Landlords must ensure that there is a carbon monoxide alarm fitted in any room that is used partly or wholly as living accommodation which also contains any appliance which burns, or is capable of burning, solid fuel. This would include log and coal burning stoves and open fires, even if they are not normally in use, but does not include gas and oil boilers. If an open fireplace is purely decorative and not useable then it is not covered by the regulations.
Gas is not a solid and so there is no requirement to fit one near a gas boiler. It is still advisable as best practice however.
The landlord is specifically required to carry out a check to ensure that smoke alarms or carbon monoxide alarms installed to comply with the Regulations are in proper working order on the day a tenancy begins where it is a new tenancy. A new tenancy is a tenancy granted on or after 1st October 2015.
For these purposes a new tenancy does not include a tenancy which was granted where the original agreement was entered into before 1st October 2015; nor does it include a periodic statutory tenancy which arises when a fixed term shorthold tenancy ends. It does not apply to a tenancy which starts at the end of an earlier tenancy where the landlord and tenant are the same as under the earlier tenancy and the premises are the same (or substantially the same) as those under the earlier tenancy. Therefore this express requirement to check does not apply to the renewal of a tenancy for the same premises by the same landlord to the same tenant. This should not be confused with the requirement to install detectors and alarms which applies to tenancies in existence before October 1st 2015.