. The SEER is a weighted average of EERs over a range of rated outside air conditions following a specific standard test method. The term is generally applied to systems less than 60,000 Btu/h. The units of SEER are Btu/W h. It is important to note that this efficiency term typically includes the energy requirements of auxiliary systems such as the indoor and outdoor fans. For purposes of comparison, the higher the SEER the more efficient the system. Although SEERs and EERs cannot be directly compared, the SEERs usually range from 0.5 to 1.0 higher than corresponding EERs.
· EER - Energy Efficiency Ratio
Room air conditioners in general range from 5,000 Btu per hour to 15,000 Btu per hour. Select room air conditioners with EER of at least 9.0 for mild climates. In hot climates, select air conditioners with EER over 10.
The Energy Efficiency Ratio - EER - is a term generally used to define the cooling efficiency of unitary air-conditioning and heat pump systems. The efficiency is determined at a single rated condition specified by the appropriate equipment standard and is defined as the ratio of net cooling capacity - or heat removed in Btu/h - to the total input rate of electric energy applied - in watt hour. The units of EER are Btu/w.h.
EER = Ec / Pa
EER = energy efficient ratio (Btu/W.h)
Ec = net cooling capacity (Btu/h)
Pa = applied energy (Watts)
This efficiency term typically includes the energy requirement of auxiliary systems such as the indoor and outdoor fans and the higher the EER the more efficient is the system.
· IPLV - Integrated Part-Load Value
The term IPLV is used to signify the cooling efficiency related to a typical (hypothetical) season rather than a single rated condition. The IPLV is calculated by determining the weighted average efficiency at part-load capacities specified by an accepted standard. It is also important to note that IPLVs are typically calculated using the same condensing temperature for each part-load condition and IPLVs do not include cycling or load/unload losses. The units of IPLV are not consistent in the literature; therefore, it is important to confirm which units are implied when the term IPLV is used. ASHRAE Standard 90.1 (using ARI reference standards) uses the term IPLV to report seasonal cooling efficiencies for both seasonal COPs (unitless) and seasonal EERs (Btu/W h), depending on the equipment capacity category; and most chillers manufacturers report seasonal efficiencies for large chillers as IPLV using units of kW/ton. Depending on how a cooling system loads and unloads (or cycles), the IPLV can be between 5 and 50% higher than the EER at the standard rated condition.
· ηc or Ec - Combustion Efficiency
For fuel-fired systems, this efficiency term is defined as the ratio of the fuel energy input minus the flue gas losses (dry flue gas, incomplete combustion and moisture formed by combustion of hydrogen) to the fuel energy input. In the U.S., fuel-fired efficiencies are reported based on the higher heating value of the fuel. Other countries report fuel-fired efficiencies based on the lower heating value of the fuel. The combustion efficiency is calculated by determining the fuel gas losses as a percent of fuel burned. [Ec = 1 - flue gas losses]
· Thermal Efficiency (ηt or Et)
This efficiency term is generally defined as the ratio of the heat absorbed by the water (or the water and steam) to the heat value of the energy consumed. The combustion efficiency of a fuel-fired system will be higher than its thermal efficiency. See ASME Power Test Code 4.1 for more details on determining the thermal efficiency of boilers and other fuel-fired systems. In the U.S., fuel-fired efficiencies are typically reported based on the higher heating value of the fuel. Other countries typically report fuel-fired efficiencies based on the fuel′s lower heating value. The difference between a fuel′s higher heating value and its lower heating value is the latent energy contained in the water vapor (in the exhaust gas) which results when hydrogen (from the fuel) is burned. The efficiency of a system based on a fuel′s lower heating value can be 10 to 15% higher than its efficiency based on a fuel′s higher heating value.
· HSPF - Heating Seasonal Performance Factor
The term HSPF is similar to the term SEER, except it is used to signify the seasonal heating efficiency of heat pumps. The HSPF is a weighted average efficiency over a range of outside air conditions following a specific standard test method. The term is generally applied to heat pump systems less than 60,000 Btu/h (rated cooling capacity.) The units of HSPF are Btu/w-h. It is important to note that this efficiency term typically includes the energy requirement of auxiliary systems such as the indoor and outdoor fans. For purposes of comparison, the higher the HSPF the more efficient the system.
