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Department for Business, Energy and Industrial Strategy

Combination Steam Ovens

1.1           Scope

Combination steam ovens are used in the preparation of food within the professional foodservice industry, and are also referred to as combi-ovens, combi-steamers or convection steamers. Combination steam ovens combine traditional convection-based cooking with steam, to offer a range of cooking options within a self-contained appliance. Convection mode is used for baking, grilling roasting and oven-frying, whilst steam mode is used for steaming and poaching food. Combination mode allows for the optimum cooking environment to be created by either adding or removing humidity to the cabinet whilst retaining the food’s moisture and flavour in delivering the browning associated with convection cooking.

1.2           Definitions

Combination steam ovens are fuelled by either electricity or gas. Heat is transferred from the heat generators to the food using separate or combined heat transfer mechanisms. These mechanisms are hot air convection (using either an electrically fuelled heating element or a gas-fired burner to generate heat, and an electrically driven fan(s) to circulate the hot air), saturated steam, or a combination of both (producing super-heated steam). Steam is generated in one of two ways. Injection systems generate steam within the cooking chamber by spritzing water onto a rotating fan(s). Boiler/tank systems generate steam outside the cooking chamber and feed the steam into the cavity as required. Temperatures vary from 30-130°C in steam mode, to 30-300°C in convection and combination modes.

Combination steam ovens contain racks, designed to host containers that correspond with dimensions from the Gastronorm (GN) – fraction-sizing – specification. A 1/1 GN sized container is 530mm (W) by 325mm (L) and a 2/3 GN sized container is 354 x 325mm. Two 1/2 GN containers (265 x 325mm) fit in the footprint of a 1/1 GN, etc. The racks will vary in dimensions, and so are only suitable for certain sizes of containers (e.g., 1/1 GN or 2/3 GN). The number of racks will vary depending on the size of the appliance.

Combination steam ovens are typically categorised into full (1/1 GN), double (2/1 GN), and two-thirds (2/3 GN) size models. They are classified by size and container capacity. The full, double, and two-thirds-size ovens are determined by the number of containers the oven can accommodate per rack position. The container capacity is based on the total number of containers each respective size of oven can accommodate. A double-size combination steam oven can accommodate two 1/1 GN containers per rack position, whereas a full-size combination steam oven can accommodate one 1/1 GN container per rack position. The two-thirds-size combination steam ovens can accommodate one 2/3 GN container per rack position.

There exist tabletop and floor standing combination steam ovens. A tabletop unit is defined as a unit either placed on a table, a stand, any countertop location or stacked on another unit. Floor standing or freestanding combination steam ovens are available with short legs or casters. Floor standing units are displayed solely on the floor as a single independent unit.

Combination steam ovens are available in a range of different designs and efficiencies. The Energy Technology List (ETL) Scheme aims to encourage the purchase of higher efficiency products.

The ETL Scheme covers three product-categories according to size:

  • Combination steam ovens full-size 1/1 GN models.
  • Combination steam ovens double-size 2/1 GN models.
  • Combination steam ovens two-thirds-size 2/3 GN models.

The ETL Scheme covers two product-categories according to type:

  • Combination steam ovens – Tabletop units.
  • Combination steam ovens – Floor standing units.

Products listed on the ETL will be presented according to the above size and type categorisation. To be eligible for inclusion on the ETL, products shall meet the requirements as set out below.

1.3           Requirements

1.3.1       Eligibility requirements

To be eligible, products shall: 

  • Be either electric (single/three phase) or gas fuelled.
  • Have an appropriate Conformity Assessment mark.

1.3.2       Performance requirements 

Eligible products shall meet the performance criteria set out in Table 1.1.

Table 1.1    Performance requirements for combination steam ovens

Fuel Maximum Energy Consumption in Convection Mode for a Loaded Oven (kWh/stone)[1]
Electricity 0.26 kWh
Gas 0.27 kWh


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

  • Heating-up time (minute: seconds) in convection mode.
  • Energy (kWh) and water consumption (litres) in convection mode for an empty appliance.
  • Energy (kWh) and water consumption (litres) in convection mode for a loaded appliance.
  • Convection mode cooking efficiency (η).

