Understanding Efficiency

System Efficiency or Boiler Efficiency?

One common issue is a widespread confusion associated with boiler efficiency and system efficiency:

• Boiler efficiency: Is how efficient the boiler is at converting the energy stored in the delivered fuel to useful energy, by transferring heat from combustion to hot water in the heat exchanger of a boiler. This can be around 90%. Read more on boiler efficiency.

• System efficiency:  Overall system efficiency includes boiler (or combustion) efficiency but also all other system components which hold, move or exchange hot water to the point of use.  This is usually no higher than about 75%. The three main factors that determine overall system efficiency are:
  • Boiler/combustion related losses (read more on boiler efficiency)
  • Plant room losses (read more on plant room efficiency)
  • District heating network losses (read more on district heat networks efficiency)

It is important to use real world system efficiency when predicting the amount of RHI income, running costs and overall savings and business case, not the boiler combustion efficiencies

Biomass installers and suppliers can use figures for boiler efficiency rather than system efficiency as part of the sales process as it improves the economic case.  Keep an eye out for this!  Any advice that suggests biomass systems will operate at 85% efficiency or higher should be treated with caution.

Levels of Expected Performance

Discounting district heating losses (as they are highly site specific), the average performance standard is 76.75% (equal to 23.25% overall losses) with a range from 72% at worst to 81.5% at best. It is important to note that these estimates are not very different from how a gas, oil or coal fired heating system would perform in a similar situation.

These figures assume that the systems are well designed, properly optimised and adequately maintained. These figures can be regarded as the performance standard to aim for by all installers and operators.

Further Information:

• Find out about including performance standards in procurement contracts
• For more details on the performance of biomass systems see ‘Desk based review of performance and installation practices of biomass boilers‘, published by the Department of Energy and Climate Change in 2014.