Since April 2002 an update to the building regulations (document j - chimneys and flues) has been in effect. The legislation was primarily concerned with addressing
" the increased risk of carbon monoxide, ill - health effects of flue gas entering living space and the risks of injuries caused by fires"

Testing procedures are based on smoke or air pressure tests of differing stringency dependant upon the fuel to be burnt.
Lining systems are required to conform to a system of performance indicators that give greater transparency when choosing the best system for the job!!

• If the flue is leaking - see why do flues leak?
  
• Appliance efficiency - many manufacturers and installers of high efficiency appliances recommend a flue lining for the most efficient operation of their appliances. E.g. Roomheating stoves / Boilers, Modern gas appliances / enclosed appliances: Where flue gas temperature is relatively low, therefore gasses cool within flue and lose buoyancy.
  
• To create a harder wearing / longer lifespan / higher temperature resistance for your flue
  
• To comply with current building regulation standards where a new appliance is being installed or fire place altered, upon failure of relevant flue flow or pressure test.