1.4           Measurement and Calculations

1.4.1       Measurement Standards

For electric products only, the product performance shall be determined by combining the two test processes outlined below. Gas products shall be tested according to DIN 18873-1, since the ETL test process is focused entirely on electric products.

  • DIN 18873-1: 2012 Methods for measuring of the energy use from equipment for commercial kitchens - Part 1: Convection steamers.
  • ETL test process for the measurement of energy performance of Combination Steam ovens. The test process is outlined in the Annex section of this document.

1.4.2       Testing processes 

The products shall be tested in different modes and conditions:

  • Models shall undergo energy performance testing in the default mode set as the models are shipped, while adhering to the standard test methodology.
  • Models shall be tested empty and under load while operating in convection mode.
  • For the testing under load in convection mode, a specified amount of weight of saturated Hipor-Stones per grid are placed in the cavity of the appliance.
  • Each test process shall be conducted a minimum of 3 times with the intent to limit any test repeatability deviations in the recorded product test metrics. The average of all three runs is the value that shall be reported.  

1.4.3       Metric measurements 

  • The heating-up time (minute: second) for the product to reach a specific temperature level while operating in convection mode shall be recorded.
  • The energy consumption (kWh) of the product shall be recorded over the series of testing procedures. 
  • The water consumption (litres) of the product shall be recorded over the series of testing procedures. 

1.4.4       Rounding   

There shall be no benefit gained from rounding of the energy performance results. For example, an electric model’s energy consumption per stone of 0.261 kWh would be considered ineligible under these criteria.

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-tested and verified or cross-checked by an independent body.
  • Witnessed testing. 
  • Independent testing. 
  • Representative testing (see clause 1.5.1 below).      

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 two or more models that are variants of the same basic design, test data may be submitted for a single ‘representative model’ provided that all variants:  

  • Use the same fuel. 
  • Belong to the same size product category (i.e., full, double, or two-thirds size models). 
  • Can accommodate the same total number of containers (e.g.,10) of equivalent dimensions. 
  • Have the same rack dimensions (e.g., 1/1 GN). 
  • Have the same or less energy consumption values (kWh) while operating in convection mode. 

It should be noted that:       

  • Models that are variants of the same basic design are defined as all professional oven equipment which belong to the same product family and are manufactured by a single manufacturer, with the same primary power or energy source. All physical and functional characteristics shall be identical. Aesthetic changes are acceptable as long as the energy consumption and performance are not impeded. 
  • 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

This specification is scheduled to be reviewed during the 2023/24 review cycle.

1.7.2       Illustrative future direction of the requirements   

The next technical review will look to strengthen the performance thresholds for electric and gas fuelled combination ovens and consider amending the current performance thresholds from an absolute to an efficiency parameter, in light of the performance data collected during the intervening period.  

The next technical review will also consider if an EN test and measurement standard has been published from the revision of the DIN 18873 carried out by the CENELEC/TC 59X WG18 in covering the energy performance measurement for this technology. Following the publication of the EN standard, the setting of performance thresholds for steam and combination modes will also be considered. 

Consideration will also be given to the addition of the following eligibility requirements, namely that products shall: 

  • Have the option to connect to an energy optimisation or management system. 
  • Have LED lighting. 
  • Have as a minimum a double-glazed door.  
  • Have Wi-Fi capability or other connected functionality. 

1.8           Annex

The ETL testing procedure for the products shall consider the DIN 18873-1 in combination with the following convection mode test process.  

  • The voltage and frequency of the products shall be confirmed during the test. 
  • For the heat-up measurement, an external thermocouple is installed on a chrome-nickel steel grid and fixed at the geometric centre of the nearest possible insertion position in the useful volume (from top to bottom, side to side and front to back of the built-in insert) of the combination steam oven. 
  • The temperature control of the empty appliance is set to maintain an average temperature, measured at the external thermocouple in the geometric centre of the useful volume, of 160 ± 2.5°C.

1.8.1       Number and position of the bricks

The weight of each GN container must be measured and documented. 

  • 2/3 units are to be loaded with 2/3 GN containers per usable rack. 
  • 1/1 appliances are to be loaded with 1/1 GN containers per usable rack. 
  • 2/1 appliances are to be loaded with 2/1 GN containers per usable rack. 

