How to deal with a carbon monoxide (CO) detector activation

The development of reliable CO alarms at a competitive price has resulted in 15% of households now having one installed. They are also being installed in new caravans and in many new-build properties. Mike Heads looks at dealing with the investigation of a CO detector activation – as dealt with in the CMDDA1 assessment.

In 1995, BS 7967 part 1 to 3 was released, providing a procedure for dealing with a carbon monoxide (CO) detector activation. At that time, the CMDDA1 assessment had not been fully certified and only became available in November 2012.

Gas engineers can currently carry out safety checks following a CO detector activation. They should ask the customer questions to ensure the activation was genuine and, following the requirements in regulation 29(3)
of the Gas Installation and Use Regulations 1998 (checking of appliances) and the procedures in BS 7967 part 1 to 3, check for CO presence using a standard Electronic Flue Gas Analyser (EFGA). All emergency service providers should be able to do this but – at present – are not correctly equipped.

This procedure for checking for CO activation was not covered as a competency and therefore is not normally taught or assessed during the normal five-year ACS cycle. This has created a problem because unless the engineer has read BS 7967 part 1 to 3 they might not be aware of how to deal with this situation.

The CMDDA1 assessment is designed to meet any shortcomings. It is not intended as a mandatory unit as not everybody will have the necessary equipment.

At present, only local authorities and housing associations have taken this assessment. This has created a problem for private customers as there are very few engineers who have the necessary qualification to carry
out a thorough investigation following CO detector activation/smell of fumes. This has been highlighted by the various CO safety charities.

The purpose of this assessment is not an incident investigation where a person has required medical attention for the effects of CO exposure or where a fatality has occurred. This would be covered by Gas Safe TB001 and TB002A regarding unsafe situations and RIDDOR.

In fact, it is designed to ensure a correct procedure has been followed for identifying sources of smells within a dwelling, the reason for a CO detector activation, or to confirm an installation is safe and provide a report to that effect.

The assessment has been developed in collaboration with all certification bodies. It consists of seven scenario tasks picked out of ten options, two practical assessments of different property types and a multi-choice question paper.

Summary of CMDDA assessment

The candidate is briefed on the procedure of the assessment. The candidate records their findings on a fume report form and must set up equipment correctly following the procedure outlined in BS 7967 parts 1 and 2.

The candidate identifies the appliances of highest risk and checks each appliance in turn. For flueless cooking appliances (Type A), the correct procedure is covered for testing the grill and hotplate. Flueless space-heaters, open-flue (Type B) appliances and room-sealed (Type C) appliances are all checked. The candidate must identify the presence of CO following the outlined procedure in BS 7967 parts 1 and 2, and
then investigate the source. The emphasis is on following the correct procedure and accurately recording relevant information on the fume report form. This is then used to diagnose the source of the problem.

There are also PAWS (Practical Assessment Workshop Scenarios), which focus on different house types with plans and completed fume reports. The candidate must establish the source of the problem from the information given. Finally, the theory paper is included to cover and reinforce the information learned through carrying out the exercises.


The accuracy of combustion analysers manufactured to BS EN 50379 part 3 used in the measurement of CO in dwellings should be guaranteed by the manufacturer and should be of a type that measures and displays
CO in parts per million (ppm). Combustion analysers that calculate CO2 levels from an oxygen level are not suitable for measuring ambient levels of CO2 in dwellings. Ambient air analysers used for measuring CO2 should be manufactured to BS 8494, although these analysers are not suitable for measuring combustion products. The period of time required for this ambient air assessment has led to several manufacturers producing a stand for the analyser.

Experience has shown that by measuring the ambient CO2, as well as the CO, the source of fumes is easier to detect. Typically, the products of combustion from a premix condensing boiler using an air/gas ratio valve is approximately 9% CO2 (equivalent to 90,000ppm, since 1% is 10,000ppm) and typically 50ppm to 100ppm for CO.

During this assessment, the measurement of both CO and CO2 is undertaken, which typically requires the use of two instruments. Kane instruments, however, has developed the ‘Kane 457’ analyser (see Figure 1) that can measure all three gases – CO2 as well as a CO and O2 – and can provide a direct reading with just one instrument.

Engineers working in customer properties should consider the use of a personal CO alarm.

General combustion information

CO is extremely toxic and has approximately the same density as the surrounding air. It can be carried around the building with warm air currents which is usually the case with gas fires – condensing boilers with lower flue gas temperatures will generally stratify. There is a rapid increase in CO generation if the CO2 level increases to around 1.5% to 2%.

Often referred to as vitiated air, this has been known to occur in some cooker grills (see Gas Safe TB085 – Beko cooker modifications).

CO can be generated from smoking, other fossil fuel-burning appliances, other carbon-based fuels, charcoal and wood, certain pellets, vehicles or generators adjacent or attached to a building or from engines and boats.

CO2 is non-toxic – it is exhaled through respiration. The normal outside atmosphere reading is usually within the region of 350ppm to 450ppm unless there is road traffic or other sources of CO2 nearby. The maximum exposure limits are 2,800ppm within the gas industry for directfired appliances. The World Health Organisation has set a limit of 5,000ppm over a maximum length exposure of 8 hours.

Levels in excess of 2,000ppm can cause sleepiness, headaches, poor concentration, nausea and increased heart rate. Levels in excess of 40,000ppm (4%) can cause brain damage, coma and eventually death.

Smells and fumes

There are numerous causes of smells and fumes that heating engineers should be aware of, including:

  • Gas escapes
  • Fibreglass log effects – bonding agents
  • Certain paints
  • Dust
  • Solvents and adhesives
  • External events – barbeques, bonfires, etc.
  • Drains
  • Certain plastic components
  • New appliances (preservatives and oils)

To sum up

  • CMDDA1 was developed to ensure engineers are competent to carry out a fume/CO detector activation report following the procedure in BS 7967 part 1 and 2.
  • CMDDA1 is not a mandatory unit if the candidate does not respond to a report of fumes/CO activation.
  • Larger installation and maintenance companies – and some SMEs – should have engineers competent to carry
    out this work.
  • All emergency service providers should have sufficient competent engineers with the correct equipment.