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Friday, 3 May 2013



1.       UK Power Flushing Industry

 In the UK, there is a thriving power flushing industry. It’s a big business.

2.       What is PF?

Power flushing involves pumping water and/or chemical cleaning solutions at high speed through a heating system to dislodge solid particles that may have settled in the pipes and fittings.

3.       How is PF carried out?

 A contractor will typically remove the central heating pump and temporarily connect hoses to a power flushing machine. The flushing machine consists of a powerful pump that draws water from a tank, circulates it through the heating system and back into the tank. The high-speed water stream will entrain loose particles of swarf and sludge and deposit them into the tank. The denser particles sink; others can be trapped in a filter or on a magnetic wand or will be dumped into the drains with the cleaning solution.  By shutting radiator valves, all the flow can be concentrated through one or two radiators at a time to dislodge more of the debris. The flow direction through the heating system can be reversed by opening or closing valves on the flushing tank.  The water in the tank will typically turn black with suspended particles of suspended sludge.

4.       Why is PF done on new installations?

Flushing is recommended for new systems, to remove any particles of swarf and residues of soldering fluxes that may have got into the pipe work during assembly.

5.       Why is PF done on existing systems?

For existing heating systems, power flushing is usually recommended to rectify sludge problems; the usual symptoms are radiators that are cold at the bottom, where sludge has settled, blocking the waterways. The sludge particles can also settle in the boiler’s heat exchanger, where they obstruct the transfer of heat from the hot flue gases to the water, causing hot spots and eventually, perforation of the heat exchanger. The sludge is mostly the products of corrosion, mainly composed of magnetite (Fe3O4) a black iron oxide. UK home-owners have been subjected to persistent advertisements for power flushing services and seem to believe that domestic heating systems will inevitably accumulate sludge and need regular power flushing every few years; this is not true. Power flushing will usually be unable to remove all the sludge in a heating system; the hardened deposits of sludge adhering to the pipe walls will be usually be untouched by even thorough power flushing.

6.       Sources of sludge

The iron (Fe3) component of magnetite usually comes from the inside surfaces of steel radiators, but may start life in other steel or iron components of the system. The oxygen (O4) component is a trespasser within the heating system, which shouldn’t be there; it is either absorbed from the air, carried in dissolved in fresh water or else it is generated within the heating system by electrolysis, which breaks the water down into oxygen and hydrogen. The oxygen corrodes steel and iron, transforming it into sludge. Keeping oxygen out of your heating system will ensure you won’t have problems with sludge and your heating system will never need power flushing. Sludge is a product of corrosion, ferrous metals reacting with dissolved oxygen. Others symptoms of sludge contamination are leaking radiators and recurring pump failures.


7.       French Power Flushing Industry

In France, there is no power flushing industry.

8.       French Sludge

You can get sludge in French heating systems and you can find power flushing pumps for hire in France, but most French plumbers are unaware of power flushing pumps and will never have had a customer who had requested, or whose system needed, power flushing.


9.       Desperately seeking French Power Flushers

 A search for “Power flushing pump” in the UK on Google gives about 3.1 million hits. A web search for the equivalent French terms give many fewer hits; “Pompe à détartrer” (51,000), “Pompe à désembouer” (22,700) or “pompes de rincage hydrodynamique” (51).


9.1.    Les pompes à détartrer’

Of these, ‘pompe à détartrer’, literally means a pump for removing lime scale, which is an entirely different problem to the black, magnetite sludge so familiar to British plumbers; the flushing machines are much the same, but they are being marketed in France for a different purpose. There is virtually no French market for services to remove sludge from heating systems.


9.2.    Les pompes de rincage hydrodynamique

Most of the pages containing the phrase “pompes de rincage hydrodynamique” are British manufacturers’ French language web pages.

