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Department for Business, Energy and Industrial Strategy

Steam Boilers

1.1           Scope

Steam boilers are used to produce steam for process heating, space heating and water heating. They consist of a burner, a pressure vessel containing a heat exchanger, and associated burner control systems and boiler control equipment.

Steam boilers are available in a range of different designs and efficiencies. The Energy Technology List (ETL) Scheme aims to encourage the purchase of the higher efficiency gas and oil fired steam boilers, including products that are designed to use liquid and gaseous biofuels.

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

1.2           Definitions

Steam boilers are products that are specifically designed to convert water into pressurised steam by means of a burner that converts fuel into heat and a heat exchanger that transfers the heat into the water as it passes through the product.

1.3           Requirements

1.3.1       Eligibility requirements 

To be eligible, products shall:

  • Be gas and/or oil fired. 
  • Use an appropriately matched forced draught burner (or burners). 
  • Automatically respond to changes in steam demand by modulating their output in a continuous manner across a minimum specified turndown ratio, as set out in the performance criteria below, without initiating a purge cycle.
  • Conform to the requirements of The Pressure Equipment (Safety) Regulations 2016 in respect of their design, manufacturer and testing procedures, or have an appropriate Conformity Assessment mark.

To be eligible, products with a thermal input greater than or equal to 1MW, and less than 50MW, shall: 

  • Comply with the minimum requirements as stated in Annex II of the Medium Combustion Plant Directive (EU) 2015/2193[1] as implemented in domestic law. 

In addition, products with a thermal output in excess of 400kW shall either use burners from the “burners with controls” list on the ETL or

  • Incorporate a microprocessor based control system that continuously modulates burner output in response to measured boiler temperature or pressure values. 
  • Use fully pre-mixed burners, or forced draught burners. Where forced draft burners are used, automatic (electronic or pneumatic) air fuel ratio controls shall be fitted.
  • Where mechanical dampers are used to modulate the air flow to the burners, they shall be operated by a precision servomotor. The servomotor shall be controlled by a positional or flow-based feedback mechanism that automatically adjusts its operation to correct for mechanical wear, valve stiction and hysteresis. 
  • Where control valves are used to modulate the cc to the burners, they shall be operated by a precision servomotor. The servomotor shall be controlled by a positional or flow-based feedback mechanism that automatically adjusts its operation to correct for mechanical wear, valve stiction and hysteresis (this requirement shall not apply to pneumatically operated modulating gas valves). 
  • Where the product is gas fired or dual fuelled, use a variable speed motor controller (or variable speed drive) to operate each fan incorporated into the product that controls air flow rate to the burner and, where relevant, the fuel-air pre-mixer.  


1.3.2       Performance requirements

Products shall have a minimum net thermal efficiency of 92.0% at the full load and part load conditions set out in Table 1.1 below. 

Table 1.1    Performance test points for steam boilers​​​

Fuel Type

Turndown ratio

Test point % MCR

Net thermal efficiency %

Gas fired or dual fuelled




≥ 92.0%


≥ 92.0%


Oil fired




≥ 92.0%


≥ 92.0%

">=" means "greater than or equal to" Where MCR is the product’s maximum continuous rating (MCR)

Suppliers shall report the following, which will be published on the ETL Product Search: 

  • The model’s operating pressure (bar) when the measured net thermal efficiency value was recorded.
  • Whether when tested the model achieved the minimum net thermal efficiency at full and part load conditions (Table 1.1) using an installed economiser.   

1.4           Measurement and Calculations

1.4.1       Measurement Standards and Test Requirements

The required minimum performance shall be demonstrated using Method A, Method B or Method C, (as set out in, and below), which are subject to the following restrictions: 

  • Method A shall only be used, where all burners incorporated in the product are listed on the “burners with controls” list on the ETL. 
  • Method B shall be used to demonstrate the performance of modular boilers. A modular boiler is defined as an assembly of two or more similar (but not necessarily identical) modules, each with their own a heat exchanger, burner, and control and safety devices. The assembly has common water feed and steam output connections, but the water flow to, and steam flow from each module is independently controlled. 
  • Method C may only be used for products with rated outputs above 600kW, where it is not possible to measure product performance in a laboratory due to product size. 

