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More to it than metal T
he industry has been involved in a longstanding debate over the metal of choice used in the construction of boilers for many
years now. The issue centres around the merits of stainless steel and aluminium, their corrosion- resistant properties and how these impact on the performance and longevity of a boiler’s heat exchanger. In this context, installers are under increasing
pressure to fit boilers with a long life expectancy, at the same time as meeting ever-more strict performance and efficiency targets. But while new models are emerging, the debate over which metal delivers the best results can deflect from the key considerations at stake.
With both materials offering excellent performance, installers need to look more closely at system design and maintenance, which can have a far bigger impact.
Stainless steel vs aluminium
Stainless steel and aluminium are the two main metals out of which boilers are made. Although historically aluminium has been the metal used to produce the first condensing boilers, there’s been a shift to stainless steel, respected for its durability, in recent years. In the case of stainless steel, choosing the correct
grade for a demanding application such as a heat exchanger is critical. While chromium is the key element in resisting the oxidisation process that causes corrosion, this is further enhanced by the addition of the metal molybdenum to create stainless grade 316/316L. This low-carbon stainless steel is a good option for a heat exchanger because it is a high-alloyed steel with a chromium content of over 18%. When the chromium is oxidised, it forms a thin surface layer which protects the underlying metal. With this thin film covering the metal surface, corrosion resistance is greatly enhanced. When it comes to aluminium heat exchangers,
versatility is widely seen as a major benefit. With this metal easy to cast into complex shapes, heat exchanger design can maximise the exchange surfaces and improve the hydraulic flow in a compact structure.
Aluminium is also known for its resistance to water in a wide range of applications and is, in fact, resistant to most environments and a variety of chemical agents. Like stainless steel, aluminium develops its own protection with an inert and protective aluminium oxide film which forms on the metal surface as soon as it comes into contact with water.
September 2018 Steven
Evans, sales director at Potterton
Commercial
the system. Meanwhile, limescale deposits on heat transfer surfaces will cause the boiler to run at higher temperatures for longer which reduces efficiency by up to 30%. The corrosion-resistant qualities of the heat
exchanger material cannot be relied on alone to combat acidity. It is therefore important to keep the water at a pH level specified by the boiler manufacturer.
So what needs to be done?
In terms of the processes involved, the engineer should perform a chemical clean, ideally in conjunction with a power flush, as the first step, even if the boiler and pipework are new. If a new boiler is being installed on an old system, then the engineer should perform a gentle, neutral pH clean four to six weeks prior to the installation, followed by a more aggressive (though still neutral pH) chemical clean over one to two days, along with power flushing.
Sizing and maintenance With both metals offering performance and
durability, the focus should instead turn to issues such as water quality and system design which can play a major part in the life expectancy of a boiler.
Ensuring water quality
Regardless of the choice of heat exchanger material, maintaining the quality of the water circulating through a heating system is essential. All boilers will perform more efficiently and reliably with good- quality water circulating in them. Chemical water treatment should be incorporated into both the commissioning and ongoing maintenance of a heating system, including a system clean/flush and regular doses of inhibitor as required. This is critical to ensure boiler efficiency and longevity in any commercial heating system, large or small. Systems that are corroded become blocked with sludge and debris, which means the boiler must work harder and for longer, as the circulating water is unable to transfer the heat efficiently throughout
System design is another important consideration for the reliability and life span of a boiler and it should never be assumed that like-for-like replacement in terms of boiler capacity always offers the best solution, especially if the needs of the building have changed. It is essential to size a replacement installation based on the current building requirements, rather than rely on the size of the boiler originally specified. A good option is a cascade system, which is flexible and modular in nature and can respond to changes in building use.
A scheduled maintenance and service regime must also be implemented from the installation stage of any heating system to safeguard long term energy efficiency and performance. This should involve analysing the flue gas to determine the right mixture of gas and air, as well as performing a deep clean of the heat exchanger. With commercial organisations increasingly under
pressure to review existing heating solutions, it is important for installers to remember that when replacing older models of boiler the type of metal it is made from is not necessarily the most important thing to consider. System design, water quality and ongoing maintenance will have the biggest impact on efficiency and performance. Get this right and installers can deliver peace of mind for customers and system longevity for years to come.
Installers can be forgiven if they’re a little confused about the debate over stainless steel and aluminium heat exchangers. With conflicting information often coming from different manufacturers, it is easy to quickly lose track of the merits of different models. Steven Evans, sales director at Potterton Commercial, argues that what really matters is the way the system is set up and maintained
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