Buildings account for around 45% of Switzerland's final energy consumption and a third of its CO2 emissions, mostly for space heating (SH) and domestic hot water (DHW), and more than half are still heated by fossil fuel boilers. Replacing these boilers with heat pumps (HPs) or connecting buildings to district heating networks (DHNs) is central to decarbonizing the sector, particularly for multifamily buildings (MFBs) in dense urban areas, where few other renewable options are available. However, the performance of these renewable heat sources is strongly penalized by the high operating temperatures inherited from fossil fuel systems.
The T-DROP project investigates how existing MFBs can shift from high- to low-temperature heat distribution through combination of targeted envelope retrofitting and heat distribution optimization, and quantifies the benefits for renewable heat integration and decarbonization. Focusing on decentralized HP systems, representative building and SH/DHW system typologies are modeled and combined into an integrated TRNSYS model that captures building-system interactions, used to carry out a sensitivity analysis across building typologies, retrofitting depths and optimization measures.
The simulations show that the achievable reduction in SH supply temperature is primarily governed by the effective heat emitter oversizing, defined as the ratio of installed emitter capacity to actual heating load. Consequently, any measure that reduces the SH demand (e.g. envelope retrofitting, lower indoor temperature setpoints, optimized ventilation) increases the effective emitter oversizing and enables lower supply temperatures, provided that the heating curve is optimized and the distribution system is properly balanced. By contrast, DHW production remains the main limitation since the temperature required to limit the legionella risk keeps the overall system temperature high, increasingly so as retrofitting raises the DHW share of the heat demand, and the studied DHW optimization measures bring only modest gains.
Lower operating temperatures directly improve HP efficiency and reduce CO2 emissions, while demand reduction simultaneously lowers total energy consumption and emissions per unit floor area. The role of system optimization depends on the indicator considered. For temperature levels and efficiency, system optimization measures can partly compensate for the absence of deeper envelope retrofitting (bringing a building to a performance otherwise reached only with further retrofitting), making it an effective short-term strategy for poorly performing buildings while waiting for deeper envelope retrofit. For the absolute emissions that ultimately drive decarbonization, however, envelope retrofit and system optimization are complementary: since optimization cannot substantially reduce the heat demand, envelope retrofitting remains fundamental, with optimization accelerating and reinforcing it rather than replacing it. The choice of the heat production unit also matters: a high-efficiency HP improves performance at unchanged temperatures, while a hybrid system can serve as a useful transitional solution as long as its control and hydraulics prevent the boiler from taking over.
Overall, lowering distribution temperatures can be considered as a co-benefit of reducing the heat demand, through measures prioritized according to the pre-existing characteristics and current state of each building.
