The environmental benefit of a heat pump strongly depends on the amount of electricity required for heat production. In other words, how much environmental energy from water, soil or air is used. The ratio of the amount of heat emitted in the course of a year to the electrical energy used is called the seasonal performance factor. Taking all EWZ plants into account, the seasonal performance factor is 3.6. This means that in our company, we generate 3.6 kWh of heat and cooling with 1 kWh of electricity. And of course, we do use electricity from renewable sources for this.
One possible energy source for heat pumps is the waste heat from wastewater treatment plants and data centres. Geothermal energy, wood, groundwater and lake and river water can also be used as heat or cooling sources. All of these renewable energy sources are locally available. Import is not necessary.
The following graph shows that heating oil and natural gas boilers account for the largest share of greenhouse gas (GHG) emissions per unit of heat produced. Heat pumps powered by renewable electricity, on the other hand, have the best values. The average greenhouse gas emissions of the ewz energy networks are approximate 69 g CO₂-eq per kWh*. The increase compared to 2024 (46 g CO2-eq per kWh) is attributable to the takeover of the former ERZ heating networks linked to the Hagenholz waste incineration plant and the Aubrugg wood-fired power station.
* According to the accounting rules of the Coordination Conference for Public Sector Construction and Property Services (KBOB)
The Environmental impact points (EIPs) shown below indicate the degree of environmental pollution caused by different types of heat generation. EIPs are based on the method of ecological scarcity, which was developed in Switzerland. Their calculation takes into account a wide range of environmental impacts. Heating oil and natural gas heating systems have the greatest environmental impact. This is a consequence of fossil fuel extraction and CO₂ emissions during operation. The lowest values are for heat pumps powered by renewable electricity. A comparison with Swiss district heating networks shows that they are significantly more environmentally friendly than fossil fuel heating systems.
Local and district heating networks can be operated with different energy sources and are therefore feasible almost everywhere. Locations with a strong demand for heat and/or cooling are particularly well suited for development. In the city of Zurich and in many other municipalities in Switzerland, the conditions are ideal.
The rapid implementation of a large number of climate-friendly heating and cooling networks is an urgent task for the city of Zurich’s climate targets. At the same time, this necessary transformation of the heating supply is one of the key challenges of the next two decades. It is a generational project, one that must not only expand new thermal networks but also promote the energy-efficient renovation of many older properties. The history of district heating in Zurich began in the 1970s with the use of waste heat from waste incineration. In the 2000s, renewable energy sources, including lake water and waste heat from treated wastewater, were added to the district heating supply.
Since January 2025, ewz’s district heating network in the city of Zurich has covered around 250 km, making it the largest district heating network in Switzerland. By 2040, the thermal network in the city is set to grow to around 400 km. Today, district heating networks cover around one third of the city, and by 2040 this figure is expected to rise to around 60%. ewz is taking on this challenge and pushing ahead with the expansion at a rapid pace. Further information on district heating and milestones can be found at District heating in the city of Zurich.
CO₂-eq is a unit of measure used to standardise the greenhouse gas impact of different climate gases. Here’s an example: Methane (CH₄) is 25 times more damaging to the climate than CO₂. The greenhouse gas potential of 1 g of methane therefore corresponds to 25 g of CO₂‑eq.
Environmental impact points (EIPs) are a method of depicting the environmental impacts of products or processes. They are calculated according to the ecological scarcity method developed in Switzerland, which takes into account a wide range of environmental impacts and is summarised in an indicator (the environmental impact points).
