Hybrid Heating System Controls: A Technical Framework for Cost Optimised Decarbonisation

Pete Seddon Head of Technical at Rinnai looks in detail at getting the max performance in terms of performance and economy of a Hybrid Heating Hot Water System.

Join Rinnai’s free newsletter to learn more about new products & systems, case histories and legislation developments.

Sign Up to Newsletter

The transition toward Net Zero is reshaping energy infrastructure across the UK and globally. As national policy accelerates decarbonisation, consumers and building operators & 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 (H&HW) 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 new‑build 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.

• Heat Pumps as a Decarbonisation Technology

Heat pumps (air‑source, water‑source, and ground‑source) 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:

By contrast, boiler efficiency is expressed as a percentage:

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.

2. Temperature Sensitivity and System Efficiency

2.1  Heat Pump Performance Variables

• Heat pump efficiency is highly sensitive to:

• Outdoor air temperature (for ASHPs)
• Required flow temperature to meet building heat demand

As outdoor air temperature decreases or the flow temperature increases, the compressor must work harder, in turn reducing the COP.

2.2 Boiler Efficiency Variables

Whilst the variables that affect the efficiency of boilers are:

• Higher return temperatures (>55–60°C) prevent condensing operation
• Lower system temperatures improve efficiency

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 at lower temperatures for heat pumps.

3. 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.

3.1 Why Hybridisation Works

• Hybrid systems:
• Reduce reliance on high‑carbon fuels without requiring full electrification
• Optimise operational cost by dynamically selecting the most efficient heat source
• Avoid expensive electrical upgrades
• Maintain resilience and redundancy
• Enable staged decarbonisation rather than disruptive system overhauls

This approach is particularly advantageous for buildings with:

• High peak heat loads
• Limited electrical capacity
• Poor insulation
• Space constraints
• Continuous heating requirements (e.g., care homes)

4. Technical Control Strategy for Hybrid Systems

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.

Above this temperature:

• Heat pump operates as primary heat source
• Boiler remains off or provides peak‑load support

Below this temperature:

• The Heat Pump and Boiler can operate together or
• Boiler takes over as primary heat source
• Heat pump may shut down or provide limited pre‑heat

This ensures the system always operates at minimum cost per delivered kWh.

5. Example: 100 kW Care Home Application

A care home with a continuous heating requirement provides a representative case study.

5.1 Operational Characteristics

• High annual heat demand
• Long heating season
• Need for reliability and redundancy
• Often limited electrical capacity

5.2 Hybrid System Benefits

• Heat pumps handle most of the heating during mild and moderate weather
• Boilers provide peak‑load support during cold spells
• System temperatures can be reduced to improve overall efficiency
• Operators gain predictable running costs and reduced carbon intensity

6. Strategic Advantages of Hybrid Controls

• Dynamic control ensures the lowest‑cost heat source is always prioritised using predetermined set points.
• Heat pumps operate during periods of high COP and lower grid carbon intensity.
• Hybrid systems reduce peak electrical demand, supporting national decarbonisation goals.
• As insulation improves or tariffs shift, control parameters can be updated without replacing equipment.

It can allow end users to gradually switch to a point of use carbon free system.

Conclusion

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
• A scalable route toward full electrification

This approach acknowledges the practical constraints of existing buildings while enabling meaningful progress toward Net Zero.

Join the free Rinnai newsletter for updates on technology and policy​.

Sign Up to Newsletter

Rinnai offers clear pathways to lower carbon and decarbonisation
Plus customer cost reductions for commercial, domestic
And off-grid heating & hot water delivery and off-grid heating & hot water delivery

• Rinnai’s range of decarbonising products - H1/H2/H3 - consists of hot water heating units in gas/BioLPG/DME, hydrogen ready units, electric instantaneous hot water heaters, electric storage cylinders and buffer vessels, a comprehensive range of heat pumps, solar, hydrogen-ready or natural gas in any configuration of hybrid formats for either residential or commercial applications. Rinnai’s H1/2/3 range of products and systems offer contractors, consultants and end users a range of efficient, robust and affordable low carbon/decarbonising appliances which create practical, economic and technically feasible solutions. 

• Rinnai is a world leading manufacturer of hot water heaters and produces over two million units a year, operating on each of the five continents. The brand has gained an established reputation for producing products that offer high performance, cost efficiency and extended working lives. 

• Rinnai products are UKCA certified, A-rated water efficiency, accessed through multiple fuel options and are available for purchase 24/7, 365 days a year. Any unit can be delivered to any UK site within 24 hours.

• Rinnai offer carbon and cost comparison services that will calculate financial, and carbon savings made when investing in a Rinnai system. Rinnai also provide a system design service that will suggest an appropriate system for the property in question.

• Rinnai offer comprehensive training courses and technical support in all aspects of the water heating industry including detailed CPD’s.

• The Rinnai range covers all forms of fuels and appliances currently available - electric, gas, hydrogen, BioLPG, DME solar thermal, low GWP heat pumps and electric water heaters More information can be found on Rinnai’s website and its “Help Me Choose” webpage. 

Rinnai full product availability 24/7 for next day delivery of all hot water heating unit models including 48-58kW units.

Average savings of:

20% REDUCTION of Opex cost

30% REDUCTION of initial cost

15% REDUCTION in carbon

75%    REDUCTION of space

For more information on the RINNAI product range visit our website.

Visit Rinnai