What types of heat pumps are there?

Heat pumps can primarily be distinguished by their heat source.

The most popular type of heat pump is the air-to-water heat pump. This extracts heat from the ambient air and transfers it to the building.

In residential buildings, air-to-water heat pumps are usually installed as split units. Ambient heat is drawn from an outdoor unit, which contains a fan, and transferred to a refrigerant circuit. This circuit then directs the heat to the indoor unit, where it heats domestic hot water and the water-based central heating system.

One type of air-to-water heat pump is the monobloc heat pump. This consists of only one outdoor unit, which directly heats the water circuit connected to the building. Monobloc heat pumps are simple systems because they are installed entirely outside the building.

Air-to-air heat pumps also extract heat from the outside air. However, instead of radiators or underfloor heating, the heat is distributed throughout the building by fans. You are already familiar with the reverse of the air-to-air heat pump as an air conditioner: it extracts heat from inside the building and releases it to the outside air. Most modern air conditioners can also heat.

The advantages of the air/water heat pump

Air-to-water heat pumps are easy to install. They do not require ground drilling or special permits. This makes them more cost-effective than other heat pumps.

Their disadvantages include the noise generated by the outdoor unit and their lower efficiency compared to, for example, geothermal heat pumps. Their seasonal performance factor (SPF) is usually between 3 and 4.

The advantages of the geothermal heat pump

Geothermal heat pumps, on the other hand, can achieve a seasonal performance factor (SPF) of up to 5. However, they require a borehole, which, at a typical depth of around 100 meters, costs between €8,000 and €12,000 (Germany; prices may vary depending on the country). In addition, there are the standard costs for the unit and the building services. For heat pumps that extract heat from the earth or groundwater, additional costs of up to €3,000 for planning and permits can quickly arise. 

What do COP and JAZ mean?

COP stands for "Coefficient of Performance." It's a good metric for comparing different devices. It's determined under laboratory conditions, but doesn't tell you much about how efficiently the device will operate under the specific conditions at your property. For that, the seasonal performance factor (SPF) is crucial. It takes into account the individual temperature conditions at your location.

The COP (Coefficient of Performance) always refers to a specific ratio of outside temperature to inside temperature, or flow temperature. The flow temperature is not the room temperature you desire in the building, but rather the temperature at which the hot water from the central heating circuit leaves your boiler and flows towards the underfloor heating or radiators. The flow temperature must always be slightly higher than the desired room temperature so that the heating system can still transfer heat to the room.

The COP is specified in manufacturers' technical data sheets as follows: A2/W35=4.0. In this example, this means that the COP is 4.0 at an outside temperature of 2°C and a flow temperature of 35°C. Most data sheets specify four or five different operating points, e.g., A5/W35, A7/W35, A7/W55. In addition to the various COPs, the maximum heat output of the units in kW is also specified.

It is important that the device still achieves the required heat output in your building at the operating point with the greatest temperature difference (in this example A-7/W55).

The annual performance factor (SPF) is calculated according to a method according to VDI 4650 sheet 1: 2024-02.

When planning a heat pump, factors such as the planned flow temperatures and the standard outside temperature according to DIN/TS 12831-1 are relevant. Furthermore, the proportion of hot water production is crucial, as the heat pump consumes more electricity for hot water generation than for heating. Your heating specialist or energy consultant can perform this calculation professionally for you.

With an existing heat pump, you can determine the seasonal performance factor (SPF) much more easily: The amount of heat delivered can usually be read in the menu on the heat pump's display. Divide this by the amount of energy consumed, as measured by your heat pump's separate electricity meter, to obtain the SPF. You should have a separate electricity meter just for your heat pump, as this is the only way to benefit from a significantly cheaper electricity tariff for heat pumps!

A seasonal performance factor (SPF) below 3.0 is poor. Among other things, it will then be difficult to obtain funding for your heat pump.

In summary:

• COP: Key figure for comparing devices (refer to the operating points in the data sheet!)
• JAZ: Key performance indicator used to determine individual efficiency on site

How much does a heat pump cost for a 150 square meter house?

A heat pump for a house with approximately 150 square meters of living space costs between €20,000 and €35,000. However, the costs depend on several factors that should be clarified before purchasing:

• What type of heat pump should be purchased? Is the somewhat cheaper air-to-water heat pump sufficient, should it be a more efficient but more expensive geothermal heat pump, or even a water-to-water heat pump that draws its heat from groundwater?
• What are the ongoing operating costs for the new heat pump?
• What funding opportunities reduce the purchase costs?
• What additional planning and approval costs are involved?
• What modifications to the existing heating system are necessary to operate the heat pump with maximum efficiency?

 What are the advantages of an air/water heat pump?

