16 June 2026
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Pete Seddon, Rinnai’s Head Of Technical, details the steps and logic for ensuring maximum practical, economic & technical results for commercial and residential hybrid heating hot water delivery systems.
Rising fuel costs, volatile energy markets plus the prices of appliances and installations in the UK as well as many other countries are all part of the equation making the achievement of NetZero a very difficult goal. One of the areas the UK is looking to decarbonise is the Heating and Hot Water delivery sector.
Heating and hot water in buildings account for approximately 37% to 38% of total UK carbon emissions (or around 32% of all greenhouse gases). Direct emissions from heating homes and commercial buildings specifically make up about 17% to 23% of the UK's total emissions. Legislation has moved towards electrification for new buildings but these only account for a small percentage of the UK’s building stock with the vast majority being existing buildings.
These buildings are going to be the hardest area to decarbonise, as simply removing gas altogether can pose challenges. Factors such as building suitability (insulation), available capacity (electrical), available space and cost (equipment and operational) are all obstacles to overcome for existing buildings.
One technology marketed as a solution to help achieve decarbonisation are Heat Pumps. These come as Air Source, Water Source or Ground Source with Air Source being the most prominent. The main reasons legislation advocates for Heat Pumps is because they can generate more heat output per unit energy which makes them more efficient than a gas burning appliance, and in addition, the UK is setting targets to decarbonize the national grid. One of the biggest issues heat pump systems faces is not the product itself or efficiency but the cost per unit of electricity versus gas.
There is a general opinion that although heat pumps have a far superior efficiency than gas appliances, they generally cost a lot more to run. I will provide a table of some examples below to demonstrate that this is not unduly true, heat pumps can be cheaper to operate but when they aren’t, the difference in cost is minimal.
Heat Pumps efficiency is shown as COP (Coefficient of Performance). This is the useful energy out / energy consumed to operate.
Boiler efficiency is shown as a percentage and uses a similar calculation, useful energy out / energy consumed.
Figure 1
* Highlighted section is a representation of modern condensing boiler efficiencies.
What this table shows is the higher the COP, the less an end user/consumer will pay for electricity per kWh of heat delivered to the system. It also portrays that as the boiler efficiency decreases, the cost of gas per kWh of heat delivered increases.
There are two main factors that affect the COP of a heat pump, the set flow temperature and the outside air temperature. Generally, the higher the flow temperature the lower the COP and the colder the outside air temperature is the lower the COP.
On the other hand, with boilers, efficiency will reduce as the flow temperature increases. Typical existing buildings operate at 80 degrees flow and 60 degrees return temperature. A boiler’s efficiency can range from as low as 70% for older models to over 90% for modern condensing boilers that recover heat from flue gases below 55 degrees, operate using advanced burner technologies and have smart controls integrated to avoid overheating.
To make buildings (systems) more efficient we are seeing these operating temperatures reduce to a level which maximises appliance efficiency. To achieve these lower temperatures, buildings may require additional works to insulation and heat emitters (radiators).
This reduction in temperature benefits both Heat Pumps and Boilers with their respective efficiencies, where flow temperature and COP will dictate whether heat pumps are more cost effective than boilers, as seen by figure 1.
For example, let’s consider a typical Care Home with a peak heat demand of 100kW, operating its heating system throughout the year (except for the hotter months).
If the Heat Pumps are used in a Hybrid system with boilers, we can maximise the efficiency of both technologies with effective design and control. This can be achieved by having the system operate at lower temperatures and by allowing the heat pumps to remotely enable the boilers when required.
Predetermined outdoor air temperatures can be set up to activate the boilers to work in conjunction with the Heat Pumps or solely to enable the boilers by themselves. These predetermined temperatures can be set up to optimize the running cost of the system as the COP of the heat pump reduces at lower temperatures. The three main working conditions of the hybrid system will be:
- External ambient temperature is high, heat pumps sized to operate alone as cost per unit of heat delivered is best.
- As external temperature starts to drop, the heat pump will enable the boiler to work in conjunction with the heat pump until the price gap becomes too large.
- At this point, the heat pumps will turn off and the boiler will solely operate as it’s the most cost-effective technology to meet the heating demand.
This allows end users to continue our collective journey to decarbonisation by introducing heat pumps, while maintaining back up boilers for instances when operational cost would be too high.
www.rinnai-uk.co.uk
