Confined space work has extremely stringent requirements that need to be carried out in order to meet with all the current health and safety regulations.
What is a confined space? It can be any space of an enclosed nature where there is a risk of death or serious injury from hazardous substances or dangerous conditions (eg lack of oxygen). Some confined spaces are fairly easy to identify, eg enclosures with limited openings: storage tanks, silos, reaction vessels, enclosed drains, sewers. Others may be less obvious, but can be equally dangerous, for example: open top chambers, vats, combustion chambers in furnaces etc, ductwork, unventilated or poorly ventilated rooms. It is not possible to provide a comprehensive list of confined spaces.
Some places may become confined spaces when work is carried out, or during their construction, fabrication or subsequent modification so we always have to asses every situation before any confined Space Entry
Here is what the HSE have to say about confined spaces http://www.hse.gov.uk/pubns/indg258.pdf