# Convection Ovens

## 1.1           Scope

Convection ovens are used in the preparation of food within the professional foodservice industry. Convection ovens can be fuelled by electricity or gas, and are typically used for roasting, baking and browning food. The effectiveness of the air flow within the oven cavity governs the quality and time of cooking. Convection ovens use forced air convection to transfer heat. Compared with natural convection, forced air convection ovens cook faster and create an even distribution of heat within the cooking chamber.

## 1.2           Definitions

Whether using an electrically fuelled heating element, or a gas-fired burner, the heat is transferred to the food by forced air convection. Forced air convection ovens deliver the air flow using electrically driven fans, which enable the circulation of the hot air within the cooking chamber.

Convection ovens contain either removable racks or wall mounted shelf hangers. The term ‘rack’ is used to cover both arrangements to simplify text within this document. Racks are typically designed to correspond with dimensions from the Gastronorm (GN) – fraction-sizing – specification. A 1/1 GN sized container is 530mm (W) by 325mm (L). 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). Gastronorm trays and pans come in standard depths (or height) of 20, 40, 65, 100, 150 and 200mm. The number of racks will vary depending on the size of the appliance.

Convection ovens are typically categorised into half (1/2 GN), full (1/1 GN), one-third (1/3 GN), one-quarter (1/4 GN) and two-thirds (2/3 GN) size models. They are classified by size and container capacity. The full, half, third, quarter, 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. As an example, a half-size convection oven can accommodate one half of a 1/1 GN container (a single 1/2 GN container) per rack position, whereas a full-size convection oven can accommodate one 1/1 GN container per rack position. The one-third-size convection ovens can accommodate one 1/3 GN container per rack position. The quarter and two-thirds size models can respectively accommodate one 1/4 GN and 2/3 GN size containers per rack position.

Note – the U.S. baking tray size norm ‘sheet’ for full or half sheet, are also used within commercial catering. These trays are 25mm deep (outer dimension) with outer width and length dimensions for full sheet being 660mm (W) x 457mm (L) and for half sheet 330mm (W) x 457mm (L).

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

Convection 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 five product categories according to size:

• Convection ovens one-third-size 1/3 GN models.
• Convection ovens half-size 1/2 GN models.
• Convection ovens two-thirds-size 2/3 GN models.
• Convection ovens full-size 1/1 GN models.

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

• Convection ovens – Tabletop units.
• Convection 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.
• Be an electrically driven, fan-assisted oven.
• 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 convection ovens

 Fuel Maximum Energy Idle Rate (kW) Electricity 1.35 kW Gas 3.12 kW

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

• Heating-up time (min:sec) and required energy consumption (kWh).
• Energy consumption (kWh) in idle mode.
• Energy consumption (kWh) in convection mode under load without humidity.
• 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 18873-4, since the ETL test process is focused entirely on electric products.

• DIN 18873-4: 2013 Methods for measuring of the energy use from equipment for commercial kitchens - Part 4: Convection ovens.
• ETL test process for the measurement of energy performance of Convection ovens. The test process is outlined in the Annex section of this document.

### 1.4.2       Testing processes

The products will 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 loaded conditions while 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 model.
• Each test process shall be conducted for 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 (min:sec) 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 in the varying load conditions while in convection and idle modes.

### 1.4.4       Rounding

There shall be no benefit from rounding of the energy consumption data, for example the maximum idle energy of 1.351 kW for an electric product will be considered being above the threshold limit.

## 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 Guidance Note 5 on the ETL product testing framework[1].

### 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, half, or two-thirds size models).
• Can accommodate the same total number of containers (e.g., 4) 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 idle 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 consider amending the current performance thresholds from an absolute to an efficiency parameter for electric and gas fuelled convection ovens 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 by the CENELEC/TC 59X WG18, or any other committee working on a relevant standard (e.g. CEN/TC 106) in covering the energy performance measurement for this technology.

Consideration will also be given to the addition of the following performance 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.
• Shall have Wi-Fi capacity or other connected functionality.

## 1.8           Annex

The ETL testing procedure for the products shall consider the DIN 18873-4 in combination with the following convection mode test process.

• The voltage 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 convection 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 °C ± 2.5 K).

### 1.8.1       Number and position of the bricks

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

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

The number of usable racks is determined according to E DIN 18866:2021-04, 5.6.1. 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.

Legend

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

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, if applicable depending on oven size.

Use the following formula to determine the number of GN containers  to be loaded:

$$n_{GN}=\frac{n_S}{2}$$

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 geometrically centred brick with thermocouple for appliances with:

- GN container 1/3 (fig. 2)

• Two bricks with thermocouples for appliances with:

- GN container 1/2 (fig. 3)

• Three bricks with thermocouples for appliances with:

- GN container 2/3 (fig. 4)

• Three bricks with thermocouples for appliances with:

- GN container 1/1 (fig. 5)

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

Legend

a. Thermocouple

Figure 2 - Arrangement of 1 brick on GN container 1/3

Legend

a. Thermocouple

Figure 3 - Arrangement of 1 brick on GN container 1/2

Legend

a. First thermocouple
b. Second thermocouple

Figure 4 - Arrangement of 2 bricks on GN container 2/3

Legend

a. Thermocouple

Figure 5 - Arrangement of 3 bricks on GN container 1/1

### 1.8.3       Convection mode - Energy and water evaporation 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 (1050 ± 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.

 GN container size Number of bricks per GN container Water absorption per GN container Maximum permissible deviation GN container 1/3 1 1050 ± 50 GN container 1/2 1 1050 ± 50 GN container 2/3 2 2100 ± 100 GN container 1/1 3 3150 ± 150

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 s 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 s. 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 s.
• 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 K, 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 in order to 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 watered again.

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:

$$Q_{ }= Q_{total} ± ΔQ_{grid}$$

Where:

 $$Q_{ }$$ 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  can be calculated according to the following formula

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

where

 $$\eta _{conv}$$ 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  according to the following formula

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

where

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

Calculate the  according to the following formula

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

where

 $$m_{H2O\_EV}$$ 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  according to the formula

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

where

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

Calculate the  according to the following formula

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

where

 $$T_{FINAL\_SURFACE}$$ 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  according to the following formula

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

where

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

Calculate the  according to the following formula

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

where

 $$m_{start}$$ 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  according to the following formula

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

where

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

Calculate the  according to the following formula

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

where

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