Types of heat pumps

Overview & differences

Air, water or geothermal heat pumps - the different types of heat pumps utilise different heat sources. There are also models for indoor and outdoor installation. Depending on the structural conditions, property size and space in the basement or plant room, one system may be more suitable than another. Knowing the differences, advantages and disadvantages will help you choose the right type of heat pump and find the right heating solution for your home.

At a glance

Types of heat pumps: Air-to-water, water-to-water and brine / water heat pumps differ primarily in the heat source they utilise - be it the ambient air, groundwater or the ground.
Particularly easy to install: Air source heat pumps do not require any development measures and are therefore also ideal for retrofitting.
Efficient, but more complex: Groundwater and geothermal pumps require drilling or the installation of collectors.
Other heat pump types: Air-to-air heat pumps without a water-bearing heating circuit and domestic hot water heat pumps for heating domestic hot water only. Hybrid heating as a combination of heat pump and existing heating system.
Selection criteria: Climate, building type, property, costs and subsidies determine the right type of heat pump.

The most important types of heat pumps

As a climate-friendly heating solution, heat pumps convert environmental energy into heat - this operating principle is the same for all types of heat pumps. The main difference between them is the energy source from which they obtain their heat.

The air-to-water heat pump obtains its heat from the ambient air and is particularly flexible to use.
The brine / water heat pump, also known as a geothermal heat pump, converts the heat from the ground into heating energy and is particularly suitable for properties with sufficient space.
The water-to-water heat pump uses groundwater as a heat source and impresses with its constant and high efficiency.

The function of heat pumps is explained in detail here.

Air-to-water heat pump in detail

How does an air-to-water heat pump work?

This type of heat pump continuously draws in ambient air via a fan. This is either outside air (for outdoor appliances) or inside air (for appliances installed in the basement). The air source heat pump extracts energy from this air, brings it to a higher temperature level through a compression process in a refrigerant circuit and then transfers it to the heating water.

This even works when the temperature outside falls below 0 °C. For outdoor units in particular, this means that the "heat" of the energy source, i.e. the outside air, is very low. Nevertheless, sufficient heat can be generated. It is only important that the boiling point of the refrigerant in the heat pump is below the outside temperature.

What needs to be considered when installing an air-to-water heat pump?

There are two installation options for this type of heat pump:

Heat pump as an outdoor unit: The air heat pump is installed weatherproof in front of the building or in the garden.
Models for indoor installation: The system is installed in a protected location inside the house.

With both versions, it is important to ensure that the airflow is not obstructed. Further development measures are not necessary.

Typical areas of application for this type of heat pump are new buildings, but also the retrofitting of existing heating systems, as the air-to-water heat pump is relatively easy to integrate.

To the air-to-water heat pumps

Advantages and disadvantages of air-to-water heat pumps

The advantages of an air source heat pump include

Air as an energy source is available free of charge and everywhere
Simple installation without drilling holes in the ground, laying collectors or digging wells
Lower investment costs compared to other systems
High flexibility in heat pump installation, either indoors or outdoors
Low space requirement for the air-to-water heat pump
Ideally suited for new builds and retrofitting in existing buildings
Possibility of an active cooling function for pleasant temperatures in summer

Conclusion

Of all the heat pump types, the air-to-water heat pump scores highly thanks to its particularly simple, space-saving and comparatively inexpensive installation. This makes it a flexible, environmentally friendly heating solution for both new builds and renovations. Although the efficiency of appliances installed outside drops slightly in very cold weather, they can still generate sufficient heat even at low temperatures.

Disadvantages of an air source heat pump:

Efficiency of outdoor units depends on the outdoor temperature
Higher power consumption at very cold temperatures
Operating noise due to fans

Water-to-water heat pump in detail

How does a water-to-water heat pump work?

A suction well pumps groundwater to the heat pump. Its constant temperature is particularly suitable for generating heat. The groundwater heat pump extracts the heat from this water and utilises it for the heating system via the compression process. The groundwater is fed back into the ground via an absorption well.

What needs to be considered when installing a water-to-water heat pump?

The following are necessary for the installation of this type of heat pump

The construction of wells for water extraction and return
Access to clean groundwater
Obtaining permits and fulfilment of requirements

Typical areas of application are properties with good access to groundwater.