· Cooling Load in - kW/ton
The term kW/ton is common used for large commercial and industrial air-conditioning, heat pump and refrigeration systems.
The term is defined as the ratio of the rate of energy consumption in kW to the rate of heat removal in tons at the rated condition. The lower, the kW/ton, the more efficient the system.
kW/ton = Pc / Er
Pc = energy consumption (kW)
Er = heat removed (ton)
· COP - Coefficient of Performance
The Coefficient of Performance - COP - is the basic unit less parameter used to report the efficiency of refrigerant based systems.
The Coefficient of Performance - COP - is the ratio between useful energy acquired and energy applied and can be expressed as:
COP = Eu / Ea
COP = coefficient of performance
Eu = useful energy acquired (btu in imperial units)
Ea = energy applied (btu in imperial units)
COP can be used to define both cooling efficiency and heating efficiency as for a heat pump.
• For cooling, COP is defined as the ratio of the rate of heat removal to the rate of energy input to the compressor.
• For heating, COP is defined as the ratio of rate of heat delivered to the rate of energy input to the compressor.
Coefficient of Performance is the ratio of cooling or heating to energy consumption. A refrigerator with a COP of 2 moves 2 watts of heat for every watt of electricity consumed. An air conditioner with a COP of 4 moves 4 watts of heat for every watt consumed.
COP may also be used for domestic heating. An electric heater has a COP of 1. Each watt of power consumed produces 1 watt of heat. Conventional heat pumps have COPs of 2 - 5, delivering 2 to 5 times the energy they consume.
COP can be used to define the efficiency at a single standard or non-standard rated condition or a weighted average seasonal condition. The term may or may not include the energy consumption of auxiliary systems such as indoor or outdoor fans, chilled water pumps, or cooling tower systems. For purposes of comparison, the higher the COP the more efficient the system.
COP can be treated as an efficiency where COP of 2.00 = 200% efficient For unitary heat pumps, ratings at two standard outdoor temperatures of 47oF and 17oF (8.3oC and -8.3oC) are typically used.
· Heat Pumps in Commercial Buildings
About 96% of all heat pumps sold in the UK are for non-domestic buildings - over 60,000 units in 1996 alone. Sports centres, particularly those having swimming pools, are ideal candidates, where the heat pump can provide both heating and dehumidification. Retail outlets and office buildings where there is a need for simultaneous heating in one area and cooling in another can benefit from substantial energy cost savings.
· Heat Pumps In Industry
Industrial heat pumps are used to recover or make best use of heat in manufacturing processes or in public utilities such as energy generation & distribution. There are relatively few heat pumps currently installed in industry world wide in comparison to those installed in the residential or commercial markets. However, as environmental regulations become stricter, industrial heat pumps can become an important technology to reduce emissions, improve efficiency, and limit the use of ground water for cooling.
Industrial heat pumps are mainly used for:
1) space heating2) heating and cooling of process streams3) water heating for washing, sanitation and cleaning4) steam production5) drying/dehumidification6) evaporation7) distillation
In Japan, Sweden and the Netherlands, multi-MW heat transformers operating on the absorption cycle are used for waste heat recovery in petrochemical and steel works.
Heat pumps are a feature of many homes in, for example, Switzerland, Norway and the Netherlands, but not many systems have been installed in the UK. The UK market is strongly influenced by first cost for heating systems, and gas heating is fairly cheap, widely available and fairly clean. Apart from some purpose-built demonstration houses, domestic heat pumps in the UK tend to be confined to rural areas without gas supplies. In these areas oil is the main alternative which is currently very economical.
The largest domestic market for heat pumps is in regions with a warmer climate such as Florida, USA. These regions make use of the dual functionality possible with heat pumps, providing both heating and/or air conditioning.
Air is the most common distribution medium in the mature heat pump markets of Japan and the United States. The air is either passed directly into a room by the space-conditioning unit, or distributed through a forced-air ducted system. Water distribution systems (hydronic systems) are predominantly used in Europe, Canada and the north eastern part of the United States. Conventional radiator systems require high distribution temperatures, typically 60-90°C. Today's low temperature radiators and convectors are designed for a maximum operating temperature of 45-55°C, while 30-45°C is typical for floor heating systems Because a heat pump operates most effectively when the temperature difference between the heat source and heat sink (distribution system) is small, the heat distribution temperature for space heating heat pumps should be kept as low as possible during the heating season.