The boiler manufacturer's instructions (and warranty) require that cleaning and flushing of the system is carried out prior to boiler installation. This is also a requirement in building regulations.

A newly-installed central heating system may contain residual debris such as metal particles, solder residue etc., while an older system which has not been correctly protected with inhibitors may contain corrosion deposits in the form of accumulated "sludge" in pipes, radiators etc.

Accumulation of sludge may be an indication of electrolytic corrosion in the heating system. This can occur because of contact between different metals used in heating system components and may be accompanied by the accumulation of flammable gas within radiators - which may be mistaken as air accumulation in very severe cases.

If a new boiler is to be installed then it is obviously important to ensure that the heating system (pipes and radiators) to which it is being connected, whether old or new, is as clean as possible.

Any internal debris or deposits could ultimately be washed into the boiler and its heat exchanger, affecting performance and possibly causing a major breakdown. Accumulated sludge in radiators can also affect the circulation of water in the primary circuit, affecting heat output and leading to a reduction in efficiency of the system as a whole.

A common symptom of sludge accumulation is that radiators are cold at the bottom but hot at the top - due to reduced water flow. The image below shows a "thermal image" of a radiator with reduced water flow due to blockage.

Treatment chemicals may range from mild detergent-based cleaners suitable for removing debris after the installation of new systems or modification of existing systems, through to more aggressive cleaners designed to remove accumulated deposits in existing systems which may not have been treated for some time.

British regulations specify a number of cleaning methods which will meet the requirements of building regulations:

1. A conventional clean and flush - using gravity to empty and re-fill the system and adding WT chemicals as required. Cleaning and flushing are accomplished by repeated filling and draining of the system, after using appropriate WT chemicals to suspend, disperse and remove accumulated material. This is then followed by a "flush" with clean water, before any inhibitors are added to protect the system.

2. Mains pressure clean and flush - This involves the connection of a mains pressure hose to an appropriate point on the heating system and another hose from the system's drain valve to a suitable foul drain. After using WT chemicals to suspend, disperse and remove accumulated material, individual radiators on the heating circuit are flushed using mains pressure water by opening/closing their isolation valves in turn, before flushing the whole system with all valves open. The system is then refilled, using inhibitor (see below) as required and returning all radiator valves to their previous settings.

A second possibility is to drain and remove individual radiators and clean them (preferably having taken them outside) by flushing with mains pressure water. This can be a time-consuming process and may result in some disturbance and mess. This method may not clean any accumulated deposits in the system pipework and further flushing may be necessary to achieve this. Use of inhibitors will again be required once the system has been re-assembled and refilled.

3. "Power flushing" - which uses a specially-designed pumping system to rapidly circulate water and treatment chemicals around the heating circuit.

If a new boiler is being installed, power-flushing should be carried out prior to installation, or with the boiler isolated from the system. If an existing system is being maintained then isolation of the boiler is normally required to avoid blockage by deposits dislodged during cleaning.

The manufacturer of the power-flushing system will provide detailed instructions and may also specify the treatment procedures and chemicals to be used. These instructions must be followed.

Note: power-flushing may not be suitable for some systems - e.g. gravity systems, single pipe systems and some micro-bore systems. More detailed guidance is available elsewhere - e.g.  from power-flushing equipment manufacturers.

With all cleaning methods it is important to ensure that cleaning agents are completely removed from the heating system as they may nullify the effect of the inhibitor. Additionally, if cleaners remain present, having mobilised the system debris the resultant mixture can lead to premature failure of system components (e.g. pumps) and can also lead to flammable gases e.g. hydrogen being formed within the central heating circuit if corrosion re-occurs due to the reduced effectiveness of the inhibitor.

The next area we cover in our comprehensive process is protection, making sure that your central heating system isn't just clean for now, but protected for many years to come.