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Department for Business, Energy and Industrial Strategy

Air to Domestic Hot Water Heat Pumps

1.1           Scope

Air to Domestic Hot Water Heat Pumps are products that are specifically designed to transfer heat from the outdoor environment into a domestic hot water tank by means of a refrigeration cycle.

1.2           Definitions

Air to Domestic Hot Water Heat Pumps use an electrically operated refrigeration system to transfer heat from the ambient source into a domestic hot water system. They can be used to provide sanitary hot water in a wide range of buildings, including commercial and leisure.

Air to Domestic Hot Water heat pumps are available in a range of efficiencies. The Energy Technology List (ETL) Scheme aims to encourage purchase of higher efficiency products. Air to Domestic Hot Water Heat pumps can realise substantial reductions in carbon emissions when used instead of fossil fuel based, or resistive electric, water heating.

The ETL Scheme covers various types of products, including:

1.     Air Source CO2 heat pumps for domestic hot water heating

2.     Air Source non-CO2 heat pumps for domestic hot water heating

1.3           Requirements

1.3.1       Eligibility requirements

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

To be eligible, products shall:

  • Consist either of a single factory built unit or of an ‘outdoor’ unit and one or more ‘indoor’ units that are:
    • Factory-built sub-assemblies.
    • Supplied as a matched set of units.
    • Designed to be connected together during installation.
  • Incorporate an electrically driven refrigeration system that uses a refrigerant which has a Global Warming Potential (GWP) ≤ 750.
  • Be designed for, and include fittings for, permanent installation.
  • Be designed primarily to provide domestic hot water heating
  • Have an appropriate Conformity Assessment mark.

GWP values will be those set out in Annex I to Regulation (EC) No 842/2006. For refrigerants not included in this reference, the IPCC UNEP 2010 report on Refrigeration, Air Conditioning and Heat Pumps should be used as the reference.

1.3.2       Performance requirements

Eligible products shall meet the performance criteria set out in Table 1.1 below for:

  • Water Heating Energy Efficiency (ηwh) at the declared load profile.

Table 1.1    Performance thresholds for air to domestic hot water heat pumps (all product categories)

Declared load profile







Water Heating Energy Efficiency (ηwh)







"≥" means "greater than" or equal to”


  • Water Heating Energy Efficiency’ (ηwh) is the ratio between the useful energy in the water provided and the energy required for its generation, expressed as a percentage.
  • Load profile is a given sequence of water draw-offs, as specified in in Annex III, Table 1 of Commission Regulation (EU) No 814/2013 “Ecodesign requirements for water heaters and hot water storage tanks” and Annex VII, Table 3 of Commission Regulation (EU) No 812/2013 “energy labelling of water heaters, hot water storage tanks and packages of water heat and solar devices”.

1.3.3    Information requirements

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

  • Whether the model has ‘smart features’. Specifically, this includes the capability to provide information on whether the products are ‘smart ready’ without the replacement or addition of any hardware, which includes the following one or more options:
    1. Demand Side Response Ready
    2. Date Collection Ready, which includes:
      • Storing data on the heat pump or a remote device.
      • The ability to store data for each calendar year as a value per day, per month or per year.
      • The following real time value parameters: electricity energy consumption (kWh); heat delivered (kWh); energy efficiency performance; operating times (days, hrs); number of on/off cycles.

1.4           Measurement and Calculations

1.4.1       Energy efficiency metrics

Water Heating Energy Efficiency wh) – means the ratio between the useful energy provided by a water heater and the energy required for its generation, expressed in %.

Reference Energy (Qref) means the sum of the useful energy content of water draw-offs, expressed in kWh, in a particular load profile.

Smart Control Factor (SCF) means the water heating energy efficiency gain due to smart control under the conditions.

1.4.2       Test Requirements

No additional testing requirements beyond the measurement standard below.

1.4.3       Measurement standards

The product’s performance data shall be determined in accordance with the procedures detailed in BS EN 16147:2017 “Heat pumps with electrically driven compressors - Testing and requirements for marking of domestic hot water units” and the water heating energy efficiency calculation, following the requirements of Commission Regulation (EU) No 814/2013 or Commission Regulation (EU) No 812/2013”.

The load profile used for the test shall be declared by the manufacturer and shall be the maximum load profile or the load profile one below the maximum load profile for the product.

1.4.4       Rounding 

For the avoidance of doubt test data should be presented to zero decimal places. As an example, an Air to Domestic Hot Water Heat Pump product with a declared load profile of XL and a water heating energy efficiency of 121.4% would be deemed to be a fail.

1.5           Verification for ETL Listing

There are five main ways that applicants can demonstrate their product’s performance:

  • In-house testing – Self-certified
  • In-house testing – Self-tested and verified or cross-checked by an independent body
  • Witnessed testing
  • Independent testing
  • 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 a range of products that are variants of the same basic design, test data may be submitted for a representative model, provided that all variants, i.e. models, share the following characteristic features:

  • Use the same refrigerant
  • Have the same compressor type (i.e. manufacturer, line of models), which should imply:
    • same method of compression (e.g. reciprocating or scroll) and
    • same type of enclosure (e.g. hermetic or semi-hermetic)
  • Use the same defrosting method (e.g. hot gas defrost)
  • Fit within the same product category (i.e. are all low-temperature air to water heat pumps, or are all non-low-temperature air to water heat pumps.

The representative models may be selected by dividing the range of products into groups of models with similar design characteristics. The performance of each model shall be predicted using a validated mathematical model. At least one model in each group shall be tested for validation purposes. A report documenting performed model calculations, showing all significant calculation steps, shall be submitted with the application.

It should be noted that:

  • If a manufacturer voluntarily removes the 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 model 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

The next technical review is scheduled for 2025-26.

1.7.2       Illustrative future direction of the requirements 

Future changes to the Specification may include:

  • Increasing performance thresholds for Water Heating Energy Efficiency (ηwh).
  • Updating the product eligibility and functionality requirements as per market demand and any UK and EU regulatory changes.
  • Investigating the efficiency of domestic hot water heat pumps with a large (L) load profile, specifically breaking down the performance of the market between 114% and 115% efficiency.  
  • The possibility of ‘smart features’ becoming an eligibility requirement for listed air to domestic hot water heat pumps. 
  • The possibility of updating the product eligibility requirements to encompass embodied lifetime carbon of heat pumps. Introducing a maximum value (measured in tonnes/ CO2) for the lifetime carbon of a heat pump.