Approved Code of Practice
This Code has been approved by the Health and Safety Executive, with the consent of the Secretary of State. It gives practical advice on how to comply with the law. If you follow the advice you will be doing enough to comply with the law in respect of those specific matters on which the Code gives advice. You may use alternative methods to those set out in the Code in order to comply with the law. However, the Code has a special legal status. If you are prosecuted for breach of health and safety law, and it is proved that you did not follow the relevant provisions of the Code, you will need to show that you have complied with the law in some other way or a Court will find you at fault.
as for your co alarm idea for a flue in void
Carbon monoxide (CO) alarms
21 CO alarms are not covered by GSIUR or this ACOP. Although CO alarms are a useful back-up precaution they must not be regarded as a substitute for proper installation and maintenance of gas equipment
Work in relation to a gas fitting
51 For the purposes of these Regulations, ‘work’ includes do-it-yourself activities, work undertaken as a favour for friends and relatives, and work for which there is no expectation of reward or gain, eg voluntary activity for charities. This means that anyone carrying out such work must have the necessary competence, as required by regulation 3(1). However, membership of an HSE approved class of persons (under regulation 3(3)) is required only by businesses carrying out gas fitting work
No person shall carry out any work in relation to a gas fitting or gas storage vessel unless he is competent to do so.
The employer of any person carrying out such work for that employer, every other employer and self-employed person who has control to any extent of such work and every employer and self-employed person who has required such work to be carried out at any place of work under his control shall ensure that paragraph (1) above is complied with in rel setup, read and interpret ation to such work.
(if you are wrking for friends and family they are the employer and you the employee, their property is a place of work)
82 Anyone who does work on a gas fitting or gas storage vessel must be competent to do so (whether or not they are required to be a member of an approved class of persons). Therefore, do-it-yourself gas engineers and those performing favours for friends and relatives all need to have the required competence. The level and range of competence should match the full extent of work done, but needs only to be sufficient for and relevant to that work
(competence is proved by acquiring the necessary qualifications , experience and equipment, if you install your own boiler how can you safely check for gas leaks, set up, read and interpret the combustion analysis readings, effectively ensure the flue is sound an correctly fitted which requires an analyser to do so.
If you fail to do these things you have not installed it competently and are therefore in breach of regulations, try explaining that to the judge "sorry i killed my old mum and her neighbours guv, but it's her house therefore i can do what i want, i am competent because i say so, prove me wrong"
I don't think that will hold up somehow.
Competence is a combination of practical skill, training, knowledge and experience to carry out the job in hand safely, and ensuring the installation is left in a safe condition for use. Knowledge must be kept up-to-date with changes in the law, technology and safe working practice.
It's not just bunging a load of pipe together and hanging a white box on the wall, it requires specialist equipment, skills and knowledge whether you are a professional or diy'er,
how can you test the safety devices?
how can test the combustion?
how can you test for leaks?
how can you test operating and working pressures?
how can you test the effectiveness of a flue?
all this needs doing to prove the installation is safe therefore proving competence, diy or not.