• No costs for an earth drilling
• Lower investment volume
• Quick and easy installation
• If you live in a region that is not too cold
• No permit required for deep drilling
• If very cheap electricity is available from a PV system

 What are the advantages of a geothermal heat pump?

• Low operating costs
• No noise emission from an outdoor unit
• They live in a very cold region and require high efficiency through constant temperatures in the ground.

The following diagram shows how the costs of an air-to-water heat pump and a ground-source heat pump accumulate over the years. We have depicted the air-to-water heat pump once for an average cold region with a seasonal performance factor (SPF) of 3.5 and once for a very cold region with an SPF of 3.1. The ground-source heat pump operates in both regions with an SPF of 4.5, as the ground is uniformly warm.

In addition, we made further assumptions:

• Annual heating demand of the property: 15,000 kWh
• Electricity price: approx. 22 ct/kWh
• Maintenance costs are approximately €250 per year for the air/water heat pump and €200 per year for the geothermal heat pump.

*This graphic is not relevant if the electricity is sourced from a PV system.

Attention: A heat pump is eligible for funding – up to 70% of the investment volume can be subsidized!

What is the biggest problem with a heat pump?

Many myths surround heat pumps, but there is no general problem with this technology. 

It's often said that a heat pump doesn't provide enough heat in winter at cold temperatures, leaving the residents freezing. This is definitely wrong. A heat pump works even at double-digit sub-zero temperatures and generates more than enough heat.

It should be noted that heat pumps operate most efficiently at flow temperatures between 25°C and 45°C. Some older buildings still have unsuitable radiators with small heating surfaces, which require higher flow temperatures. This results in a less efficient heat pump.

You will also need a higher flow temperature if your building is poorly insulated.

However, there is a solution: The heating surface can be enlarged through modernization measures, thereby lowering the flow temperature into the efficient range of the heat pump. And this can often be achieved with simple and cost-effective measures. 

Can I operate a heat pump with normal radiators?

Yes, a heat pump can always be operated with normal radiators.

BUT – this is not always efficient and economical.

With standard radiators, you usually need a flow temperature of over 55°C – the coefficient of performance (COP) then often drops below 3, and electricity consumption increases exorbitantly.

Overview of current heat pump electricity prices

Electricity for heat pumps is currently offered by the cheapest providers for approximately 22 cents per kilowatt hour. For this, you will need a separate electricity meter from your metering point operator.

To find a supplier of electricity for heat pumps, simply use the relevant comparison portals.

Can I install a heat pump in an old building, and what do I need to consider?

This depends on the flow temperature at which you can operate your heat pump to ensure efficient and cost-effective operation. This, in turn, depends on whether you have suitable existing radiators and the heating load of your older building. And this, in turn, is influenced by the building's insulation.

Older buildings often have finned radiators installed – as can be seen in the following picture.

These do not release enough heat at flow temperatures of 35°C.

Instead, you need underfloor heating or panel radiators with the largest possible heating surface. The heat output of panel radiators can be further improved by using convection fans.

How does a heat pump work? 

The way a heat pump works can be compared to a refrigerator. A refrigerator pumps heat from the inside to the outside. A heat pump pumps heat from the surrounding environment into the interior of the building.

Heat transfer occurs via a refrigerant circuit between the outdoor unit and the indoor unit of the heat pump.

The refrigerants circulating in the heat pump evaporate even at very low temperatures (-40°C to -50°C) and absorb heat from the surroundings. These surroundings can be the outside air, the ground, or groundwater.

Here is an example of an air-to-water heat pump:

Assume an outside temperature of -10°C. The liquid, very cold refrigerant (a few degrees below -10°C) in the outdoor unit of the heat pump absorbs heat from the environment through evaporation.

The refrigerant then flows to the indoor unit, where it is compressed using electrical energy. This compression raises its temperature (above the desired flow temperature) so that it can transfer a large portion of its heat to the building's heating system (underfloor heating, radiators, hot water). During this process, the still gaseous refrigerant condenses in the condenser, releasing heat into the heating system.

Finally, the compressed refrigerant is expanded again until its temperature drops below the outside temperature, allowing it to absorb heat from the environment once more. The cycle is complete, and the process begins anew.

In this process, most of the energy is absorbed from the ambient air into the cycle. Only a small amount of energy needs to be supplied to the process in the form of electrical energy for compressing the refrigerant.

A geothermal heat pump takes ambient heat from the earth, and a water/water heat pump takes ambient heat from groundwater or another body of water. 

As a rough estimate, one kilowatt-hour of electricity can "generate" approximately four kilowatt-hours of heat. Whether more or less heat is provided depends on the efficiency of the heat pump. The heat pump operates most efficiently at low flow temperatures between 25°C and 35°C.

