Gas boilers positioned behind a fireplace are regulated for use in the U.K. In 2011, the U.K. Building Regulations recommend the replacement of back boilers for properly positioned boilers with higher fuel efficiency ratings.

Gas back boilers are commonly made of cast iron and heat water for a home using the heat generated by a fireplace. Ventilation for a back boiler is often completed using the chimney of a home to exhaust fumes.

The building regulations in England and Wales require installation and repair of a gas back boiler by a person registered with the U.K. government as a competent person. It is the responsibility of the competent person to decide if a ventilation system for a gas fireplace is sufficient for the type of appliance.

When using a gas back boiler the chimney used for ventilation does not need to be lined unless a problem is detected by the installer or repair person. Chimney liners are sometimes needed when there is a leak or to improve the draw of the chimney.