To the water-to-water heat pumps

Advantages and disadvantages of water-to-water heat pumps

The advantages of a groundwater heat pump include

Environmental friendliness through the use of renewable energy sources
High efficiency of groundwater heat pumps
Low operating costs due to the constant temperature of the heat source
Constant power output regardless of outside temperatures
Possibility of passive cooling for energy-efficient building temperature control in summer

Conclusion:

Groundwater heat pumps are among the most efficient types of heat pump - with low operating costs and constant output. Despite the somewhat more complex installation, which requires authorisation, they are therefore a particularly powerful and environmentally friendly heating solution if the structural conditions are suitable.

Disadvantages of a groundwater heat pump:

Higher costs and more complex installation due to well construction
Permits required for drilling
Utilisation dependent on groundwater level
Possible environmental regulations during installation

Brine / water heat pumps in detail

How does a brine / water heat pump work?

A brine / water heat pump requires geothermal probes or geothermal collectors. A liquid, the brine, circulates through these, which absorbs the heat from the ground and feeds it to the heat pump. Inside the heat pump, the energy is then utilised for heating by means of a compression process.

Sole-Wasser-Wärmepumpe Erdwärmekollektoren

What needs to be considered when installing a brine / water heat pumps?

There are two versions for extracting heat from the ground:

Geothermal probes: Are installed in drill holes about 100 metres deep. The possibility and suitability for drilling on the property are decisive here. Regional regulations must also be checked. The advantage: from a depth of around 10 metres, the ground temperature remains constant all year round.
Geothermal collectors: Are laid in a serpentine pattern at a depth of around 1.5 metres and require more area than geothermal probes. A sufficiently large, unsealed plot of land is therefore required.

Typical areas of application for this type of heat pump are new buildings and properties with sufficient undeveloped land.

To the brine / water heat pumps

Advantages and disadvantages of brine / water heat pumps

The advantages of the rine / water heat pump include

Conserves resources by utilising natural geothermal energy
High, constant efficiency and therefore low operating costs
Independence from outside temperatures
Brine / water heat pumps have a lengthy service life
Option for passive cooling in the warm months

Conclusion

The geothermal heat pump is considered a particularly efficient and resource-saving type of heat pump. However, its installation requires more complex development measures. If the property is suitable, the geothermal heat pump as a heating solution offers low operating costs, a constant heat supply and a long service life.

Disadvantages of a geothermal heat pump:

It is not possible to drill boreholes or install collectors on every property
Higher space requirements and installation costs
Higher investment costs due to structural development measures

Species in comparison

Heat pump type

Air-to-water heat pump

Water-to-water heat pump

Brine / water heat pumps

Operating principle

Extracts heat from the ambient air

Utilises groundwater as a heat source

Utilises geothermal energy via geothermal probes or collectors

Installation site

Indoor or outdoor installation possible

Indoor installation + 2 fountains

Indoor installation + geothermal collectors or geothermal probes

To be observed during installation

Free air circulation, for outdoor installation: specified distances

Access to clean groundwater, authorisation

Sufficient space on the property for geothermal probes/collectors

Investment costs

Low to medium

Medium to high

Operating costs

Moderate, for outdoor units also dependent on outdoor temperature

Low, very efficient

Advantages

Simple installation, no need to drill holes in the ground

Very high efficiency, constant high output

Very high efficiency, long service life

Disadvantages

Performance drops at very cold temperatures (especially with appliances installed outside)

Authorisation procedure for groundwater required

Boreholes or collectors require space

Typical areas of application

New construction and refurbishment, also in densely built-up areas

New construction and refurbishment with access to groundwater

New construction and refurbishment with sufficient land

Funding opportunities

State subsidies and loans possible

Government grants and loans possible

Heat pump as an indoor or outdoor unit?

The most commonly installed type of heat pump is the air-to-water heat pump. Thanks to its flexible installation options, it is also suitable for different structural conditions on or in the house.

Outdoor installation: If there is little space in the basement or indoors, outdoor units can be installed flexibly in front of the house or in the garden. However, distances from neighbouring properties and bedrooms must be taken into account due to the noise generated. Note: Weather-resistant materials and a robust housing.

Indoor installation: Air source heat pumps can also be installed inside the building - this installation option is considered to be one of the quietest. The prerequisite for this is, of course, sufficient space in the house. Especially when switching to renewable energies, the room of the old oil heating system or oil tank can be used, for example.

Please note: Stable, solid walls in the installation room

Tip: Volume of outdoor heat pumps

The noise level also plays a role with the different types of heat pumps. This is because the fans of air heat pumps in particular generate noise. But there is no need to worry that an outdoor unit will operate very loudly and disturb neighbours or residents: Thanks to particularly effective sound and vibration damping, heat pumps from KERMI run as quiet as a whisper. The noise level is comparable to the rustling of leaves in the wind.

