The heat pump dates back to the early twentieth century, with the invention of the refrigerator. At the basis of the operation of the heat pump, a series of phenomena and laws of physics compete: The second principle of thermodynamics: „Heat can never go by itself from a lower temperature body to a higher temperature one ”(Clausius' statement) as well as the discovery of the physicist Watt: a compressed gas emits heat and vice versa, a relaxed one - absorbs heat.

Why are we interested in the heat pump?

In constant search of the best power generation solutions we analyze in this article heat pump in all its variants.

All the efforts of the specialists to find non-polluting thermal resources or at least that pollute to a small extent and the transition to their mass use, for heating the houses have drawn attention to the implementation of heat transfer and the appearance of the increasingly famous heat pump. The heat pump is, due to its efficiency as well as the use of less polluting resources than other heating or cooling systems, one of the preferred solutions as an alternative installation system for projects houses or buildings with almost zero consumption and depending on the situation is used in the active house.

What is a heat pump ?

In a nutshell we can say that the heat pump attracts and transfers heat from one of the sources for which it is designed (whether we are talking about earth, water or air) and using a compressor, exactly on the principle of the refrigerator, raises the temperature of a refrigerant with a very low boiling point (somewhere at about -2 ° C), transferring the temperature of the heating medium circulating through the heating system regardless of the system to the static bodies used (radiators, floor, ventilation, etc.). The heat pump is characterized very simply by the mode of operation very close to that of refrigerators with compressor: a compressed gas releases heat and in the opposite direction, a relaxed one, absorbs heat.

How the heat pump works

The heat pump needs a permanent source of electricity. It is mandatory to connect to a permanent source, if possible to the public network, if not preferable, due to low consumption to a system of photovoltaic solar panels or, why not to wind turbines, alternative systems, etc. The electric source will supply the compressor that compresses the coolant, to reach the temperature used for heating and domestic hot water preparation. When the coolant vapors reach the condenser in the heat pump, it will be surrounded by heat. The temperature of the thermal agent is lower than the temperature of the coolant, in the vapor state, so the vapors will cool and condense. At this time, the heat is taken up by the evaporator including the heat generated during the compression process, a considerable amount of which is released into the condenser and transferred to the thermal agent in the form of useful energy for heating. The transfer agent is then returned to the vaporizer, often using an expansion valve. The coolant thus changes from the high pressure of the condenser to the low pressure of the evaporator. With the entry into the evaporator, here are reached again values ​​found at the beginning of the process and the circuit closes.

Refrigerators as well as air conditioners are classic examples of heat pumps that already exist in households and work only in cooling mode. A refrigerator is actually just a well-insulated box from a thermal point of view and with a heat pump system connected to it. The evaporator coil is located inside and is usually found in the freezer compartment. Heat is absorbed from the freezer and sent outside, usually behind or under the unit where the condenser coil is located. An air conditioner that transfers heat from the inside of a house to the outside works almost similarly. So we have had the heat pump system in our homes for some time, only it has not been used in both directions.

The heat pump can be used without the existence or support of other heat sources.

According to the operation criterion, we find regime heat pumps

  • Monovalent: the heat pump is the only heating source - it uses electricity from the public network or other sources to operate the compressor
  • Bivalent - parallel: can be mounted and used in parallel with the heat pump and another heat source; when the parallel heat source is electricity, the system is called bivalent - parallel monoenergetic;
  • Bivalent - alternative: the heat pump works alternately with another heat source;
  • Bivalent - partial - parallel: the heat pump works in parallel or alternatively with another heat source.

The heat pump has a completely reversible cycle and can provide control over the indoor environment for heating in winter and cooling or dehumidification if necessary in summer. Since the ground or the outside air always contains heat, why not use a heat pump to heat and cool the house? One of the principles used in the heat pump is that the air at a temperature of -18 ° C has in its composition about 85% of the energy it contains at 21 ° C.

What are the types of heat pumps

There are two main types of heat pump named according to the system used

  • Heat pump with compression that always works using mechanical energy through electrical energy
  • Absorption heat pump that can also run on heat as a source of energy (through electricity or fuel).

A number of sources have been used as heat sources for heating.