10.   Black sludge; a British epidemic

 It seems that black magnetite sludge is a British disease and power flushing is a uniquely British phenomenon. Why should this be so when French heating systems use the same materials as those in Britain and use boilers from the same manufacturers? What do les plombiers français do that British plumbers don’t do? To answer that question, you need to note that there are two critical differences between French and British plumbing and look at how oxygen, the cause of sludge, gets into a heating system in the first place.

11.   Vive les différences

                There are two critical differences between French and British plumbing;

     11.1.French domestic heating systems generally do not use feed and  expansion tanks.

      11.2.French plumbers do not join copper pipes by soldering, they braze them.

12.   Sources of oxygen in heating systems 

There are 3 routes through which oxygen usually gets into a heating system, namely;

      12.1.dissolved in fresh water, that enters the system to make up water lost due                       to leaks;

       12.2.absorbed from the atmosphere, and;


13.   THE F&E TANK

The feed and expansion (F&E) tank is, like power flushing, a British institution. The F&E tank is a small tank, typically of 4-gallon’s capacity, that is found in the loft of many UK homes. Its function, as its name suggests, is first to provide a cold feed, a supply of cold water (via the cold feed pipe, connected near the bottom of the tank) to ensure the heating system is always full and, secondly, to provide a space to accommodate the expansion of the system water when it is heated. There is also an open vent pipe that terminates over the F&E tank which serves as an escape route for air and as a pressure relief device, allowing steam to be discharged harmlessly from the boiler in the event that the water does boil. This is very rare with modern gas or oil boilers, but it can happen with solid fuel boilers where the flame cannot simply be turned off. The water level in the F&E tank is maintained by a float valve, connected to the mains water supply. There is nothing inherently wrong with F&E tanks, they are cheap and reliable devices which do the job of several more complex, but less reliable components on sealed systems. The problem is that they can conceal a fault, or create a fault where they have been incorrectly installed. Two of the common sources of oxygen, mentioned above, are usually attributable to the use of F&E tanks.

14.   F&E tanks in Europe  

              F&E tanks are rarely used in France, or in most of Europe, or in the USA; although they are not unknown, their use is mainly restricted to solid fuel systems. The main reason is that most of Europe endures winters in which the periods of cold weather are both colder and longer than in the UK, so that any tanks or pipes in a loft, outside the insulated envelope of the building, are much more liable to freeze. In the UK, we enjoy the benefit of heat imported from the tropics by the Gulf Stream, our winters are warmer than other places on the same latitude and the cold spells are usually brief. We can usually (but not always) get away with having a tank of water in the loft without it freezing, providing the tank is insulated. Conversely, we cannot cope with any significant snow fall, which invariably brings the country’s transport systems to a halt and provides other Northern Europeans with regular amusement.


15.   Leaks and F&E tanks 

             On a properly installed system, the exposed water surface in the F&E tank will absorb some atmospheric oxygen, but the amount is negligible and will not cause significant corrosion. A problem arises if there is a leak from the system. A leaking sealed system (with an expansion vessel but no F&E tank) would lose pressure and the boiler would lock-out regularly. The chore of regularly refilling the system should alert the home owner to the fact that there is a leak that must be found and sealed. However, a heating system with an F&E tank will keep itself topped up automatically; if the leak is under the ground floor, or into the hot water storage cylinder, it will not be apparent and the system can keep running, with no obvious problems for years, or even decades. The fresh, make-up mains water contains dissolved oxygen (and dissolved lime-scale in hard water areas). The dissolved oxygen corrodes the radiators from the inside; the first the home-owner knows of the problem is when he notices the cold spots on his radiators caused by sludge.