And if it needs to be particularly quiet, the additional silent mode ensures extra quiet operation. This means that the appliances can usually be installed outside without any further structural measures.

Other types of heat pumps

Air-to-air heat pump: This type of heat pump works in a similar way to an air conditioning system: it extracts heat from the outside air, brings it to a comfortable temperature level and releases it directly into the room via a fan system. Unlike other heat pumps, there is no water-bearing heating circuit and no hot water preparation.

Domestic hot water heat pumps, or drinking water heat pumps - the name says it all: This type of heat pump does not heat rooms, it only heats the drinking water required in the household, for example for showering, washing hands or cooking. Similar to the air-to-water heat pump, it utilises the energy of the room air, outside air or extract air. The domestic hot water heat pump is often used in combination with existing heating systems and enables noticeable energy savings compared to conventional water heaters.

To the domestic hot water heat pumps

What is hybrid heating?

A hybrid heating system combines a heat pump with a conventional heating system to utilise the advantages of both technologies: The heat pump provides energy-efficient and environmentally friendly heating in mild temperatures, while the conventional boiler kicks in when the outside temperature is very cold.

Advantages of hybrid heating:

Height flexibility and operational reliability
Legal protection in extreme outside temperatures, particularly relevant for old buildings
Utilisation and preservation of existing heating infrastructure

Typical application scenarios:

Refurbishment of existing heating systems
Transitional solution on the way to complete heat generation with the heat pump

You can find more information about hybrid operating modes as a supplement to existing heating systems here.

Choosing the right type of heat pump

Ultimately, your individual situation will determine which type of heat pump is the best choice for your home. To summarise, you should keep the following factors in mind:

Climatic conditions: The milder the climate and the higher the average outdoor temperatures, the more suitable an air-to-water heat pump is.
Building type and insulation standard: All heat pump types are suitable for new builds. For older buildings, the air-to-water heat pump is often the simplest solution due to the retrofitting options.
Structural conditions: The availability of land area, access to groundwater or geothermal energy influences the choice. Water-to-water and brine / water heat pumps often offer the higher efficiency here, but require appropriate conditions.
Cost-effectiveness: Air-to-water heat pumps generally have lower investment costs, as no complex development measures are required, but can incur higher operating costs.
Subsidy programmes and legal framework conditions: There are funding opportunities for all three types of heat pumps, some of which are linked to specific requirements.

FAQ

What type of heat pump is best for my home?

Which heat pump is best for your home depends on several factors, including the climate, building type, structural conditions and your budget. In mild climates and where space is limited, the air-to-water heat pump is often the most flexible and cost-effective solution, especially for older buildings and retrofits. However, if you have sufficient land area or access to groundwater, brine / water heat pumps offer higher efficiency and lower operating costs. An individual consultation will help you find the optimum heat pump to suit your needs and general conditions.

What are the differences between air, ground and water heat pumps?

The main differences between air, ground and water heat pumps lie in the heat source used and the associated properties: air-to-water heat pumps extract heat from the outside air and are easy to install. However, the efficiency of outdoor units decreases in cold temperatures. Brine / water heat pumps utilise constant geothermal heat and are very efficient, but require sufficient space and suitability for drilling geothermal probes or laying geothermal collectors. Water-to-water heat pumps extract heat from groundwater, offer high efficiency, but require access to clean groundwater and official authorisation. The choice of the right heat pump therefore depends primarily on local conditions and available space.

Which heat pump is suitable for old or new buildings?

As a rule, all types of heat pumps are well suited to new builds, as they often fulfil optimal requirements such as good insulation, sufficient space and suitable site conditions. In older buildings, the air-to-water heat pump is often the most practical solution, as it is flexible and comparatively easy to retrofit and does not require any special property conditions. Brine / water heat pumps offer high efficiency, but require sufficient land area for geothermal probes or access to clean groundwater as well as the appropriate permits.

Further information here.

Which type of heat pump has the lowest operating costs?

Water-to-water heat pumps generally have the lowest operating costs as they utilise a very constant and efficient heat source. Although air-to-water heat pumps are cheaper to purchase, they require more electricity for heat generation. Brine / water heat pumps are usually somewhere between the other two systems in terms of operating costs.

Buy a KERMI heat pump

We attach great importance to the highest quality and flawless installation of our products. That is why we sell KERMI heat pumps exclusively via the three-stage sales channel. This ensures that you not only receive a premium product, but also benefit from the best possible advice, planning and professional installation. Your qualified local specialist partner is your direct contact and ensures the optimal integration of our efficient and sustainable heating solutions into your home.

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