From a systemic perspective, selecting suitable heating technologies for buildings involves a trade-off between heating costs, infrastructure costs, and emission reduction. In districts with many renovated buildings, electric heat pumps are often economical. In districts with many unrenovated buildings, hybrid heating systems—combinations of heat pumps and gas condensing boilers—could be an alternative in the considered time period: Compared to purely electric heat pumps, hybrid heating systems require less electricity grid expansion and achieve two-thirds of the emission reduction. This is shown in a new analysis by the EWI, which compares hybrid heating systems and single-energy-source technologies in terms of greenhouse gas (GHG) emissions, grid fees, and heating costs. The trade-off between these three dimensions for different heating technologies was also considered.
The German Buildings Energy Act (GEG) specifies various technologies that can be used to cover the 65% share of renewable energy requirement. An analysis by the Institute of Energy Economics at the University of Cologne (EWI) shows the costs and carbon footprint of hybrid heating systems compared to electric heat pumps. The analysis assesses the advantages and challenges in the context of the heating transition. It was funded by the Society of Benefactors to the EWI (Gesellschaft zur Förderung des Energiewirtschaftlichen Instituts an der Universität zu Köln e. V.).
In air-to-water heat pumps with heating rods, the heat pump covers most of the heat load. During peak load periods, system efficiency is low due to low outdoor temperatures and the use of the backup heating rod. Simultaneously, the emissions factor of electricity is usually high during these peak periods. As a result, the heat pump system can cause more emissions during peak load times than a gas boiler. Hybrid heating systems combine an air-to-water heat pump with a gas condensing boiler, which takes over during peak loads. “We compare the average annual GHG emissions and full costs of different heating systems between 2025 and 2045 for different building types,” says Michael Moritz, co-author of the analysis. “Hybrid heating systems could halve GHG emissions in unrenovated buildings compared to gas heating systems. Heat pumps with heating rods could reduce emissions by around two-thirds – limited by the emissions intensity of the electricity. The renovation of buildings has the greatest savings potential and could reduce GHG emissions by around 80 percent,” says Moritz. These figures are based on the assumption that GHG emissions in the electricity sector are subject to a cap through emissions trading and that climate neutrality is achieved in 2045.
The introduction of a second emissions trading system (ETS2), which also includes private heating emissions, could influence the economic viability of these technologies. According to the analysis, heat pumps and hybrid systems would become cheaper than gas condensing boilers at an ETS2 price of around 200 EUR/tCO₂. The ETS2 price has less impact on the cost ratio between heat pumps and hybrid heating systems than the capital and operating costs.
The choice of heating technology has consequences for infrastructure requirements. Heat pumps with backup heating rods increase both electricity demand and peak power the grid must provide, potentially necessitating electricity grid expansions in many districts. Hybrid heating systems, on the other hand, rely on the gas grid during peak load periods, potentially reducing the need for electricity grid expansion. However, this may lead to rising gas grid fees. “The dependence of hybrid heating systems on the gas infrastructure poses a challenge in the long term, as the fixed costs of the gas grids would have to be passed on to fewer and fewer consumers,” says Tobias Leibfritz, co-author of the analysis. Hybrid heating systems could be economical primarily in neighborhoods with a large number of unrenovated buildings if they can reduce electricity grid expansion.
In the analysis, no single heating configuration performs best across all three dimensions (infrastructure costs, emissions, and heating costs). While hybrid heating systems have higher GHG emissions than heat pumps with backup heating rods, they could be economical in districts with many unrenovated buildings if they help avoid necessary electricity grid expansion. In districts with mostly renovated buildings or new constructions, hybrid heating systems seem less interesting.
This means that the decision in favor of a heating technology is linked to a individual trade-off between heating costs, infrastructure costs, and climate protection.
