Central Heating Water Quality – Why It Matters and What To Do

Facts, Puzzles, Conjecture and Differing Approaches

Ian Braithwaite, 19 October 2014, updated 15 July 2016

The mere fact you’re reading this means that you have more than average curiosity – congratulations! Heating water is hidden and generally forgotten, but since it transfers heat from the boiler to radiators and since it’s capable of causing costly repairs due to corrosion, some attention to the subject is worthwhile.

My interest stems from what I see in the course of my work, and my curiosity is generally piqued by conflicting information and difficulty at getting to what can be regarded as facts.

Make no mistake – poor heating water quality can stop boilers working – particularly combis, and causes pumps and other components to fail or lose efficiency.

The facts (as far as I can tell):

Here is the recipe for heating system corrosion:

Mix water + oxygen + ferrous metal, heat and leave.

Heating systems generally have the ingredients – radiators and the cast iron heat exchangers of older boilers provide the ferrous metal. Air provides the oxygen, the one ingredient that isn’t inevitable – exclude oxygen and there’s no corrosion.

Corrosion debris commonly takes the form of a black sludge of “magnetite” which is attracted to a magnet. (The latest low energy pumps contain permanent magnets so we can expect them to attract magnetite sludge.)

Modern boilers are more prone to failure or problems due to corrosion, because their more efficient heat exchangers have smaller water-ways which are easier to block.

(Hot products of combustion from burning gas pass over the main heat exchanger, exchanging heat with the water within – hence the name.)

Combi boilers generally (though not all) have a plate heat exchanger in which the heated water in the closed system in the boiler is transferred to cold water from the main on its way to tap or shower. Plate heat exchangers are extremely efficient, have very narrow water-ways and so are particularly prone to blockage.

As already noted, boilers used to have cast iron heat exchangers, whereas now, aluminium or stainless steel is employed. Care is needed with the acidity/alkalinity of heating water as measured by pH – aluminium in particular is attacked by alkaline (high pH) solutions. Most sources recommend keeping heating water pH below 8.5 or 9 (pure water is pH 7).  Low cost pH meters are available but I’ve found there’s a complication. If water is sampled, especially from a pressurised sealed system, and immediately measured, a high pH, perhaps above 9, is typically obtained. This pH is real – a piece of aluminium foil is rapidly pin-holed. However, over a few days the pH declines, usually to 8 or less, and aluminium, is no longer attacked. As aluminium is not found in the water sample, it appears that the lower, safer pH is the correct one. I don’t understand the mechanism, which may be to do with dissolved carbon dioxide, but the upshot is that on-the-spot measurements of pH are meaningless – some days need to elapse.

In hard water areas such as mine, the build up of lime scale within heating systems can be a problem, despite the fact that heating water is in a closed system, so scale is not being continually deposited. I have encountered a plate heat exchanger blocked, not by the expected corrosion, but lime scale.


Received wisdom:

Sealed systems are, it is said, superior to open-vented systems.

[Open-vented systems have a feed and expansion (F&E) tank at the highest point in the system, which contains a float valve which automatically replenishes from the cold main any water lost from the heating system. The tank also provides a volume into which the heating water can expand as it warms. Open-vented systems have a safety vent pipe looping over the F&E tank to discharge water in the event of excess pressure occurring.

Sealed systems replace the F&E tank with an expansion vessel. Water lost must be replaced manually using a filling loop – there’s no other way for water to get in. The vent pipe is replaced by a pressure relief (safety) valve and discharge pipe.

Both systems can benefit from air vents, automatic or manual, that allow air to be released.]

The reason for this supposed superiority is firstly that the open-vented system has an air-water interface at the F&E tank that allows air to dissolve in the water. While this seems self-evidently reasonable, provided there’s no loss of water from the system, I’m not sure how the dissolved oxygen would be drawn in. Secondly, sealed systems tend to operate at higher water pressure which helps prevent the ingress of air.

The following images (figures 1 & 2) show a sectioned radiator taken off after 36 years (it was mine – I take a while to get round to things) – the waterways, while having a thin black deposit of magnetite, are clear and the valves are in reasonable condition. It would be nice to take credit for looking after the heating water so carefully, but in truth, the heating water was taken for granted and as neglected as any other system for many years following original installation.

In contrast I have spent 20 to 30 minutes flushing thick black corrosion sludge from radiators serving a sealed system that was power-flushed two years earlier, before the water finally ran clear.

It’s clearly not as simple as “sealed systems good, open-vented systems bad”.

Use of softened water: until recently, the received wisdom was that artificially softened water should not be used to fill central heating systems, and some boilers even have a label to this effect. However (see References below), this is no longer the case and the UK Water Treatment Association code of practice has been revised accordingly.

Figure 1: radiator sectioned after 36 years service on an open-vented system

Figure 2: radiator valves from the radiator in figure 1

Different national approaches – German & British:

There is a widespread problem with poor quality heating in the UK and an industry has grown up around it, including:

  • Power-flushing of sludged systems.
  • Chemical cleansing.
  • Corrosion inhibitors (chemicals).
  • Magnetic filters which trap corrosion debris, preventing it doing harm by collecting in boiler heat exchanger or pump.

