What are heat pumps and how do they work?

Heat pumps are systems that absorb energy from nature and transfer it to the heating medium. The main component of such a system is the heat exchanger, which consists of an evaporator, a compressor, a condenser and an expansion valve. A refrigerant circulates between these elements. This is a substance that has a very low boiling point (becoming gaseous even at double-digit temperatures below zero).

Heat pump heating works as follows: thermal energy absorbed from the air, earth or water meets the liquid refrigerant in the evaporator, which absorbs the heat and causes it to evaporate. The gaseous refrigerant is then compressed in the compressor, causing it to heat up rapidly. This phenomenon can also be seen in everyday life. When inflating bicycle tyres, for example, the valve becomes very warm. The gaseous refrigerant reaches the condenser and expansion valve, where it releases its heat to the heating medium (water). The cooled refrigerant is no longer under pressure and returns to liquid form. The cycle begins again. This is also how refrigerators and air conditioning systems work, but in the opposite direction: they transfer heat out instead of in.

What are the different types of heat pump heating systems?

There are four types of heat pump heating systems. They all work according to the principle described above, but they use different heat sources and heating mediums to distribute the heat throughout the house. The name always mentions the heat source first, followed by the heat transfer medium. Overview:

• Air-to-water heat pump: this type of heat pump draws heat from the ambient air via a fan and transfers it to the heating water. Hot exhaust air from industrial buildings or data centers can also be used as a heat source. The air-to-water heat pump is relatively easy to install and is therefore the most common and cheapest solution for single-family home owners. It is also the least efficient, however.
• Air-to-air heat pump: this pump also uses ambient air to generate heat, but the heat is transferred directly to a ventilation system and distributed throughout the building. This type of heat pump is used primarily in buildings that are of very high quality in terms of energy and construction, such as houses with the Minergie-P standard.
• Brine-to-water heat pump/geothermal energy: here, the heat is obtained from the ground (geothermal energy), either with collectors below the Earth’s surface (up to approx. 1.5 metres) or with probes inserted up to 300 metres into the ground. The deeper the probe, the more heat is generated. This type of heat pump is particularly efficient and has a long service life. However, it is relatively expensive due to the complex installation work and the required special approval.
• Water-to-water heat pump: this heat pump obtains heat from groundwater, natural bodies of water such as rivers or lakes or from the sewer system. It is the most efficient option, but requires access to a body of water and filter systems. It is therefore usually too complex for a single-family home.

Is it worth the investment?

The investment costs for a heat pump heating system are usually higher than for an oil or gas heating system. However, due to the lower operating costs and heating cost savings, it is usually quicker to provide a return on investment and saves money in the long term. So it provides both an environmental and economic benefit.

The cheapest version is the air-to-water heat pump; the most expensive is the water-to-water heat pump. The following is an overview of the approximate costs for a house requiring 20,000 kilowatt hours of heat per year, which is realistic for an average Swiss single-family house with a living space of around 140 m²:

How efficient are heat pumps?

Heat pump heating systems require electricity to operate their electric motors (fan, compressor, pumps, etc.). For this reason, their efficiency is calculated based on the ratio between the thermal energy obtained in kWh and the electricity used in kWh. This results in the CoP (coefficient of performance). On average, the systems produce around 3 to 5 kilowatt hours of heat with 1 kilowatt hour of electricity. The average CoP is therefore 3 to 5, although this can vary a great deal in individual cases. These are the most important efficiency factors:

• Type: air-to-water heat pumps generally perform somewhat worse than the other types.
• Flow temperature: this is the temperature of the water when it enters the heating circuit. The higher the required flow temperature, the greater the power requirement and the worse the efficiency. The required flow temperature depends on:
• Insulation: poorly insulated houses usually require a higher flow temperature.
• Size/type of heating element: large heating elements – such as for underfloor heating – require a lower flow temperature than devices such as classic smaller radiators.

Does it make sense to use heat pump heating systems in older buildings?

Heat pump heating systems are ideal for houses with good insulation and underfloor heating. These require only low flow temperatures of 30 °C or less, and correspondingly little power is required to operate them. But even poorly insulated or uninsulated old buildings can be heated with a heat pump – most systems achieve flow temperatures of up to 70 °C, although power consumption is significantly higher in these cases. It is best to discuss with a specialist whether a heat pump heating system is suited to your old building and what form would be best.

Advantages and disadvantages of a heat pump heating system

Advantages

• Environmentally/climate-friendly: they do not use fossil fuels to generate heat and therefore do not emit any greenhouse gases when heating.
• Cost-saving: you use free energy sources (air, water or geothermal) to generate heat. Costs are generated only from maintenance and the electricity required for operation.
• Efficient: they generate more energy than is needed to operate them.
• Predictable: apart from the electricity costs, there are no price fluctuations or dependencies like with oil or gas heating.

Disadvantages

• Rather high investment costs, but these pay off relatively quickly thanks to the heating cost savings.
• The initial costs are mitigated, as the environmentally friendly heating system is subsidized by the state.

Tip

If you are operating your heat pump with electricity from your own photovoltaic system or other renewable energies, you can significantly increase the sustainability and climate-friendliness of your heating system.

The biogas alternative – how viable is it?

Biogas offers another option for heating with renewable energy. This is a gas mixture that is created when bacteria break down organic waste such as manure, plant or food waste. The fermentation process takes place in a special biogas plant. Biogas consists mainly of methane and CO₂.

When biogas is burned, CO₂ is produced, just like natural gas or oil. So biogas is not emission-free, but it is a carbon-neutral _{heating fuel. This is because the plant-based raw materials captured the same amount of CO2 that is released later during combustion.}

According to the Swiss Federal Office of Energy, biogas can make an important contribution to achieving climate goals. Compared to natural gas heating, it is possible to reduce emissions by up to 40 percent with biogas heating. This figure can go as high as 60 percent when compared to oil heating systems.

In Germany, maize and rapeseed are planted specifically for the production of biogas. This is not the case in Switzerland, which makes sense, because it means that fields can still be used for food production. The biogas production sector in Switzerland is quite small and has very limited growth potential. Experts do not believe it is possible to cover all gas requirements with domestic and imported biogas. The dependence on natural gas (especially from Russia) will remain.

The share of biogas in the total volume of gas consumed in Switzerland is just over 6 percent. There are now providers that offer a supply of 100 percent biogas, but as a rule, biogas is merely mixed with natural gas in certain proportions. As a result, the climate-friendliness of this mixed solution is limited. Furthermore, biogas is more expensive – significantly more expensive than natural gas. According to a study of gas providers conducted by Kassensturz in 2022, the smallest price difference was 34 percent, while the largest was 70 percent.

Conclusion

If you want to switch to a climate-friendly, long-term cost-effective heating system with 100 percent renewable energy, you are better served with a heat pump rather than a (new) biogas heating system.

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