16.   Pumping-over

The other problem created by the widespread use F&E tanks in Britain is called ‘pumping-over’; this happens when the F&E tank has not been connected correctly and there is a pressure difference between connections points of the open vent and cold feed pipes.  If the pressure difference is big enough, water will trickle from the open vent pipe and into the  F&E tank, from where it flows back down the cold feed pipe and into the heating system. This mini-waterfall, trickling into the F&E tank, churns up the surface of the water; it is very effective at aerating the water, allowing it to absorb air from the atmosphere. The dissolved air then gets carried into the heating system where the oxygen converts the inside surfaces of the steel radiators into sludge. The remainder of the dissolved air is nearly all nitrogen; the nitrogen is less soluble is hot water, so bubbles of the gas reappear when the water is heated in the boiler. Nitrogen is unreactive and will either escape through the open vent pipe, or will cause air-locks in the radiators, requiring frequent venting.

17.   Detecting Pumping-over

      Pumping-over can be elusive and intermittent, for example only happening when there is a demand for hot water but not space heating, or occurring briefly whilst zone-valves are closing. Since the F&E tank is out of sight in a loft, it may not be noticed by the home-owner who, in addition,  may not know that water trickling from the open vent pipe indicates a fault. It may also start happening on a system where a partial blockage, of sludge or lime scale, has created an obstruction. In nearly every case, it happens because the F&E tank has been incorrectly installed.


18.   Electrolysis

The third source of oxygen is electrolysis; passing an electric current through water will cause the water to break down, releasing oxygen at the anode (positive) and hydrogen at the cathode (negative).  This should be irrelevant, since there shouldn’t be any electrical currents passing through the water in a heating sytem. However, since heating systems are generally made of different metals, typically copper pipes and steel radiators, the system can act as a galvanic cell, generating DC electricity at a very low voltage. This can only happen if the water is conductive and can act as an electrolyte. If the water is acidic, the conductivity of the water and the likelihood of galvanic corrosion is greatly increased. Obviously, no acidic compounds should be allowed to get into the system water, if galvanic corrosion is to be avoided.

19.   Soldering 

  In Britain, plumbers usually join copper pipes with soft-soldered joints. Soldering requires cleaning the two metal surfaces, applying a soldering flux and heating until a solder wire melts on contact with it; the liquid solder is drawn by capillary attraction into the fitting where it cools and solidifies.


20.   Active Soldering Fluxes

The cleaning can be reduced, or eliminated, if an active, acidic flux is used; the flux turns into acid compounds on heating and the acid removes the oxidation tarnish on the surfaces of the copper pipes and joint. The acidic flux residues have to be cleaned from the outside of the joint or they will continue to cause corrosion, typically leaving stains of green verdigris around the joint. The flux residues can only be removed from inside the pipes by flushing; this is often neglected, since it costs the installer time and money and the home-owner/customer usually doesn’t know that it should be done. Residues of active flux will cause accelerated corrosion within the system, but the effects usually will not become apparent for some years. Active fluxes are prohibited on UK mains gas systems, which obviously cannot be flushed clean, because of the dangers of later corrosion.


21.   Soldering v Brazing 

  This is the other major difference between French and British plumbing techniques. French plumbers usually do not solder copper pipes, they braze them, at a higher temperature. There are no acid fluxes used and so no soluble acid compounds that may cause corrosion and sludge problems if they should be allowed to contaminate the water in the heating system.  Soldering is adequate, reliable, cheaper and causes less heat distortion of the pipework. I have never seen a correctly soldered joint that had either leaked or separated. There is only a problem if the installer uses an active flux and fails to thoroughly flush the system out before commissioning it.



UK consumers spend millions on power flushing services which should never be required.
 The reasons are;
  • the lower standards of technical competence amongst heating installers in the UK than in France
  • the installation and amendment of many UK systems by DIYers, and
  •  the lack of preventative maintenance by maintenance contractors.
Power flushing to remove sludge from a heating system is not the confidence trick here; it is the best that can be done to restore a system that has been contaminated with sludge.
  The confidence trick has been perpetrated on the UK public by some UK heating installers using installation techniques that will cause irreparable damage to a heating system after several years.
 If you were confident in the technical competence of your heating installer(s) and you've later suffered sludge problems, then it's bad news; you were conned. 

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