Central heating flushing

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"Flushing" a central heating system referees to a process designed to removing contaminates (sludge and other corrosion by products) from the pipework and radiators of a wet central heating system. It is one of the aspects of "Water treatment" required at certain times to maintain the correct working of a heating system.

There is a British Standard document (BS7593:2019 Code of Practice for treatment of water in domestic hot water central heating systems). Some of this document is based on the previous (2006) version of this standard.

What is flushing?

Flushing is a process of running or pumping water (possibly with the addition of chemical cleaning agents) through the pipework and radiators of a heating system, so as to help dislodge and remove accumulated deposits of sludge and other undesirable contaminants that will be detrimental to the life or performance of the heating system. The flushing water is ultimately sent to a foul drain - along with any contaminants carried in it.

Sludge - Different types of contaminant

The term "sludge" is often used as a "catch all" to describe all the different contaminants in a central heating system. Different types can be caused in different ways, and in some cases, require different treatment to remove.

Sludge is usually a mixture of corrosion by products such as rust or magnetite (magnetite is a black magnetic corrosion by product one one gets from steel when it corrodes in a low oxygen environment), hard water scale, and particulate impurities introduced into the system make up water. It can also include microbiological growths / fungus etc.


When water is heated, insoluble calcium carbonate is formed which can then come out of suspension to form deposits on surfaces within the system (this often referred to as "lime scale").

While it is most likely to form in the boiler's heat exchanger, it can also accumulate elsewhere in the system, often at places where the water circulates more slowly.

Under normal operating conditions non-bicarbonate or "permanent" hardness salts, e.g. calcium sulfate, will remain in solution, but at the higher temperatures of the heat exchanger surfaces their solubility reduces rapidly and precipitation can occur.

The potential for scale formation is greatest in those hard water areas of the United Kingdom where the bicarbonate alkalinity is high. Scale formation will be most pronounced if there is a high rate of water loss from the system, requiring frequent addition of fresh water to the system.

Scale in the heat exchanger will have a detrimental effect on boiler heat transfer efficiency. Scale can also be responsible for boiler noise or "kettleing".


Microbiological organisms range from simple bacteria to fungal and yeast spores. All of these can cause problems when they enter a central heating system.

The most likely place for microbiological growth exists in the feed and expansion cistern of an open-vented system. Here the temperature conditions are more favourable for bacterial growth and there is contact with the air. Aerobic bacteria, fungi and slimes which have developed in the cistern can enter the system with the make-up water and produce debris. Such debris can cause blockages and is liable to foul the boiler heat exchanger.

Under-floor heating and other systems which operate at lower temperature (typically below 60 °C) can also be prone to microbiological fouling. (although note that even the high temperature in the boiler heat exchanger might not be sufficient to kill all micro-organisms).

Anaerobic bacteria can thrive in both open and sealed systems fouled with corrosion and other debris, beneath deposits where the temperature might be lower and there is an absence of oxygen. This can give rise to microbiological corrosion of iron or steel compoents.

Which parts of the system are most likely to be affected?

System areas most prone to failure due to fouling by scale or sludge include the boiler heat exchanger and circulator pump. Fouling can also restrict flow through components such as thermostatic radiator valves, zone valves, drain valves, etc., and any parts of the system where there is a low water velocity or small pipe diameters (e.g. in radiators or fan convectors and microbore circuits).

When should a system be flushed?

A system will need an initial flushing after installation to remove any residue of flux or pipe jointing compounds, plus any metallic particles such as swarf left over from recent pipework installation or changes.

If the system is correctly maintained, well designed, and regularly treated with a corrosion inhibitor, then it is unlikely to need deliberate flushing until significant changes are next made to the system.

However its common for not all of these ideals to be met, and excessive sludge can be the result. Clues that sludge may be a problem include:

  • Cold patches at the bottom of radiators.
  • Parts of a system that no longer appear to heat correctly.
  • Excessive "kettling" noise (bangs, clicks, pops) being produced by a boiler once it heats up.

Systems should also be flushed thoroughly prior to the installation of a new high efficiency condensing boiler, since the heat exchangers in these are particularly susceptible to damage from contaminants in the heating system.