Air source heat pump (extracts heat from outside air)

  • air-to-air heat pump (transfers thermal energy from the air inside)
  • air-to-water heat pump (transfers heat to a water tank)

Geothermal heat pump (extracts heat from the ground or similar sources)

  • geothermal-air heat pump (transfers thermal energy to indoor air)
  • ground-to-air heat pump (the ground is a heat source)
  • rock-to-air heat pump (rock is a heat source)
  • water-to-air heat pump (water as heat source)

Geothermal-water heat pump (transfers heat to a water tank)

  • ground-to-water heat pump (the ground is the heat source)
  • rock-water heat pump (rock is the heat source)
  • water-to-water heat pump (body of water as heat source)

Heat pumps Ground-water

The antifreeze solution called glycol circulates through the circuit of pipes buried in the ground of the heat source and takes the heat of the earth through a horizontal or vertical heat exchanger. This heat is absorbed and transferred to the heating system of the transfer housing by means of a compressor and transported to the condenser. In the condenser the water of the heat transfer system incorporates the heat absorbed in the evaporator and is supplied with energy to the compressor.

Depending on the way the collectors are located, we distinguish several types of heat pumps with collectors:

- flat: they have the system of collectors in various shapes (U or spirals) buried at an average depth of 1-2m. It is a highly efficient economic system but cannot be buried in any soil and needs an area almost double the living area.

- depth: these components include wells mounted at a depth of 50-100m and has a very high efficiency. The installation costs are significant due to the drillings that have prices varying between 100-300 lei / meter. They are usable only if the soil allows it.

- with direct vaporization: they have a special capture circuit in addition to the flat ones and for this reason they are more efficient or can occupy smaller spaces.

Water-to-water heat pumps


Heat from pumped groundwater or surface water is used to transfer heat transported through the compressor to the condenser. Once there, the Sol-Apa heat pump system is used. These are the most efficient using the fall from geothermal sources with temperatures above 8 degrees Celsius but also the most difficult to use both due to the availability of the resource but also due to the legal regulations for its excavation. The water-to-water heat pump has a very important role in the industry or in the maximum exploitation of geothermal springs. Wastewater or geothermal water with temp. maximums of 28-30 º C can be successfully capitalized In the case of geothermal waters the source can be multiplied by the cascade use of several PDCs. Obviously the water quality will be taken into account, this impediment can be avoided by using suitable heat exchangers (anticorrosive).

The water-to-water heat pump can also be used by exploiting water from lakes, rivers, tunnel waters, dams (which have temperatures> 8 º C). The water-water system is also called the open loop capture system. The speed of water flow through the evaporator must not exceed 0.8m / s. This type of heat pump can achieve the highest COP of all the types we refer to. Such a water-water system can easily reach a COP = 5 and it can even surpass it if it is well made and correctly dimensioned. It can also provide impressive power reaching thousands of kW, on a single unit or by coupling several units of lower power. However, to date, at least in Europe, it is not the most common heat pump. The reasons are several: water quality must practically meet the quality of drinking water; the water extracted from the groundwater layers must be reinjected into the soil (the injection well must be located at least 15m downstream from the direction of water flow in the groundwater) for each kW of thermal installation a minimum volume of 160 liters / hour is required , ie 0.16 m8 / hour (at min XNUMX º C), the flow must be ensured at any time by the extraction well ;. In the EU there are very strict regulations on this type of drilling.

Air-Water heat pumps


The heat from the outside air supplied with a fan or the air from the ventilation system is transported by evaporator to the heat transfer medium and then by the compressor to the condenser. This heat transfer system consists of a liquid circuit, used for heating the home and / or for producing domestic hot water. These systems are inefficient, in their current version water-to-water heat pumps have the lowest efficiency ratio so the cost is lower. These pumps cannot operate monovalently at temperatures below 15 degrees Celsius.

Air-to-air heat pumps

The heat from the outside air supplied with a fan or the air from the ventilation system is delivered by evaporator to the heat transfer medium and transported by compressor to the condenser. The heat transfer system consists of a system of air ducts or specific devices used to heat the building. They are used in rare cases in heating systems, due to the low efficiency of the air heater (the systems known as the generic name of "air conditioning" are actually air-to-air heat pumps and their COP is below 3).

Heat pumps with Ultraflex heat exchangers

The Ultraflex system no longer has an intermediate exchanger as found in most heat pumps on the market, which is why the system has a very high COP compared to other heat pumps, much higher reliability and very long operating and warranty periods. and their purchase price is very attractive. However, it has an extremely wide power variation (such systems are currently not limited in terms of power and offer unparalleled performance).

  • the size of the objective to be heated (living and heated surface, the average height of the rooms). Available (unbuilt) space around the lens.
  • the geographical area in which the objective is located;
  • the possibilities of installing other heating systems (gas, liquid fuel, liquefied gas, etc.);
  • the possibilities on the spot of capitalization of the “cold source” (water, air, soil); the existence of other heating sources (to establish the operating mode of the heat pump)
  • power supply conditions (single- or three-phase network, maximum electrical power allowed on the electrical connection).