The number of usable racks is determined according to E DIN 18866:2021-04, 5.6.2. The lowest level in the appliance is the first level to be loaded with a GN container. There must be a minimum distance of 30 mm between one brick and the next GN container to ensure good air flow between them. To fulfil this requirement, it may be necessary to use only every other level. An example of this loading scheme is shown in Figure 1. 



a. Oven opening
b. Thermocouple 
c. Free rack 
d. Empty rack 
e. Distance between top edge of brick and bottom edge of insert > 30mm 
f. Uppermost rack 
g. Lowermost rack 

Figure 1 - Example of a loading scheme

Starting from the lowest level, the GN containers on each level are loaded at a distance of at least 130 mm from the previously loaded container. 

Use the following formula to determine the number of GN containers nGN-v2.png to be loaded:


The following applies: 

$$n_{GN}$$ Number of GN containers to be loaded
$$n_{S}$$ Number of insertion levels

The result must be rounded up to a whole number. 

1.8.2       GN container spacing and arrangement

Each GN container used must be loaded as follows: 

A brick with thermocouple for appliances with: 

  • GN container 1/1 (fig. 3) 

Two bricks with thermocouple for appliances with: 

  • GN container 2/3 (fig. 2) 
  • GN container 2/1 (fig. 4) 

The bricks in the GN container must always be arranged with the maximum distance from the GN container walls or the neighbouring bricks. 



a. First thermocouple 
b. Second thermocouple 

Figure 2 - Arrangement of 2 bricks on GN container 2/3



a. Thermocouple

Figure 3 - Arrangement of 3 bricks on GN container 1/1



a. First thermocouple 
b. Second thermocouple 

Figure 4 - Arrangement of 6 bricks on GN container 2/1 

1.8.3       Convection mode - Energy and water consumption with load 

The bricks must be completely soaked in water at a temperature of 23 ± 2 °C for at least 12 hours before testing. 

After removing the bricks from the water, excess water must drip off for 1 minute. Load the containers. The preparation time per loading must not exceed 120 s. The water absorption must be 1,050 ± 50 g per brick. Then the remaining bricks are positioned, and the wet bricks are weighed. The water absorption per loading is the difference between the weight of the wet bricks and the weight of the dry bricks. The water absorption per loading must be in accordance with Table 1.2.

Table 1.2 Maximum tolerance per loading

GN container size Number of bricks per GN container Water absorption per GN container Maximum permissible deviation
GN container 2/3 2 2,100 ± 100
GN container 1/1 3 3,150 ± 150
GN container 2/1 6 6,300 ± 300

As soon as the last GN container is prepared, the preheated appliance is loaded. During loading, the heating elements must be switched off. 

The total loading time must not exceed 15 seconds per GN container. For units with mobile tray rack trolleys, an empty trolley must remain in the interior during the temperature stabilisation period and be replaced by a loaded trolley. The total loading time must be 60 seconds. The door must remain fully open before the end of the loading time, even if the GN containers or the tray rack trolley are already loaded. 

After the door is closed, the following values must be determined: 

  • Time in seconds. 
  • Power consumption in kWh. 
  • Water consumption in L. 
  • Temperature of the bricks with thermocouple in °C. 

When all the bricks have reached a core temperature of + 60 °C, stop the convection operation and open the door immediately. The GN containers must be weighed immediately after opening the door. Only the GN containers of the upper, middle and lower racks are to be weighed. The GN containers must be weighed within 60 seconds to effectively minimise the steaming of the bricks. Subsequently, the weight loss is extrapolated to the complete number of loadings. 

Hot bricks must cool down to ambient temperature before they are saturated. 

NOTE 1: Hot bricks watered directly in water absorb significantly more water due to capillarity and a change in water viscosity at different temperatures. 

NOTE 2: If the bricks are not used for a longer period of time, they should be stored in a dry place. The water for the bricks should be stored hygienically. 