What comes out of a heating system:


General Considerations

Before cleansing, the system should be examined to determine the system configuration and the age and overall condition of components, in order to ascertain the cleansing regime required. For example, the procedure could remove corrosion debris covering pin-holes in radiators and this could result in leaks. If there is any doubt as to whether a system will withstand any cleansing methodology, then replacement or repair of relevant components will be necessary before continuing.

Flushing Procedures - complete system

There are a number of ways that a system can be flushed, and there are a number of treatment products on the market designed to aid in cleaning and flushing heating systems. Many products are designed to be added to the circulating water some time prior to flushing, and aid the mobilisation of contaminants prior to flushing. Follow the manufacturers instructions.

The primary flushing procedures are:

  1. Powerflushing
  2. Mains pressure flushing
  3. Gravity flushing with the assistance of a circulator pump

Powerflushing is arguably the most effective procedure (although it is important to check the boiler manufacturer's instructions to establish whether powerflushing is acceptable with the boiler in circuit). Powerflushing is a particularly effective method of cleansing existing systems, especially those containing a high level of black magnetite sludge.

Note with all techniques, reversing the flow will help to remove debris which might otherwise remain trapped.


Prior to flushing by any method there are some common preparation steps. These will typically include adding an appropriate cleaning chemical to the working system and running it normally for a time prior to the actual flushing. The cleaner is usually dumped (while hot) from the system, and the system refilled to start the flushing process.

Prior to the flushing itself you may also need to:

  1. Turn off all electrical controls and electrically isolate the system.
  2. Isolate the cold water supply to the central heating system.
  3. Manually close any automatic air vents.
  4. For open-vented systems, cap-off or temporarily join together the open vent and cold feed to the feed and expansion cistern.
  5. When draining a vented system, the header tank can have quite a crust of floating sludge. Skim this off using a jug first, so it doesn't get washed into the pipework.
  6. Mark the position or note the setting of lockshield or other regulating valves, then fully open all valves.
  7. Remove any thermostatic radiator valve (TRV) heads to ensure maximum flow through the valve.
  8. Set any diverter or zone valves to their manual, open position.
  9. Where practical, anti-gravity and non-return valves should be bridged, by-passed or temporarily removed as failure to do so will prevent flow reversal.


While there are many companies that specialise in offering this service, it is also possible to hire the equipment and do it yourself. Since the exact procedure will depend on the equipment this section only outlines the basic procedure, please follow the detailed instructions supplied for your equipment.

Follow the manufacturers instructions for connection of the machine. Note that powerflushing is normally more effective at higher temperatures. Some systems allow the boiler to be operated at the same time, others may include some heating capacity of their own.

  1. operation of the unit for at least 10 min (circulation mode) with all radiator and system valves open, reversing the flow regularly;
  2. dumping the dirty water to a foul drain whilst mains water is continually added via the powerflushing reservoir tank until the water runs clear;
  3. addition of the chosen cleansing chemical to the reservoir of the powerflushing machine and circulating to disperse throughout the system;
  4. circulating the cleanser through each radiator for at least 5 min in turn by isolating the other radiators and the hot water circuit, reversing the flow regularly; NOTE Tapping of the radiator with a rubber hammer will help to remove any loose material.
  5. cleansing of the hot water circuit for at least 5 min (circulation mode) by isolating the radiators, reversing the flow regularly;
  6. flushing of each radiator in turn for at least 5 min by isolating the other radiators and the hot water circuit, and dumping to foul drain until the water runs clear;
  7. flushing of the system with all radiator and system valves open for at least 5 min and dumping to foul drain until water runs clear.

Once the procedure is complete see the recommissioning section below.

Mains pressure flushing

Mains pressure flushing may require temporary alterations to the system to make appropriate connections to the system.

Temporary pipework lashup to allow flushing

Most systems should already have a drain point, and this can be used here.

Not all systems will have a connection to a supply of mains water however, and even those that do (i.e. typically sealed systems with a "filling loop"), that does not typically permit the direction of flow around the system to be controlled.

Hence it is often worthwhile making up some adaptors to enable a hose to be connected to the tails of a radiator. Thus by removing a radiator and then connecting to either the flow or the return tail (and capping or shutting off the other tail!), its possible to inject mains water into the system from each direction.

