Hybrid heating systems are a fairly logical step to reducing reliance on fossil fuels and carbon emissions, even though until now electrification has rather been assumed to be the way in which tighter Part L requirements will be met in future. However, this looks potentially set to change with the Department for Business, Energy and Industrial Strategy (BEIS) evaluating hybrid heating systems and their potential to assist with heat decarbonisation – electric heating is no longer seen as a ‘silver bullet’, plus the prospect of bio-methane and hydrogen could see the hybrid system have greater longevity than thought after natural gas is no longer in use.

A hybrid heating system is simply a renewable energy source, most usually an air source heat pump, coupled with a boiler – and the controls automatically switch between the two, to ensure that the most efficient heat source is always in use.

Although a heat pump runs on electricity, it produces around three to four times the amount of energy it consumes, which is 300-400% efficiency, compared to the 94% efficiency of a condensing gas boiler – and of course this means that the cost of running a heat pump is much lower than a gas or oil boiler for the same amount of heat produced, as well as carbon emissions being reduced significantly.

The addition of a boiler to work in tandem with the heat pump reduces some of the challenges posed by a system solely featuring a heat pump. During the colder months a heat pump loses efficiency – it has to work harder to extract the same amount of heat from cooler air – while some households find that a heat pump doesn’t always suit their needs, perhaps because the property is large, or because their demands are high (there are also some people who are uncertain about sole reliance on a heat pump, and peace of mind can be valuable in itself).

It’s also worth noting that a hybrid heating system should have greater longevity than the conventional boiler-fired system lifespan of 10-15 years – with the two appliances sharing the load, the boiler doesn’t have to work so hard, and so remains in better condition for longer.

Although gas boiler hybrid heating systems are available, the low cost of gas means this type of bivalent system is currently unlikely to be a common choice in areas connected to mains gas – but the combination of a heat pump and an oil-fired boiler is a very realistic option for meeting Part L when building off-grid homes (not to mention the cost savings involved, and potentially RHI payments).

New-build homes are the most likely candidates for a hybrid heating system – rather than a retrofit in a property which has previously had a gas- or oil-fired system – because there is a difference in the supply temperature between a boiler (80˚C) and an efficiently running heat pump (45˚C maximum). This means that the heat emitters, the radiators and/or underfloor heating, need to be designed accordingly – and this is more cost-efficient when starting from scratch than when working with an existing system. The most efficient hybrid heating system is designed on the basis of the heat  pump output temperature as the ‘norm’,  with the boiler just there to provide additional heat output on the coldest days, and offer some back-up/comfort for those clients who are more wary of heat pump technology.

Of course, this doesn’t mean that hybrid heating systems aren’t suitable for replacement heating systems in existing houses, but it does mean that the heat emitters and distribution pipework would need to be carefully reviewed, and potentially replaced or amended, during the installation process. For example, if we use the supply temps of 80˚C for oil boilers and 45˚C for heat pumps, we would only get 40% of the design heat output from the radiators when using the heat pump, because it’s most likely that the radiators were designed to operate at 80˚C – and it’s unlikely that the occupants would find this acceptable!

To find out more about how we can help you to choose the best off-grid solution for a property (or properties), or you would like advice on the design of hybrid heating systems, please call us on 01206 266755 or email mail@ajenergy.com.