Carbon Monoxide/Carbon Dioxide Atmosphere & Appliance Testing

In our series of articles explaining the ACS modules, Steve Mulvany, Logic Certification Scheme Director, provides an overview of the recently introduced ACS assessment, CMDDA1.
According to Health & Safety Executive (HSE) figures, on average, 15 people die from Carbon Monoxide (CO) poisoning every year, caused by gas appliances and flues that have not been properly installed or maintained. CO is invisible, silent and smell-free, posing a great danger to those who are exposed to it.
Consumers are advised to minimise the risk of exposure to CO by ensuring that all gas appliances and flues within their household are regularly maintained and all ventilators are clear. Fitting an audible CO alarm is highly recommended.
Even with planned and regular maintenance things can go wrong and CO can be released into the atmosphere. Apart from an alarm, other indicators include signs of spillage around the appliance, discoloration of room decoration and the occupants feeling unwell. Where signs of CO have been reported or an alarm has activated, a detailed examination of the gas appliances installed within the property will need to be carried out in accordance with the guidance and procedures detailed in BS7967.
The ACS assessment CMDDA1 has been introduced to assess the competence of the gas engineer to carry out this detailed examination and determine the cause of the CO.
What CMDDA1 covers
CMDDA1 is limited to responding to reports of CO from domestic gas appliances, fitted in dwellings used for habitation including; terraced, detached and semi-detached houses, flats, maisonettes, bed-sitting rooms, residential park homes, leisure accommodation vehicles and boats. The range of appliance types include room sealed, open flued and flueless.
If there has been an incident where the occupier has been taken to hospital or a fatality has occurred from suspected CO poisoning, the requirements of The Reporting of Injuries, Diseases and Dangerous Occurrence Regulations 1995 (RIDDOR) apply. In this instance, the gas installation must not be disturbed, only made safe e.g. cap the gas supply at the meter. CMDDA1 does not cover the competencies required to carry out investigations in this case.
Pre Requisites
The dwelling type will determine what ACS certificates must be held prior to taking CMDDA1. For examples, gas engineers intending to investigate suspect Natural Gas installations in domestic dwellings must hold valid ACS certificates for CCN1 and domestic appliances, plus CPA1 or equivalent gas services N/SVQ.  Likewise, where the dwelling uses Liquefied Petroleum Gas (LPG) the appropriate core or changeover and appliances must be held.
As with all ACS assessments CMDDA1 is available as assessment only. It is strongly recommended that gas engineers familiarise themselves with the content, procedures and guidance provided in BS7967, parts 1-4, in preparation. This can be done as either self-study or guided training within an approved centre.
Gas engineers should also ensure that they are familiar with the electronic equipment that will be used during atmosphere sampling – it may well be different from the type they currently use for combustion performance analysis.
What’s involved?
During the assessment the gas engineer will be provided a series of scenarios where CO in the atmosphere has been reported, they will then carry out a detailed investigation of the gas appliances installed and identify the cause of the CO. The investigation will require the engineer to select the appropriate equipment, carry out a risk assessment, conduct a series of atmosphere sampling tests, confirm the presence of CO and identify the faulty appliance.
Once the presence of CO is confirmed and the suspect appliance identified, the gas engineer will carry out a series of tests including combustion performance analysis and either repair or make safe the faulty appliance. All relevant information, risk assessments, results of atmosphere sampling and appliance tests are recorded on an Incident Report Form.
The assessment also covers the presence of CO in atmosphere by causes other than a gas appliance. Gas engineers will be assessed on their underpinning knowledge of BS7967 by correctly answering a number written multiple choice questions.
For the contact details of your local CMDDA1 approved Logic Certification centre, visit

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