This list excludes repairs on items such as circulating pumps and boiler plate heat exchangers.

The German approach, embodied in VDI 2035, focuses on prevention:

  • Exclusion of oxygen from heating water.
  • Sealed systems and the maintenance of adequate pressure.
  • Some use of de-mineralised filling water (what some would call “distilled water”).
  • Regular checks.

The use of corrosion inhibitors is discouraged on the grounds that under- or over-dosing are likely to occur.

While generally an admirer of the German approach to many things, I’m not able to compare the two approaches, having no direct experience of German systems. Based on my own findings, I’m unable to muster the courage to ditch the use of corrosion inhibitor. Where inhibitor has been used (and one can generally smell it) the heating water generally runs clear. Where no attention has been paid to heating water for years, the water commonly runs black or brown.

What to do – what seems to make sense:


Preventing corrosion – dealing with oxygen:

  • Bleed radiators whenever they show signs of need (mostly warm but cooler at the top).
  • Sealed systems – keep the water pressure up (around 1 to 1.5 bar) – keep an eye on the pressure gauge and top up using the filling loop when needed (always do this when bleeding radiators).
  • Sealed systems – ensure the expansion vessel is checked regularly (a service activity often neglected).
  • Open-vented systems – make sure there’s no “pumping over” – unwanted circulation with water coming out of the vent pipe and into the feed & expansion tank. This pulls in air and results in a hot tank.
  • Fit a de-aerator (see references below). When I modified my heating system in July 2014 I fitted a Vortex de-aerator (see figure 3 below).


Preventing corrosion – chemical inhibitors:

  • Dose the system with inhibitor according to the manufacturer’s instructions.
  • Test regularly to ensure an adequate concentration.

Dealing with corrosion that’s happened:

  • Power-flushing: if there’s evidence that corrosion has been happening for some time, such as radiators that are cool bottom centre, and heating water that runs black, a power-flush, though expensive, is the most effective approach. It’s relatively quick, being done in a working day. A powerful pump is attached to the system, and cleaning chemicals circulated at high speed to strip the corrosion deposits. Magnetic filters trap the black magnetite debris and the dirty water is dumped out. The system is finally filled with clean water and inhibitor added.
  • Chemical cleansing: a cleaning chemical is added to the heating water – this may be the same chemical used for power-flushing. The chemical is circulated using the system’s normal circulating pump. After some days, the system is drained, flushed with clean water and chemical inhibitor added. In principle, it uses time and temperature to do what the power-flushing machine does more quickly. Note though, that unless a magnetic filter is fitted to the system, this ingredient that would be present in a power-flush, is absent.
  • System filter (including a magnet): a filter is installed to trap corrosion debris and prevent it reaching the boiler. It should be maintained (cleaned) annually.

My own system:

My heating system has an old-fashioned open-flued Baxi Bermuda back boiler which has a large cast iron heat exchanger. The system is fully pumped.  A new pump was fitted in 2012 which required the system to be drained down. It was re-filled and dosed with Sentinel X100 corrosion inhibitor.

The system was modified in July 2014, requiring another drain-down. Prior to this I tested a sample the water, which had a significant chloride content, characteristic of solder flux, despite the fact that no soldering had been done on the system for a very long time – a sign that the system wasn’t properly flushed. The water was clear but coloured brown, and some dissolved iron was found.

I decided to put the emphasis on prevention and fitted a Vortex de-aerator (see figure 3 below), which is claimed to reduce not only trapped, but also dissolved oxygen. I dosed the system with Fernox MB1 inhibitor.

Figure 3: Vortex de-aerator fitted to my heating system

Due to the position of the Vortex and the pressure difference across it, there was pumping over, which was cured by raising the height of the open vent pipe above the feed & expansion tank.

I didn’t fit a system filter as the boiler’s heat exchanger can be expected to be fairly tolerant, and in any case, they are to an extent an admission of defeat, an acceptance that corrosion will happen and that the debris needs to be kept from clogging the boiler (I expect if I had a modern boiler my courage would fail and I’d fit a filter).

After some weeks, I tried testing the water for dissolved oxygen using a chemical titration method. This didn’t work, I guess because of the inhibitor, so I acquired a relatively low-cost electronic dissolved oxygen meter. This failed to detect oxygen in the heating water, a hopeful sign that the Vortex is doing its job. The system certainly runs quietly. Time will tell.

After more than two months, the sampled water was clear and colourless.

This article is very unlikely to be the final word – it reflects my current state of knowledge and will be updated as I learn more. I’d be pleased to learn your thoughts, particularly where you disagree.


An informative but commercial article on heating water, advocating filling the system with de-ionised water. Allowing the pH to be as high as 10 is somewhat at odds with more general advice which is that the pH should not be higher than 8.5 to 9:


An unusually detailed specification from a manufacturer:


Short, informative article from BSRIA consulting organisation: https://www.bsria.co.uk/news/article/the-hidden-menace-of-corrosion-in-heating-and-cooling-systems/

Dirt separators & filters:



De-aerators – getting rid of oxygen:



Aluminium corrosion:



Use of softened water in heating systems: