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Department for Business, Energy and Industrial Strategy

Biomass Boilers

1.1           Scope

Biomass boilers are used to heat water for process or space heating. 

Biomass boilers are available with a wide range of efficiencies. The Energy Technology List (ETL) Scheme aims to encourage the purchase of products with the highest thermal efficiency. 

The fuels used in biomass boilers are renewable so their use will also reduce the amount of fossil fuel that might otherwise have been consumed. 

To be eligible for inclusion on the ETL, products shall meet the requirements as set out below. 

1.2           Definitions

Biomass boilers are products that are specifically designed to burn solid biomass fuels in order to heat water.  

1.3           Requirements

1.3.1       Eligibility requirements 

To be eligible, products shall: 

  • Be designed to burn wood and solid fuels derived from them. 
  • Heat water for process or space heating. 
  • Automatically respond to changes in hot water demand by modulating their output in a continuous manner without initiating a purge cycle. 
  • Have an appropriate Conformity Assessment mark. 
  • Meet the following air quality emission limits: 

          - Particulate matter (PM) emissions shall be ≤50mg/m3
          - Oxides of nitrogen (NOx) emissions shall be ≤200mg/m3
          - Carbon monoxide (CO) emissions shall be ≤500mg/m3
          - Organic Gaseous Compound (OGC) emissions shall be ≤20mg/m3

Conformity with these emissions limits shall be demonstrated by provision of valid test reports, in line with section 1.5.  

1.3.2       Performance Requirements

Eligible products with a rated heat output of 500kW or less shall meet or exceed the Seasonal Space Heating Energy Efficiency (ηs) value, as defined by Ecodesign Commission Regulation (EU) 2015/1189, of 80%.  

Eligible products with a rated heat output of more than 500kW shall meet or exceed the minimum thermal efficiency, when tested at an output that is between 60% and 100% of Maximum Continuous Rating (MCR), of 90%, based on the net calorific value of the test fuel. 

1.3.3       Information requirements

Suppliers shall report the following parameters for each model, which will be published on the ETL Product Search: 

  • The turndown ratio

1.4           Measurement and Calculations

1.4.1       Energy Efficiency Metrics 

Seasonal Space Heating Energy Efficiency (ηs) – the ratio between the space heating demand for a designated heating season supplied by a biomass boiler and the annual energy consumption required to meet that demand, expressed in %, as defined by Ecodesign Commission Regulation (EU) 2015/1189.

1.4.2       Test Requirements & Measurement Standards

1.4.2.1     Energy

Products with a rated heat output of 500kW or less shall be tested in accordance with: 

  • EN 303-5:2012 “Heating boilers for solid fuels, hand and automatically fired, nominal heat output of up to 500 kW. Terminology, requirements, testing and marking”. 
  • The tests shall be done using a biomass test fuel (designated A, B1, B2, C & D) in accordance with Table 7 of EN 303-5:2012 that is appropriate to the advertised usage of the product. 

Products with a rated heat output of 500kW or more shall be tested in accordance with: 

EITHER 

  • BS 845-1:1987 “Methods for assessing thermal performance of boilers for steam, hot water and high temperature heat transfer fluids: Concise procedure”. 

OR (for shell boilers only) 

  • BS EN 12953-11:2003 “Shell boilers — Part 11: Acceptance tests”.

OR (for water tube boilers only) 

  • BS EN 12952-15:2003 “Water-tube boilers and auxiliary installations. Acceptance tests”. 

OR 

  • The testing procedures set out in EN 303-5:2012. 

The tests shall be done using a biomass test fuel (designated A, B1, B2, C & D) in accordance with Table 7 of EN 303-5:2012 that is appropriate to the advertised usage of the product. 

Where BS 845-1:1987 is used, the standard test conditions are: 

  • A maximum ambient air temperature of 25 degrees Centigrade. 
  • An excess combustion air level certified as being representative of normal commercial operation. 
  • The boiler shall be operating at a rating of at least 60% of its maximum continuous rating (i.e. 60 – 100% MCR) during the tests. 

