Chemical flush simply put, is adding a detergent to the central heating water to help loosen and suspend any junk or debris in the existing pipe work and radiators. The boiler is used to circulate the chemical normally for at least one hour up to a maximum of 4 weeks dependant on the chemical, after that time the chemical can begin to release any suspended debris. Using the boiler has its advantages as hot central heating water works more effectively than using just cold water. The chemical gets circulated for a hour minimum then its emptied out and clean cold water is used to flush and neutralize any remaining chemical out before a system protector(inhibitor) is added to prevent any metals from rusting and causing sludge.
There's a chemical for every type of system. New central heating systems have specific chemical that helps remove flux. The cleaner will be designed specifically to remove copper swarf, flux residues, casting sand and any rust.
Older systems need a high performance cleaner which will break up, lift and suspend any sludge. On systems with slight circulation problems this chemical works well, although a longer period up to 4 weeks is recommended.
Systems with circulation problems, normally caused by the lack of inhibitor allowing corrosion to take hold of the central heating system. A fast acting chemical is needed. Aimed at removing limescale and iron oxide deposits from radiators, pipe work and the boiler. These chemicals work with either a hot or cold system.