Note to comply with water regulations, it is important that any hose used includes (or is fed from a tap which includes) a non return valve (aka a "doublecheck" valve), to prevent any possibility of contamination of the mains supply.

Unless one is specifically attempting to clear a boiler blockage problem, it is usually better to isolate the boiler via its service taps prior to flushing, to save the risk of pushing more sludge etc into the boiler itself.

  1. Connect your mains supply to the system and ensure that water fed in via the mains connection is dumped to the foul drain.
  2. Use the system pressure gauge to check the system pressure does not rise too far - if necessary throttle the rate of inlet to kep it under 3 bar. This will prevent any pressure release valves still in the system from operating unexpectedly, and also prevent stressing any system components that are not used to working at elevated pressures. (in most cases with the drain valve open, you can run the inlet at full rate and not have any difficulties)
  3. Flush each radiator in turn, dumping to foul drain for at least 5 min until the water runs clear, by isolating the other radiators and the hot water circuit. Note: when switching to the next radiator, turn its valve on first before you shut off the valve on the previous rad - this will prevent accidental pressurisation of the whole system to mains pressure!
  4. Flush the hot water circuit, dumping to foul drain for at least 5 min until the water runs clear, by isolating the radiators.
  5. Flush the system with all radiator and system valves open, dumping to foul drain, for at least 5 min until the water runs clear.

If it is possible with your flushing arrangement, repeat the flushing process with the flow in the opposite direction.

Once the procedure is complete see the recommissioning section below.

Gravity flushing

Gravity flushing is the least effective cleaning system, and may not work at all for clearing some blockage problems - especially if the system has been designed such that it includes pipe loops that do not readily fill and drain under gravity alone.

  1. Dump system water to foul drain using all available drain points. (All radiator and other air vents should be opened to ensure complete removal of system water.)
  2. Refill the system, bleeding all radiators and any other vent points where necessary.
  3. Reinstate the electrical supply.
  4. Circulate the system water.
  5. Repeat the draining, refilling, circulating steps a minimum of two further times.
  6. Repeat as necessary until the water runs clear at all drain points. Refer to the manufacturer's instructions.

Extra steps

One can improve the effectiveness of any of the above flushing procedures at removing sludge from radiators by tapping / vibrating each rad with a rubber faced mallet as its being flushed - concentrating the taps around the bottom and mid sections of each radiator will usually prove most effective. The vibration combined with the flow of the flushing water helps free more sludge that may have settled and congealed in the radiator.

You can also buy rubber faced accessories that fit into a SDS drill. Used in "roto stop" mode, these allow the radiator to be vibrated mechanically using the drill during the flushing process.

Flushing procedure - individual radiators

Rather than flushing all radiators in place, its also possible to use the "hosepipe method" to flush an individual rads that are giving indications of being badly sludged. To do this, first isolate the rad with the valves either side, and then drain into a suitable tray by (partially) undoing one of the rad valve unions to the radiator. Once the rad is empty, undo both connections and place plastic bags over each tail, secured with rubber bands (this is to catch any extra sludge falling out when the rad is moved, which is particularly important in carpeted areas, since the black iron laden sludge will cause significant staining if spilt).

Now the rad can be unhooked from its mounting brackets carried outside to a suitable drain etc, turned on end, and flushed with through with a garden hose (and repeated from the other end). This will often shift more sludge than is possible flushing in situ.

Refit and refill once flushed.

If desired all the rads in a system can be flushed like this.

Individual radiator flushing can make the flushing process more effective overall, especially when limited to a less effective system flushing process like gravity flushing.


The system configuration and all components should be returned to their original settings.

Inhibition devices should be installed in the system pipework after the final flush and before the final fill with fresh water. Chemical inhibitors should be added at the time of final fill with fresh water. Refer to the manufacturer's instructions.

Under no circumstances should the system be left empty for extended periods as this could accelerate corrosion of ferrous components like radiators.


A good quality corrosion inhibitor suitable for the system should be used.

The system water should be checked at intervals specified by the product manufacturer to ensure adequate ongoing protection. Unless the manufacturer's instructions state otherwise, products from different manufacturers or different products from the same manufacturer should not be mixed.

It is helpful to record the product used and the date it was added for future reference.

See Also