As an alternative to measurement of losses other than flue gas losses, a standard deduction of 2.0% x 100 % load may be used. 

1.4.2.2     Emissions

Testing for emissions of PM, NOx, CO and OGCs shall be carried out in accordance with the relevant provisions specified in EN 303-5: 2012.

Testing shall be carried out in accordance with:

  • PM: EN 13284-1: 2017 or BS ISO 9096: 2017
  • NOx: BS EN 14792: 2017
  • CO: BS EN 15058: 2017 or PD CEN/TS 17337: 2019
  • OGCs: BS EN 12619: 2013 or BS EN ISO 13199: 2012

1.4.3       Rounding 

For the avoidance of doubt Seasonal Space Heating Energy Efficiency and net thermal efficiency test data shall be presented to one decimal place. As an example, a biomass hot water boiler with a rated heat output above 500kW and a net thermal efficiency of 89.9% when tested at between 60% and 100% of its maximum continuous rating (MCR) would be deemed to be a fail. 

1.5           Verification for ETL Listing

For products up to and including 500kW all tests shall be carried out by, or witnessed by, an accredited laboratory, where “accredited” means accredited by the United Kingdom Accreditation Scheme (UKAS), or the International Accreditation Forum, or the International Laboratory Accreditation Co-operation (ILAC) agreements. 

For products above 500kW, products can be either tested in an accredited laboratory OR performance may be determined from measurements made during field trials or acceptance tests, provided that the measurements have been made by, or witnessed by, an accredited laboratory or contractor that is accredited to make those measurements. The product’s net thermal efficiency shall be calculated by an independent body that is competent to verify the measurement data.

Any of the following testing routes may be used to demonstrate the conformity of products against the requirements:

  • Witnessed testing (test reports shall be prepared by, or verified by, an independent accredited test laboratory) 
  • Independent testing (test reports shall be prepared by, or verified by, an independent accredited test laboratory) 
  • Acceptance Tests or Field Trials (category 2&3 only; test reports shall be prepared by, or verified by, an independent accredited test laboratory) 
  • Representative testing (see clause 1.5.1) 

Further information regarding the first three routes can be found in Guidance Note 5 on the ETL product testing framework[1].

1.5.1       Representative testing

Where applications are being made for products of the same constructional design, which are less than or equal to 500kW, to be included on the ETL, the type testing procedures set out in Annex F of BS EN 303-3:1999 or section 5.1.4 of EN 303-5:2012 or Annex C.2.1 of BS EN 304:1992 (as amended) may be used to select representative models for testing and to reduce the overall number of performance tests that shall be completed.

Where applications are being made for products of the same constructional design, which are greater than 500kW, to be included on the ETL, test data may be submitted for a single representative model provided that the maximum rated output of the products being applied for is not more than twice, or less than half, the maximum rated output of the product tested. Where the range of rated outputs exceeds these limits, products should be grouped into size ranges that comply with these rules, and test data submitted for one representative model for each group.

Where representative testing is used, details of the design calculations and data used to predict the performance of products that have not been tested shall be submitted.

It should be noted that:

  • If a manufacturer voluntarily removes a representative model from the ETL then other products linked with that representative model may or may not be permitted to remain on the ETL. 
  • If any product submitted under these representative model rules is later found not to meet the performance criteria when independently tested; then all products based on the same representative models will be removed from the ETL.

1.6           Conformity testing

Products listed on the ETL may be subject to the scheme’s conformity testing programme in order to ensure listed models continue to meet the ETL requirements.  

1.7           Review

1.7.1       Indicative review date

This specification is scheduled for review during the 2024/25 review cycle. 

1.7.2      Illustrative future direction of the requirements 

The next technical review will consider performance thresholds for energy efficiency and emissions as well as establishing turndown ratio as a performance requirement.


[1] https://www.gov.uk/government/publications/energy-technology-list-etl-product-testing-framework