Heating systems are big energy users in existing commercial buildings. Replacing boilers with heat pumps provides an opportunity to improve building energy performance, increase energy efficiency and reduce carbon emissions, but it can be a complex process. This is because older systems typically operate at flow and return temperatures of 82/71°C or potentially 80/60°C, with the emitters sized accordingly.

The cost to upgrade, available budget, time, space and electrical capacity, as well as running costs, are just some of the typical refurbishment challenges. And as each building will have its own unique requirements, there can be no one size fits all solution. Successfully replacing boilers with air source heat pumps therefore requires a detailed and methodical process to ensure optimal performance and deliver energy savings.

Understanding the existing heating system

Having a full understanding of the existing heating system and real measured data from the building is essential to achieving optimal outcomes. Using this information, experienced manufacturers like Baxi will be able to engineer a series of solutions, simulating the energy, carbon and CAPEX impact of each option and building upon them to improve the outcomes within the project parameters.

What do we mean by this? Let’s consider an example where an existing building heating system includes gas boilers operating at 82/71°C, with a 300kW constant temperature (CT) distribution system that includes a DHW calorifier and a 100kW variable temperature (VT) distribution system, indirectly weather compensated. Both run at the same 82/71°C boiler flow temperature. Cost and operating limitations rule out replacing emitters in the CT system, however the VT circuit emitters could be replaced to allow a lower flow temperature.

One approach to decarbonising this example heating system could begin with replacing the existing gas boilers with high-temperature heat pumps (HT HPs), such as the Baxi Auriga HP+, on a kW-for-kW basis. This would maintain the 82/71°C operating temperature year-round, with minimal modifications to the existing circuits. However, while this method is relatively straightforward, it involves high capital costs and would push the HT HPs to their performance limits.

Hybrid system

A more practical solution could involve adopting a hybrid system, where the VT circuit is separated and assigned to medium or high-temperature heat pumps, while gas boilers continue to serve the CT distribution system. By consolidating the heat system and bracketing the VT circuit, weather compensation can be handled directly at the plant, eliminating the need for mixing valves. This hybrid approach significantly reduces capital costs, lowers electrical demand, and broadens the range of heat pump technologies by lowering the flow temperature. However, it would still leave some reliance on gas boilers, only partially decarbonising the peak load.

To further enhance this hybrid system, a pre-heat tank for domestic hot water (DHW) could be added, powered by an air source heat pump (ASHP). In this setup, gas boilers remain for the CT system, while heat pumps manage both the VT circuit and DHW pre-heating. This addition decarbonises a larger portion of the heat load while still maintaining lower costs and electrical demands. However, it introduces greater complexity, requiring additional space for the DHW pre-heat tank and more advanced control systems.

This combined approach offers a path to progressively decarbonise the heating system, balancing cost-efficiency, sustainability, and system complexity.

Understanding the options

As clients begin to consider boiler replacement projects, understanding the various options for decarbonising their heating systems and the impacts of each is vital. By clearly explaining the options in a step-by-step process, expert solutions providers like Baxi can help break down the complexities for clients. Working together with heating professionals, we can ensure that the right decision is made for the project to set commercial buildings on their net zero journey.