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Hot Water Boilers

1.1           Scope

Hot water boilers are used to produce hot water for space heating, process heating and domestic uses. They are available in a wide range of different designs and efficiencies.

The Energy Technology List (ETL) scheme aims to encourage the purchase of the higher efficiency, modulating, gas and oil fired hot water boilers, including products that are designed to use liquid and gaseous biofuels.

The ETL Scheme covers four categories of products:

1.    High temperature, high pressure, high efficiency hot water boilers with rated outputs greater than 400kW.

Boilers designed to operate with a water pressure greater than 6 bar and/or outlet water temperature greater than 105°C, and that are not designed to recover latent heat from flue gases by condensing water vapour.

2.    Low temperature, low pressure, high efficiency hot water boilers with rated outputs greater than 400kW.

Boilers designed to operate with a water pressure up to and including 6 bar and/or an outlet water temperature up to and including 105°C that are not designed to recover latent heat from flue gases by condensing water vapour.

3.    Condensing hot water boilers with rated outputs greater than 70kW:

Boilers designed to recover latent heat from flue gas water vapour.

4.    Condensing hot water boilers with rated outputs up to 70kW: Boilers designed to recover latent heat from flue gas water vapour.

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

1.2           Definitions

Hot water boilers are products that are specifically designed to heat water by means of a heat exchanger that transfers heat from combustion 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 (where gas includes biogas and oil includes liquid biofuels).
  • Automatically respond to changes in hot water demand by modulating their output in a continuous manner across a minimum specified turndown ratio, as set out in Table 1.1 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[3] as implemented in domestic law.

 ​​​In addition, products with a rated output greater than 400kW shall either use burners from the “burners with controls” part of the Energy Technology Product List 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 fuel 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. (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.
  • Provide product performance specification data in line with ETL Guidance Note 13.

Products with a rated output of up to 400kW shall be fitted with integrated burners.

For category 1 and 2 products, the respective thresholds as detailed in Table 1.1 may be achieved with or without an economiser designed to recover specific heat from the exhaust flue gas.

1.3.2       Performance requirements

Eligible products shall meet or exceed the minimum thermal efficiencies set out in Table 1.1 below at the specified part and full load test conditions, which vary according to product category and fuel type.

Table 1.1    Performance requirements and test points for hot water boilers

 

Product Category

Fuel Type

Turndown ratio

Test point

(% of Maximum Nominal Output)

Gross thermal efficiency %

Seasonal Space Heating Energy Efficiency %[1]

 

1.

High temperature, high pressure, high efficiency hot water boilers

Gas, oil or dual fuelled

≥ 3.33:1

30

≥ 83.8%

 

 

100

≥ 83.0%

 

2.

Low temperature, low pressure, high efficiency hot water boilers

Gas, oil or dual fuelled

≥ 3.33:1

30

≥ 83.8%

 

 

100

≥ 83.0%

 

3.

Condensing hot water boilers >70kW

Gas fired or dual fuelled

≥ 3.33:1

30

≥ 97.3%

 

 

100

≥ 87.4%

 

 

 

Oil fired

30

≥ 94.6%

 

 

100

≥ 89.0%

 

 

4.

Condensing hot water boilers ≤70kW

Gas, oil or dual fuelled

≥ 3.33:1

 

 

≥ 93%

 

"" means "greater than or equal to"

"" means "less than or equal to"

1.4           Measurement and Calculations

1.4.1       Measurement Standards and Test Requirements

Product performance shall be demonstrated using Method A, Method B or Method C (as set out in 1.4.1.2, 1.4.1.3 and 1.4.1.4 below), which are subject to the following restrictions:

  • Method A shall only be used, where all the burners incorporated in the product are listed on the “burners with controls” part of the Energy Technology Product List.
  • Method B shall be used to demonstrate the performance of modular boilers, or where any of the burners incorporated in the product are not listed on the “burners with controls” part of the Energy Technology Product List. A modular boiler is defined as an assembly of two or more similar (but not necessarily identical) modules, each with their own heat exchanger, burner, and control and safety devices. The assembly has common water feed and return connections, but the water flow to, and flow from each module is independently controlled.
  • Method C may only be used for category 1 and 2 boilers with rated outputs above 400kW and category 3 boilers with rated outputs above 900kW, 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, or more, of the test standards recognised by the ETL as set out in Table 1.2, or in accordance with equivalent procedures for assessing gross thermal efficiency within applicable British Standards. The selected test standard(s) shall be appropriate to the specific type of boiler tested.