For whom is a heat pump worthwhile (or not)?

• If excessively high flow temperatures (>55°C) are required, a heat pump will operate inefficiently. This would result in high electricity costs, making a heat pump uneconomical.
• A poorly insulated building with a very high annual heat demand of over 100kWh/m² makes a heat pump inefficient.
• Undersized radiators also make a heat pump inefficient.

 However, even if one or more of these reasons apply to your project, there is a high probability that a solution can be found!

What are the monthly electricity costs for a heat pump?

 In summary, an air-to-water heat pump in a relatively new 150 sq m detached house built from the year 2000 onwards can be operated for under €100 per month. This calculation assumes triple-glazed windows and that most rooms are equipped with underfloor heating or large radiators.

Special electricity tariffs are offered for heat pumps. These are currently available for approximately €0.22/kWh. 

The monthly electricity costs of a heat pump depend on various factors:

• How efficiently is the device operated?
• How well insulated is the building?
• What room temperature is perceived as comfortable?
• What is the ventilation behavior?

What is the electricity price for heat pumps?

For €100 you currently get approximately 450 kWh of heat pump electricity per month – that's around 5,400 kWh per year. With an annual performance factor of 3.5, that equates to roughly 19,000 kWh of heat per year – easily enough to heat a detached house.

How much money can you save with a heat pump?

With current energy prices, you can already save up to €800 per year by heating your detached house with a heat pump instead of oil and gas. That adds up to up to €16,000 over 20 years.

Furthermore, the costs of fossil fuels are expected to rise further. Unlike electricity, oil and gas prices will be subject to additional CO2 levies in the coming years.

The savings will be even greater if the heat pump is combined with a photovoltaic system.

Unlike fossil fuels, the energy used by a heat pump is freely available in the surrounding area. And with a photovoltaic system, the energy to power the pump can also be provided very cheaply. 

And last but not least, please consider the following comparison: A modern condensing gas boiler has an efficiency of approximately 90%. This means that from 10 kilowatt-hours of gas, you can obtain 9 kilowatt-hours of heat.

A heat pump has a coefficient of performance (COP) between 300% and 500%, depending on ambient conditions. This means that you can generate 30 to 50 kilowatt hours of heat from 10 kilowatt hours of electricity.

What else do I need to consider when planning a heat pump?

First, it's important that your property is suitable for efficiently operating a heat pump. You've already read about the requirements for this above.

If this is the case, you should also consider the following:

If you plan to purchase an air-to-water heat pump as a split unit (which is usually the case), your unit will consist of an outdoor unit and an indoor unit.

You will likely place the indoor unit in the same way you would a gas or oil boiler: in the boiler room, utility room, or laundry room. As with a traditional boiler, the hot water storage tank is also located within the indoor unit area. Sometimes it's a separate unit, sometimes it's integrated into the indoor unit. The central heating circuit for the entire building originates from the indoor unit, and hot water is also distributed from here to all taps in the building.

The indoor unit typically requires a three-phase high-voltage power connection. This is because it has a backup heating element that, like a conventional instantaneous water heater, kicks in if the outside temperature drops so low that the heat pump cycle no longer functions, or if there is a technical defect in the compressor or the outdoor unit.

However, it is extremely rare for the process to fail due to excessively low outside temperatures. For this to occur, the temperature would have to drop well below -20°C.

And of course, a technical defect should be extremely rare in a high-quality and regularly maintained device.

The indoor unit of a high-quality heat pump typically has a quiet scroll compressor, which is no louder than a refrigerator. Nevertheless, we recommend not placing this unit against the back wall of your bedroom. 

The same applies to the placement of the outdoor unit. It is somewhat louder than the indoor unit. In summer and during the transitional seasons like autumn and spring, you will hardly notice it. However, in sub-zero temperatures, the fan may run at a slightly higher speed. In such cases, you wouldn't want the outdoor unit next to your bedroom window. Popular locations for the outdoor unit include the back walls of garages or carports. If you can't find a suitable spot on your building, the outdoor unit can also be placed further away from the house in the garden. A helpful tip: To maintain good relations with your neighbors, involve them in the planning of the outdoor unit.

The outdoor unit also requires a power supply. Standard single-phase alternating current is sufficient.

A refrigerant circuit must be installed between the outdoor unit and the indoor unit. As with water pipes, you don't want these pipes exposed on the surface in relevant areas.

If possible, an outdoor unit should be located close to the building.

The longer the refrigerant lines, the more disadvantages arise, such as thermal losses or potential environmental damage in the event of leaks. If refrigerant lines are laid through unheated rooms, they can quickly reach a diameter of 70 mm, including insulation. In outdoor areas, a separate protective conduit is then added.

Your installation company will lay the refrigerant lines in accordance with standards.