The water in a central heating system serves as the medium of heat transfer from the boiler to the radiators and other heat exchangers. The central heating system can contain several components with different materials such as stainless steel, copper, cast iron, aluminium, brass, plastics, and rubber. Some of these materials can be affected by the quality of water over a long period. This can lead to damages such as leakages and blockage of the heating system. The quality of water is therefore one of the most important things to consider in the management of the central heating system.
There is an ideal quality of water to maximise efficiency, reduce energy consumption, and for a long-lasting operation of the central heating system. To understand what constitutes a good water quality, we would review the possible properties of water and how can they affect a heating system. We will also suggest solutions to prevent the water quality from deviating from ideal and offer remedies when the water quality is no longer to specification.
The presence of dissolved gases like oxygen is potentially the most significant parameter in measuring the quality of water. Dissolved oxygen in the water will lead to the rusting of iron and its alloys in the heating system. This is arguably the most common and difficult challenge because the consequences of metal rusting in the central heating system are many. These can be:
To prevent rusting of metals, the water must be free of oxygen. These will be achieved with the combination of equipment and methods in the operation of the central heating system. Deaerators are devices that mechanically remove dissolved gases including oxygen from water. It often uses a vacuum principle to extract the dissolved gases while allowing the continuous operation of the heating system.
There are also chemical corrosion inhibitors that scavenge for trace particles of dissolved oxygen. And finally, the water pH can be regulated to be slightly alkaline (8.5 – 9). Low pH values will encourage corrosion of iron parts, while high pH will lead to corrosion of copper and aluminium materials. The pH can be controlled by the addition of sodium hydroxide and sodium phosphate salts after the water pH is accurately measured.
Once the corrosion process has started and it is significant enough to affect the performance of the system, there is a need to quickly perform corrective maintenance to prevent complete system failure. Corrosion debris usually collates together to form a black sludge of magnetite. This sludge must be flushed out of the system using power-flushing with a mixture of cleaning chemical and water. Magnetic filters can also be installed to trap the sludge and prevent the movement to heat exchanger surfaces and magnetic pump which attract the magnetite.
The hardness of water can affect the quality of water in a central heating system. Hard water contains the mineral calcium which when heated forms calcium carbonate. Limescale is a white substance that deposits on the surfaces of the boiler pipes, radiators and other heat exchanger reducing the heat transfer efficiency of the system. If excessive, it can block water flow. It is found on hot surfaces in the system because water is more likely to evaporate there and leave behind deposits of the mineral.
Usually, in a closed system, limescale should not be a problem as there are limited calcium carbonates that can build up. However, repeated hard water refill of the system due to periodic maintenance can introduce further minerals into the system.
The presence of limescale in a central heating system can be detected from noisy water flow in radiators, noisy operation of the boiler, cloudy water, or discoloured water from radiator bleeding.
There are several ways to prevent the formation of limescale. The most common are described below:
A power-flush will get rid of the limescale deposits and should be applied in both directions. An alternative is an acid-based cleaner. Both methods will require significant maintenance time. It is therefore important to prevent the formation of limescale.
The importance of the quality of water in a central heating system cannot be overstated. Low-quality water will directly increase the fuel consumption of boilers, maintenance frequency of the system and reduce the quality of comfort from the radiators and tap water. The corrosion damages caused by the presence of dissolved gases like oxygen in the water and the limescale formation from hard water are the most likely because that reduces the quality of water. High water quality can be maintained by simply ensuring it is oxygen-free and calcium-free.
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