Meeting the challenges of low carbon heating

Hoval boilers in plant room

With combustion remaining a key source of heating for many buildings there is a great deal that manufacturers can do to mitigate associated environmental impact. Ian Dagley, General Manager at Hoval Ltd, discusses the opportunities. 

Now that all but a few die-hards accept the threat of climate change there is growing pressure on the building services industry to ‘do its bit’ in facing up to these challenges. Crucially, it is the expertise of the building services engineer that will ensure the various heating technologies available are applied in the most effective way.

For example, the UK government’s Heat and Building Strategy includes a heating upgrade programme for residential applications that strongly promotes heat pumps, alongside other low carbon technologies. There is also scope to make wider use of heat pumps in non-residential applications.

However, we need to be realistic about the limitations of heat pumps and play to their strengths alongside other options. As a company that manufactures a range of heating products, we are well aware of the strengths and weaknesses of each technology.

The appeal of heat pumps comes largely from their ability to provide heat with relatively low electricity consumption – ideally using electricity from renewable sources. There are limitations, though, to our renewable generation capacity, exacerbated by competition from other growing uses, such as electric vehicles.

Capacity is also influenced by weather conditions. Figures released by the Department for Business, Energy and Industrial Strategy (BEIS) show that renewable production generated 40.2% of the electricity generated in the UK as of December 2020 but this fell to 37.3% in the second quarter of 2021. The decline was a result of lower wind speeds and fewer sun hours, leading to greater reliance on gas-fired power generation – much of it from highly inefficient power stations.

In the first quarter of 2022 this contribution rose to 45.5%, the second highest quarterly share on record, yet still leaving a significant gap to be filled by other power generation technologies.

A further limitation, familiar to many MBS readers, comes from the low water temperatures at which heat pumps operate if they are to deliver a sensible Coefficient of Performance (CoP). Increasing output temperatures serves to reduce CoP so getting the right balance is essential.

One result of these lower water temperatures, compared to typical boiler outputs, is that existing heat emitters will not emit as much heat as they were designed to. This means that for heat pumps to be used as the sole source of space heating, heat emitters may need to be enlarged. In many (perhaps most) cases, the building’s thermal insulation will also need to be upgraded. The issue of lower flow temperatures also limits the potential for heat pumps in the industrial sector, as process heating typically requires high temperatures.

Moreover, many homes would require their electricity supply to be upgraded to make it suitable for heat pumps.

So there are limitations, but we shouldn’t lose sight of the fact that heat pumps still have an important role to play in the UK’s transition to net zero carbon. Their inclusion in a mixed heating system, for instance, can make significant inroads into carbon emissions.

For example, there are now air/water heat pumps entering the market that are designed specifically for high output applications in commercial and industrial applications, used on their own or – more likely - as part of a hybrid system alongside gas-fired condensing boilers.

It’s also worth noting that in industrial, logistic and ‘shed’ style retail premises, heat pumps can be incorporated into ventilation systems to provide highly efficient space heating and cooling.

Optimum efficiency, minimum carbon

For all of the reasons above, there are many commercial, industrial and residential applications where inclusion of combustion heat sources is the only way to generate the required temperatures. Therefore, it’s important to make combustion plant as efficient as possible, and also make sure that today’s boilers are ready for tomorrow’s alternatives to fossil fuels.

One key element in this respect is control. Whereas a traditional boiler installation could be operated efficiently using some fairly basic control strategies, a mixture of heat sources requires a more sophisticated control strategy.

Fortunately, newer control systems are designed to address this issue whilst also offering higher levels of modularity and connectivity – making it easier for end users and/or their sub-contractors to better understand their energy consumption.

Alongside improved control, boiler design needs to incorporate features that reflect their potential use in a mixed heating system, as well as improving their inherent efficiency.

For example, boilers in a mixed heating system will inevitably be firing less often and for shorter periods of time, often at part-load. Some new boiler designs, therefore, incorporate special technology in the heat exchanger to generate turbulence on the heating gas side. This turbulence increases heat transfer and boosts efficiency by as much as 20% compared to conventional gas boilers.

Crucially, when part of a mixed heating system, the boilers need to work with any flow/return temperature difference (Delta T) for easy integration with existing systems. This is particularly important for retrofit efficiency projects.

Also, we shouldn’t lose sight of the growing use of heat networks, driven by mixed source energy centres, where a maximum operating temperature of 95oC by the boiler is often necessary.

Fuel alternatives

Given that combustion for heating is with us for the foreseeable future, there is a strong imperative to find suitable alternatives to fossil fuels. Hydrogen and biomethane are the main contenders for replacing natural gas, and hydrogen was included in the Prime Minister’s 10-point carbon plan – the second point being to ‘drive the growth of low carbon hydrogen’.

Whilst the switch to 100% hydrogen may still be some way off there is every chance that many heating systems will be burning some hydrogen in the very near future. This assertion follows the announcement by the Energy Networks Association (ENA) – a trade association for energy companies – that hydrogen could be blended into the gas mains supply – up to around 20% by volume - by 2023. A similar approach is likely to be taken with biomethane.

In the case of oil-fired plant, a promising candidate appears to be hydrotreated vegetable oil (HVO). Recent research has shown HVO to be a direct ‘drop-in’ replacement for gas oil used in large combustion plant, and a partial ‘drop-in’ replacement for kerosene in residential heating systems, with some minor adjustments to the burner.

These changes have clear implications for specifiers and building operators considering new heating plant, as they will need to ensure a level of ‘future-proofing’ that can accommodate fossil fuel alternatives as they are gradually introduced.


Currently, there are several unknowns but it is clear that we will see many more mixed heating systems in non-residential applications, exploiting the benefits of heat pumps whilst mitigating their limitations. Over a longer period we can expect to see alternative fuels gradually replacing fossil fuels. As yet, we haven’t got precise timescales to work with so Hoval is proactively addressing these challenges with new boilers and heat pumps that will give end clients the future readiness they require.

Ian Dagley is General Manager at Hoval

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