1.8.4       Convection mode – Energy calculations 

With the following formula the total energy consumption can be calculated:

$$Q_{ }= Q_{total} ± ΔQ_{grid}$$


$$Q_{ }$$ Total energy consumption plus / minus the energy divergence of the grid
$$Q_{total}$$ Measured total energy consumption
$$ΔQ_{grid}$$ Energy difference admission of grid to standard mass

The cooking efficiency for the convection mode nconv-v2.png can be calculated according to the following formula 

$$\eta _{conv}=\ \frac{\left(Q_{sensible}+\ Q_{latent}\right)}{Q_{oven}}$$


$$\eta _{conv}$$ Cooking efficiency in convection mode, defined as the energy given to the specified food expressed as percentage of the energy consumed by the oven during the cooking process
$$Q_{sensible}$$ Quantity of heat added to the bricks and GN-container which causes their temperature increase
$$Q_{latent}$$ Latent heat of vaporization added to the bricks which causes a part of the water contained in the bricks to evaporate to steam plus quantity of heat added to the steam
$$Q_{oven}$$ Total heat consumed by the oven during the cooking process

Calculate the Qsensible-v2.png according to the following formula

$$Q_{sensible}= m_{Brick}× ΔT_{Brick}×c_{Brick}+ m_{Container} × ΔT_{Container} × c_{Container}+ m_{H2O} × ΔT_{H2O} × c_{H2O}$$


$$m_{Brick}$$ Mass of dry brick  
$$m_{Container}$$ Mass of the stainless steel containers  
$$m_{H2O}$$ Mass of the water absorbed by the bricks  
$$ΔT_{Brick}$$ Temperature increase of the bricks  
$$ΔT_{Container}$$ Temperature increase of the stainless steel containers  
$$ΔT_{H2O}$$ Temperature increase of the water absorbed by the brick  
$$c_{Brick}$$ Specific heat capacity of the brick


$$c_{Container}$$ Specific heat capacity of stainless steel


$$c_{H2O}$$ Specific heat capacity of water


Calculate the Qlatent-v2.png according to the following formula

$$Q_{latent}= m_{H2O\_EV} × ΔT_{H2O\_EV}× c_{H2O} + m_{H2O\_EV} × r_{H2O} + m_{H2O\_EV} × ΔT_{Steam} × c_{Steam}$$


$$m_{H2O\_EV}$$ Mass of the water evaporated during the cooking process  
$$ΔT_{H2O\_EV}$$ Temperature increase of the water before evaporation  
$$ΔT_{Steam}$$ Temperature increase of the steam after evaporation  
$$c_{Steam}$$ Specific heat capacity of Steam


$$r_{H2O}$$ Latent heat of vaporization of water


Calculate the Tbricks-v2.png according to the formula

$$ΔT_{Brick}= T_{FINAL\_AV}− T_{INITIAL}$$


$$T_{FINAL\_AV}$$ Average final temperature of the brick
$$T_{INITIAL}$$ Core temperature of the brick measured before the test

Calculate the Th2o.png according to the formula

$$ΔT_{H2O}= T_{FINAL\_AV}− T_{INITIAL}$$


$$T_{FINAL\_AV}$$ Average final temperature of the brick
$$T_{INITIAL}$$ Core temperature of the brick measured before the test

Calculate the Tfinal_av-v2.png according to the following formula

$$T_{FINAL\_AV}=\ \frac{T_{FINAL\_SURFACE}+\ T_{FINAL\_CORE}}{2}$$


$$T_{FINAL\_SURFACE}$$ Boiling point
$$T_{FINAL\_CORE}$$ Average core temperature of the M-brick measured at the end of the test

NOTE: As long as there is vaporization the temperature will not rise higher than 100°C on the surface.

Calculate the Tcontainer-v2.png according to the following formula 

$$ΔT_{container}= T_{Container}− T_{INITIAL}$$


$$T_{Container}$$ Temperature of the GN-container (160°C)

Calculate the mH2O_EV-v2.png according to the following formula

$$m_{H2O\_EV}=m_{Start}-\ m_{End}$$


$$m_{start}$$ Mass of the wet bricks before the test
$$m_{End}$$ Mass of the brick at the end of the test measured at the end of the test

Calculate the TH2O_EV-v2.png according to the following formula 

$$ΔT_{H2O\_EV}= T_{Boil}− T_{FINAL\_AV}$$


$$T_{Boil}$$ Boiling temperature of Water

Calculate the Tsteam-v2.png according to the following formula 

$$ΔT_{Steam}= T_{Final\_steam}− T_{Boiling}$$


$$T_{Final\_steam}$$ Final temperature of the steam (160°C)

[1] Term “stone” and “brick” are used interchangeably; the dimensions are equivalent.