Table 1.2    ETL recognised test standards

 

Test standard

Applicable product categories

1

2

3

4

BS EN 625:1996 ‘Gas-fired central heating boilers. Specific requirements for the domestic hot water operation of combination boilers of nominal heat input not exceeding 70kW’

 

 

 

þ

BS EN 483:1999+A4:2007 ‘Gas-fired central heating boilers. Type C boilers of nominal heat input not exceeding 70kW’

 

 

 

þ

BS EN 677:1998 ‘Gas-fired central heating boilers. Specific requirements for condensing boilers with a nominal heat input not exceeding 70kW’

 

 

 

þ

BS EN 13836:2006 ‘Gas fired central heating boilers. Type B boilers of nominal heat input exceeding 300kW, but not exceeding 1 000kW’

 

 

þ

 

BS EN 15417:2006 ‘Gas-fired central heating boilers. Specific requirements for condensing boilers with a nominal heat input greater than 70kW but not exceeding 1000kW’

 

 

þ

 

prEN 15420:2005 ‘EN 15420. Gas-fired central heating boilers. Type C boilers of nominal heat input exceeding 70kW, but not exceeding 1000kW’ (CEN document code 06/30144913 DC)

 

 

þ

 

BS 845-1:1987 ‘Methods for Assessing thermal performance of boilers for steam, hot water and high temperature heat transfer fluids — Part 1: Concise procedure’.

þ

þ

þ

þ

BS 7190:1989 ‘Method for assessing thermal performance of low temperature hot water boilers using a test rig’

 

þ

þ

þ

BS EN 303-3:1999 ‘Heating boilers — Part 3: Gas-fired central heating boilers — Assembly comprising a boiler body and a forced draught burner’.

 

þ

þ

þ

BS EN 303-7:2006 ‘Heating boilers — Part 7: Gas-fired central heating boilers equipped with a forced draught burner of nominal heat output not exceeding 1,000 kW’.

 

þ

þ

þ

BS EN 304:1992 ‘Heating boilers — Test code for heating boiler for atomising oil burners’ (as amended).

þ

þ

þ

þ

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

þ

þ

þ

þ

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

þ

þ

þ

þ

BS EN 14394:2005+A1:2008 “Heating boilers. Heating boilers with forced draught Burners. Nominal heat output not exceeding 10 MW and maximum operating temperature of 110°C”.

þ

þ

 

 

BS EN 15502-1:2012+A1:2015 “Gas-fired heating boilers. Part 1: General requirements and tests”

 

 

þ

þ

BS EN 15502-2-1:2012 “Gas-fired central heating boilers. Specific standard for type C appliances and type B2, B3 and B5 appliances of a nominal heat input not exceeding 1000 kW”

 

 

þ

þ

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%.

1.4.1.2      Method A - separate testing 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 gross thermal efficiency at full load (100%) shall be measured in accordance with the procedures set out in an ETL recognised standard (Table 1.2).

4.    The boiler’s gross thermal efficiency at part load (30%) is then inferred from burner performance data and design calculations of burner/boiler matching.

1.4.1.3      Method B – integrated testing at full and part loads

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

1.    Measuring the gross thermal efficiency at the test points specified in Table 1.1, in accordance with the procedures set out in an ETL recognised standard (Table 1.2).

1.4.1.4      Method C – validated design calculations

Under this test method:

1.    The product’s gross thermal efficiency at the full and part load conditions specified in Table 1.1 and the test conditions specified in one of the ETL recognised standards (Table 1.2) is determined from design calculations.

2.    The accuracy of these design calculations shall be confirmed by using an indirect method (flue gas loss method) from one of the ETL recognised standards (Table 1.2) to measure the product’s actual gross thermal efficiency:

a)    At least one test point between 60 % and 100 % of product’s maximum rated input at the temperature conditions specified for the 100 % test point, and:

b)    At least one test point between 20% and 40 % of product’s maximum rated input at or near the temperature conditions specified for the 30 % test point.

3.    To be eligible, the product’s gross thermal efficiency shall exceed the performance thresholds specified in Table 1.1.

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.         Gross thermal efficiency.

e)    Details of the burners used during the test.

1.4.2       Rounding

For the avoidance of doubt gross thermal efficiency test data shall be presented to 1 decimal place. As an example, a gas fired condensing hot water boiler >70kW with a gross thermal efficiency of 87.3% at 100% of its maximum rated output 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 (Category 1 & 2 above 400kW and Category 3 above 900kW)
  • Representative testing (see clause 1.5.1)

Further information regarding the first five routes can be found in Guidance Note 5 on the ETL product testing framework[2].

1.5.1       Representative testing

Where applications are being made for hot water boilers that are variants of the same constructional design and rated outputs up to and including 400kW to be included on the ETL, test data may be submitted for a representative selection of models. The representative models shall be selected by dividing the range of products into groups of models with similar design characteristics, and testing a model in the lowest quartile of predicted performance in each group. The performance of each model in the group shall be predicted using a validated mathematical model. As a minimum, at least one model shall be tested in each range of products.

Where applications are being made for products of the same constructional design and rated outputs greater than 400kW 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.

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] As defined in EU Regulation No 813/2013

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

[3] 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.