How much does the heat pump cost?

Undoubtedly, the cost of using a heating system with a heat pump is far significantly lower than that of a conventional heating system. The difference comes from a consumption of 1kw electric power to the heat pump is transformed or can produce 3-5 kw of thermal power. A very good ratio compared to other classic solutions. The energy performance observed at the heat pump is given by the COP and is defined as the ratio between the amount of heat removed from the condenser and the electricity consumed by the compressor. From the point of view of environmental protection, compared to the heat pump but also considering the comfort in use, we can say that the residues left from heating with solid fuels are not negligible and neither the prospect of their constant and predicted increase in natural gas prices. liquids or electricity.

In various programs to encourage responsible consumption such as "Green House", the owners who equipped their homes with a system based on the heat pump were able to benefit from state funding, the last time it was 8000 lei. The expenses for the assembly or commissioning of the system were also eligible for settlement, as well as the value added tax, the VAT of these expenses.

The price of the Sol-Air type heat pump varies between 2400 euros for a power of approximately 7,5 Kw and can reach 12.000 euros for those of approximately 50Kw. It can be considered a price of approximately 280 euro / Kw depending on the need and the surface to be heated.

The price of the Apa-Apa heat pump is average compared to the other heat pumps and is between 1900 euros for the small ones of 6 Kw and can reach 15.000 euros for the 50 Kw ones. It can be considered a price of approximately 260 euro / Kw depending on the need and the surface to be heated.

The price of the air-to-water heat pump is the lowest compared to the other systems of the heat pumps and varies between 2100 euro / 6 Kw and can reach 9000 euro for 35 Kw. It can be considered a price of approximately 240 euro / Kw depending on the need and the surface to be heated.

Other important remarks

According to European norms, the calculation of the required thermal load is related to the square meter of surface, [W / sqm], taking into account the maximum height of the room H = 3m (height typical of most rooms - In atypical cases the necessary corrections will be made).

For example, if a building has a thermal requirement of 50W / sqm and a living area required to be heated by 350sqm (H = 3m) the thermal power of the thermal power plant (regardless of its type) will be 250mpx50W / sqm = 12500W = 12.5kW . (Height is specified only in atypical cases)

The correct sizing of a pump is essential for its service life. An oversized heat pump, in addition to being more expensive, has an incorrect operating regime with more frequent starts and stops. An undersized heat pump works more with short breaks. However, it is preferable, if there is no alternative, to undersize the pump (within certain limits) than to oversize.

For a house of 200 sqm usable area, where a ground - air heat pump of 8 - 10 Kw is sufficient, the total cost is about 60.000 lei and differs depending on the chosen brand.

Due to the relatively high price of heat pumps, it is uneconomical to heat poorly insulated spaces that require high thermal power. It is better to insulate the building than to increase the power of the heating source. The cooling power (EER) is taken into account when determining the power of a reversible heat pump. The cooling power is always higher than the heating power.

For example, the DIN 4701 norms regarding the conditions that heated buildings in Germany must meet are, in their evolution:

  • 45-60 W / sqm new constructions (regulation since 2002)
  • 50-60 W / sqm new constructions (regulation since 1995)
  • 70-90 W / sqm constructions made before 1995
  • 120 W / sqm old constructions made without any regulations

In the case of a building with very poor thermal insulation, the investment in a heat pump is not profitable.

Heat pump boilers

In addition to those mentioned earlier in this article, we believe that it is important to treat heat pump boilers separately due to the wide coverage potential they offer. They work on the principle of air-to-air heat pumps, the heated water being stored in a boiler that forms a common body with the heat pump. The air-to-air heat pump has maximum performance when the ambient temperature is above 15 degrees Celsius, but can also operate at temperatures of -5 degrees Celsius. If you choose the right place to mount the boiler with heat pump, in a room where there is a central heating or other equipment that generates heat, from refrigerator to servers, etc., you can ensure the optimal operation of these boilers with heat pumps, during the winter with low costs. The boilers are also equipped with electric heaters for situations where a rapid heating of the water is required. Another useful option is the fact that the forced air flow can be used to lower the air humidity or to cool the room in which the heat pump boiler is installed by a few degrees.

Sources used for this article

  1. Remus Răduleț et al. Romanian Technical Lexicon, Bucharest: Technical Publishing House, 1957-1966.
  2. The Systems and Equipment volume of the ASHRAE Handbook, ASHRAE, Inc., Atlanta, GA, 2004
  3. Fundamentals of Engineering Thermodynamics, by Howell and Buckius, McGraw-Hill, New York
  4. Heat pump - complete details


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