All performance measurements shall be carried out in accordance with the procedures set out in one of the following test standards: 

  • BS 845: Part 1:1987 ‘Methods for assessing thermal performance of boilers for steam, hot water and high temperature heat transfer fluids — Part 1: Concise procedure’. 
  • BS EN 12953-11:2003 ‘Shell boilers — Part 11: Acceptance tests’. 

Where BS 845- 1:1987 is used, the following standard test conditions shall be observed: 

  • A maximum ambient air temperature of 25ºC. 
  • An excess combustion air level of not less than 15%. 

1.4.2       Rounding 

For the avoidance of doubt net thermal efficiency test data shall be presented to 1 decimal place. As an example, a product with a net thermal efficiency of 91.9% at 100% of its maximum continuous rating (MCR) would be deemed to be a fail. 

1.5           Verification for ETL Listing

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

  • In-house testing – Self-certified 
  • In-house testing – Self-tested and verified or cross-checked by an independent body 
  • Witnessed testing 
  • Independent testing 
  • Acceptance Tests or Field Trials 
  • Representative testing (see clause 1.5.1) 

Further information regarding the first three routes can be found in the ETL Testing Framework.

1.5.1       Representative testing

Where applications are being made for products of the same constructional design 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 three times, or less than one third of, 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. 

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.      Method A – separate testing of boilers and burners

Under this test method:

  1. Boiler and burner performance are demonstrated separately. 
  2. Boiler performance can be assessed using any burner (or burners) that can provide the heat input and operational stability needed to complete the test. 
  3. The boiler’s net thermal efficiency at 100% of product’s maximum continuous rating (MCR) in accordance with the procedures in one of the specified standards. 
  4. The boiler’s net thermal efficiency at part load (30%) is then inferred from burner performance data and design calculations of burner/boiler matching.      Method B - Integral testing at full and part loads

Under this test method, overall product performance shall be demonstrated by: 

  1. Measuring the net thermal efficiency at the test points specified in Table 1.1, in accordance with the procedures in one of the specified test standards.      Method C – validated design calculations

Under this test method:

  1. The product’s net thermal efficiency at the full and part load conditions specified in Table 1.1 is determined from design calculations. 
  2. The accuracy of these design calculations shall be confirmed by interpolation and extrapolation of measurements, obtained using the procedures in one of the specified test standards, of the product’s net thermal efficiency: 

          a)    At least one test point between 60% and 100% MCR, and
          b)    At least one test point between 20% and 40% MCR. 

The test report shall include (or be accompanied by): 

          a)    Details of the calculations used to determine product performance. 
          b)    A copy of the published performance data for the product. 
          c)    Manufacturer’s design data for the product. 
          d)    The following test data, which shall be obtained with the product operating under stable conditions at each selected test point: 

                             I. Analysis of flue gas composition, including as a minimum levels or oxygen or carbon                                          dioxide and carbon monoxide.

                             II. Ambient and flue gas temperatures. 

                             III. Total conductive, convective and radiative loss rate. 

                             IV. Net thermal efficiency. 

e)    Details of the burners used during the test. 

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 introducing product categorisation into the criteria, to distinguish between small and large steam boilers, thereby allowing appropriate turndown ratios to be introduced for the size of the boiler.  

The next technical review will also consider expanding the scope of the criteria to include water tube steam boilers.

[1] The Environmental Permitting (England and Wales) (Amendment) Regulations 2018,
The Pollution Prevention and Control (Scotland) Amendment Regulations 2017,
The Pollution Prevention and Control (Industrial Emissions) (Amendment) Regulations (Northern Ireland) 2018.