How To: Diagnose the Cause of Circulation Issues in Central Heating Systems

My customer's central heating system has poor circulation

About the problem

If your customers are experiencing poor circulation in their homes, such as ineffective radiators, it could be caused by hydrogen in radiators and so it is important to accurately identify the cause of the issue in order to properly resolve it. This guide can help you to conduct checks which will lead to an accurate diagnosis of the circulation problem.


Identifying the Issue

  • Check if the boiler is operating correctly. Check the pressure of the burner. Does it remain alight?
  • Take a sample of radiator water. Examine it for suspended solids or discolouration which can indicate corrosion.
    • If there is a reddish tinge to the water, it indicates that iron oxide is present, suggesting active corrosion. The system may be drawing in air.
    • If there are black particles present in the water, these are most likely to be magnetite. As this is the final stage of corrosion, large deposits can be expected within the system.
    • If the water is clear water, it suggests that the system is free from active corrosion, but does not eliminate the possibility that corrosion has occurred in the past. It is possible that the circulation rate is not enough to keep debris in suspension. In this instance, you should check that the cold feed is not blocked.
  • If you are dealing with an open system, check the feed and expansion cistern. Is it clean, with no signs of pumping over or sucking-in through the open vent?
  • Check the pump. Is it correctly orientated and performing satisfactorily? Open all radiator valves, and, if necessary, vent the radiators. Note if the problem appears to be localised or general.
  • See if increasing the speed of the pump improves circulation. It is highly important to identify if there any radiators which fail to heat at all. If this is the case, it indicates that the system is blocked. Check all valves for free operation. If none are sticking, they are not the cause of the blockage. If you have established this, draw a sample of water from a cold radiator to check for sludge build up.
  • Do the system radiators need frequent venting? If gas is accumulating in one or more radiators, it is important to determine whether the gas is hydrogen sulphide, hydrogen or air.
    • Hydrogen sulphide is a product of bacteriological activity, and its occurrence is unusual. A toxic gas, it has an unmistakeable odour akin to rotten eggs which should not easily be confused with the smell of stale water or chemicals. 
    • Hydrogen is a product of corrosion and this is common in new systems where an excess of flux has been applied, causing copper plating. Corrosion also occurs in older systems which have not been cleaned properly after the installation of new components, such as additional radiators or a replacement boiler.
    • If air is present in the system, this suggests that the problem is a mechanical one, such as pumping over or poor joints under negative pressure.

The solution

If you have identified the problem as being one of a mechanical nature, you can resolve the issue with an appropriate mechanical fix.

Issues with hydrogen in radiators and corrosion debris should be tackled using chemical cleaning. Before proceeding with a system clean, check that the feed and expansion cistern is clean and free from bacteriological or microbiological contaminations. If necessary, isolate, drain, clean and disinfect it.

You should select the appropriate cleaning chemical for the system and the issue in question – in this instance, Sentinel X400 System Restorer and X800 JetFlo Ultimate Cleaner are recommended – and commence cleaning, using a powerflushing device where possible, but only if it is appropriate for the system. Powerflushing is not suitable for systems which contain microbore or non-barrier plastic piping. If the system is new or less than six months old, X300 System Cleaner is the recommended cleaning product and a manual flush is the recommended cleaning method for this product.

If poor circulation as a result of debris such as sludge was the problem, the powerflushing should resolve the issue. The water should be examined for the presence of debris on a regular basis and if further fouling is discovered, the system should be flushed to drain and a further dose of Sentinel X400 System Restorer applied.

If hydrogen production was the issue, the cleaner ought to be circulated for a minimum of 24 hours. However, if the system is old or there is a great deal of debris accumulated; allow at least a week for the best effects.

If you have identified hydrogen sulphide in the system, the system should be disinfected with a suitable sanitiser, such as X700 Sanitiser & Biocide, prior to the final flush.

Once you have cleaned the system, you should treat it with a multi-metal corrosion and scale inhibitor such as Sentinel X100 Inhibitor. If the system is producing hydrogen gas, you may wish to add a double dose of inhibitor. As it takes time for active corrosion surfaces to become passivated, it takes time for hydrogen production to cease. For this reason, you should allow a minimum of 6 weeks before considering further action.

If this process has not restored satisfactory flow and there is no full blockage, it may be necessary to use a more forceful approach and a stronger product, such as Sentinel Deposit Remover. This chemical, designed specifically for the removal of stubborn iron oxide deposits, ought to be used in conjunction with a JetFlush, provided that powerflushing is suitable for the system.

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