Heat pump or Gas boiler?

What are the decision factors for heating and hot water systems for large buildings?

For many years natural gas boilers have been the option of choice for providing heat in buildings of all types. They are cheap, reliable, easy to maintain and economical to run. However even with the widespread use that we see today they are often installed on poorly designed systems that do not enable the boiler to achieve its efficiency potential. Additionally, as the electricity grid de-carbonises, gas boilers are now a higher carbon heat source when compared to other technologies.

Combined heat and power generation units, generally reciprocating engines at the relevant scale, have been the low carbon choice over the last 10 years or so for larger buildings. However, even these are becoming unsustainable in terms of electricity carbon intensity when compared to a heat pump supplied with low carbon electricity.

The reality is that the grid is de-carbonising (for example, through increasing wind/solar farms and decreasing electricity usage) faster than is reflected in the calculations used by regulating bodies. Therefore the actual real world carbon performance of technologies is not reflected in decisions made using standard regulatory calculations. This means that when choices of technology (i.e. heat pump or boiler) are made, the actual carbon performance in real world conditions is not considered.

It can be seen on the graph below that around this year it is expected that a cross over point is reached and that even a modestly well performing heat pump will out perform a boiler in carbon emissions.

As global temperatures rise and we experience greater numbers of days at extremes, the requirement for cooling will also increase. A well designed heat pump system can also provide cooling. Combined with thermal storage, it becomes a highly efficient system both for primary energy cost and carbon emissions. This type of system is prevalent in Europe, with many being installed successfully.

Installing a heat pump? There are generally three challenges;

1. Space – heat pumps and associated equipment are generally larger, especially ground collectors and thermal storage. However ground collectors can be situated under fields if boreholes are used. Thermal stores can be weather proofed and sited outside.

2. Capital cost – The purchase and installation costs are significantly higher than gas boilers. The general solution to this is to arrange a finance package based on repayments that do not exceed expected Opex (e.g. green deal) and many companies offer this. Buildings such as hospitals, schools, libraries etc. have the advantage that they are not going to change use or ownership and will use consistent amounts of energy and so should be able to attract favourable terms. Alternatively a community energy scheme could be considered.

3. Expertise – Heat pumps are new to most M&E companies; who do not have the expertise to correctly design the system. It is vital that the system (central heating and hot water generation) are designed to support the efficient operation of the heat pump. Expertise is available from specialist companies who should be given the authority to dictate design decisions.

The ideal situation is for the implementation of a heat pump to be considered at the very start of a project, as once past the procurement of the main contractor, incorporating at this stage can be very challenging and costly. 

Installing a gas boiler?  Why not commit to a 100% green gas tariff? This would make the heat and hot water very low carbon.  Additionally, why not consider designing the system in a low temperature format? This means that in the future there is the possibility to change to a heat pump should this become an option.

Green gas originates from bio-digestors and in the future is likely to be supported by hydrogen from renewable electricity plants (for example, excess wind power is converted to hydrogen at source and injected in to the gas grid). However there are some ethical concerns regarding the source of the bio-digester feedstock; where it is grown specifically for gas production and in competition with food. Bio-gas is sold using a market of certification of inputs and outputs, the supply is limited and price likely to rise given the expected increase in demand. Whilst green gas presents an easier choice now it may be open to challenge and increased costs in the future. 

(Tim Rook, previously Tech Director, Woodpecker Biomass Boilers, Director, EcoFirst Renewbles,  Head of design E.ON)