Cleaning is always less than 100% perfect in practice!
The older the system, the more sludge that is likely to be present to remove during cleaning, but the paradox is that on the older systems you also have to clean more gently due to risk of leaks. This means that less aggressive cleaners have to be used, and even with Powerflushing perhaps only 80% of the debris is removed. You could remove 100% of the debris by acid cleaning, but with risk of leaks and consequent damage to the property, plus cost of replacement leaking radiators perhaps. The amount of acid (say Fernox DS-40) is also directly related to the amount of sludge. It might take 5 packs or more of DS-40 to 100% clean an old system at a considerable cost in chemicals and extra labour = five flushes. So, there is also a consideration of cost-effectiveness here. Powerflushing is a labour intensive process as we know. Therefore, the up-shot is that you have to look upon cleaning and Powerflushing as a risk management process not a means of returning the system to totally pristine condition. The more debris that you take out, the more efficient will be the system (a 15% gain in efficiency from experimental evidence) and the less the risk of breakdown. One reasonable clean plus a filter is more cost effective than many cleans to achieve 100% sludge removal.
The purpose of the filter is therefore to give additional protection to the system, especially to the boiler, pump and motorised valves, from any remaining debris that is loose and mobile in the system. Design changes to the system at the time of cleaning such as installation of a new boiler or pump, TRVs etc., may have changed flow patterns within the circuit and this can lead to re-distribution of remaining debris.
Also, it is found that there is a tendency for suspended debris to be present in the water in the immediate period after cleaning and this too will be captured by the filter. Thin films of adherent debris may break off plastic pipes after cleaning – so called ‘tea leaves’ - and cause blockages.
To assess if the cleaning job has been done well, check the filter periodically. In the first month there will be the most deposits but this should reduce significantly by month 3. If a high level of deposit continues to be removed by the filter even say 6-12 months on from the clean then this is obviously not satisfactory and would indicate that further cleaning would be desirable.
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