Going Direct

Going Direct

HETAS has developed a simplified technical guidance document which covers the relevant direct air assessment and commissioning procedures that installers can refer to in conjunction with any manufacturers requirements.

For self-certification of direct external air supply appliances, it is imperative that the installer understands the relevant safety provisions. Where a copy of the manufacturer installation instructions can be obtained this must be read and understood before work begins. If there is no information saying that the appliance can be used with a direct air supply and there is no air supply kit specified, HETAS recommends that the appliance is not fitted with a direct air kit. Alternative ventilation must be provided if required.


The following considerations should be taken into account:

  • Whether the manufacturers’ instructions confirm that the appliance can be safely installed in this way and the manufacturer has provided clear guidance on installing the appliance and air kit together.
  • Whether the dedicated air kit used meets the manufacturer’s requirements and the appliance is fitted in accordance with manufacturer’s instructions.
  • The installer must assess the installation and commission the appliance in accordance to manufacturer’s instructions and ensure HETAS commissioning and risk assessment guidelines are adhered to and evidenced correctly.
  • If at commissioning stage the appliance cannot be confirmed as operating safely, it should be disconnected from the chimney and placed somewhere that it can’t be used.
  • In some cases where spillage testing indicates poor flue performance, it may be that an ADJ vent is the only solution to supplying additional air for safe and correct operation. In this case the consumer should be advised of this compliance requirement and changes made to make the installation safe.

As well as manufacturer installation guidelines, it is essential that there are relevant assessments of the installation environment. These will involve assessment of the constructional attributes of the property, including any previous heat loss improvement measurements (cavity wall insulation, draft proofing etc.), as well as the current method of providing habitable and combustion ventilation, appliance type and the installation design providing efficient operation of the appliance.

Property Type

Properties built after 2008 are likely to incorporate more stringent energy efficiency measures, and as a result have more airtight construction characteristics; these will need to be factored in during any assessment of the installation. Careful consideration should therefore be taken to confirm the type of appliance selected is suitable for installation and operation under roomsealed conditions, and verified by the manufacturer as taking 100% of its air for combustion directly from outside.

Appliances verified by the manufacturer as non-roomsealed may require additional permanent ventilation in the room to ensure continued draw on the flue, especially under conditions of refuel. In these cases it is important to reference the applicable installation instruction guidance or seek further clarification from the appliance manufacturer on appropriate size and positioning of this additional ventilation requirement.

As well as constructional characteristics of the property, ventilation interference should be carefully considered, particularly in cases where mechanical and heat recovery extract systems are incorporated into the dwelling. When operated, these may cause depressurisation in the room or adjoining rooms in which the appliance is installed. Assessing these areas and carrying out appropriate spillage tests during initial light up and refuel phases allows for confirmation and sign off of the safe operation of the appliance during both extraction and non-extraction scenarios.

Installation Design

The final area of assessment is in relation to installation design. It is imperative at this stage for the installer to follow manufacturer specifications for the design of the air supply duct to the outside atmosphere, paying particular attention to the following areas:

  • Minimum diameter or cross sectional area of the external air duct
  • Maximum total length of the duct
  • Maximum number of bends permitted
  • Specification of the air inlet terminal
  • Measures to be taken to prevent the air supply becoming blocked by debris, flooding, insect habitats etc.

The area designated by the hearth dimensions is a non-combustible zone, and any material used for the duct within this area must be of a non-combustible material and not affected by heat. It is advised to only install a dedicated external air kit supplied or specified by the manufacturer of the appliance who will have taken this into account, ensuring the product can be installed in a way that is deemed compliant.

Additional Guidance

To aid installers further, HETAS have issued a Direct External Air Supply (DEAS) technical note for guidance on the installation, risk assessment and commissioning of solid fuel batch fed appliances. A copyof the document can be obtained from the HETAS Technical Area.

Get in touch

To discuss this article or any other technical related questions contact the HETAS Technical Helpline on 01684 278194.

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