HEATING, PLUMBING AND VENTILATION
The transition to net zero is reshaping energy infrastructure across the UK and globally. As national policy accelerates decarbonisation, consumers, building operators and owners face increasing pressure from rising fuel prices, volatile energy markets, and the growing cost of goods and services. One of the most challenging sectors to decarbonise is the heating and hot water sector, which represents a significant proportion of national emissions.
Heating and hot water in buildings account for 37–38% of total UK carbon emissions, equivalent to roughly 32% of all greenhouse gas output. Direct emissions from residential and commercial heating alone contribute 17–23% of the national total. While legislation increasingly favours full electrification in newbuild properties, these developments represent only a small fraction of the UK’s building stock. The overwhelming majority consists of existing buildings, many of which are thermally inefficient, space constrained, or limited by electrical capacity.
Because of these constraints, a hybridised approach – integrating heat pumps with existing boiler systems – offers a technically robust and economically viable pathway for reducing emissions without imposing prohibitive operational costs.
Modern condensing boilers typically achieve efficiencies in the 88–94% range, where under optimal conditions, a boiler only achieves its highest efficiency when it is in condensing mode.
COST EFFICIENCY MISCONCEPTIONS
A common misconception is that heat pumps are inherently more expensive to operate due to the higher unit cost of electricity relative to gas. However, when CoP values exceed approximately 2.5 the price disparity between heat pumps and boilers is minimal, as seen by Figure 1 (below).
As outdoor air temperature decreases or the flow temperature increases, the compressor must work harder, in turn reducing the CoP.
In many existing buildings, legacy systems operate at 80°C flow/60°C return, resulting in real world boiler efficiencies as low as 70% for older units and mid 80% for modern condensing boilers.
Lowering system temperatures benefit both technologies, but heat pumps gain disproportionately, often surpassing boiler cost efficiency at moderate outdoor temperatures due to the big increase in CoPs when heat pumps run at these lower temperatures.
THE CASE FOR HYBRID HEATING SYSTEMS
A hybrid system can integrate heat pumps and boilers under a unified control strategy. This configuration leverages the strengths of each technology while mitigating their weaknesses.
The core of a hybrid system is the control logic that determines when the heat pump, boiler, or both should operate. We can use predetermined outdoor air temperatures to bring on the boilers to work with the heat pumps or just the boilers by themselves.
A calculation can be carried out using the end user’s unit rate to determine the economic balance point – determining the outdoor air temperature at which heat pump operation becomes more expensive per kWh of heat than boiler operation.
Hybrid heating systems provide a technically sophisticated, economically rational pathway for decarbonising the UK’s existing building stock. By integrating heat pumps with boilers under intelligent control logic, operators can achieve lower operational costs, reduced carbon emissions, improved system resilience and a scalable route toward full electrification. www.rinnai-uk.co.uk
HEAT PUMPS AS A DECARBONISATION TECHNOLOGY
Heat pumps are widely promoted due to their ability to deliver more thermal energy than the electrical energy they consume. This performance advantage is quantified using the Coefficient of Performance (CoP), defined as:
Figure 1: Cost of energy per kWh of heat delivered.
By contrast, boiler efficiency is expressed as a percentage: