Appliances: Water Heaters

The boiler: a unit that’s worth more than you think

So you think that one boiler is pretty much like another? Think again! Boilers with an enamel lining are worth far more: they last longer and keep your water more hygienic. Here is some information to help you decide which type of boiler is the most efficient choice for you – and help you get the best value for your money.

A boiler is a boiler… and everyone knows all about it…. Wrong! Most users have no more than a fleeting acquaintance with what goes on inside this essential domestic fitting.
That’s because there are some boilers that last much longer, produce far more hygienic water and guarantee a higher degree of protection against rust than others: boilers with an enamel lining.

Boilers come in all sizes, for home or community use; you can even get customised boilers for a yacht or a motor boat.

Technical characteristics
Sheet steel of a composition suitable for enamelling is used to make the boiler body, the upper and lower seal caps, the serpentines and the connecting pipes, which are all welded together to make the basic boiler. After suitable preliminary treatments, one or two layers of enamel are then applied. This enamel is of a special type that is chosen because its excellent resistance to water and steam guarantees a high degree of protection for human health and the environment.
Inside the boiler, the heated water is replaced with a fresh supply every time that we draw hot water off for use. This leads to the inevitable risk that the oxygen and mineral salts normally found dissolved in fresh water will corrode the boiler and eventually perforate it.
The only safe and economically feasible solution to this problem is to line the steel with vitreous enamel: the simple fact is that a boiler lined with enamel is guaranteed to last from five to ten years, while one treated inside with epoxy resin paints, an organic substance, is only guaranteed for two or three years and a galvanised boiler’s guarantee only lasts for one or two years.

This is why vitreous enamel is so reliable:

:: its composition is inorganic;
:: there is a chemical bond between the enamel layer and the metal surface.

After firing and vitrification at over 800° C, the steel contributes mechanical resistance, while vitreous enamel guarantees the rustproof protection, because it does not absorb water or conduct ions.
The enamel provides the boiler with 99.9% rustproof protection, as there may be the occasional uncoated point. The remaining 0.1% is guaranteed by inserting magnesium anodes: once these have been installed inside the boiler, they control the action of the agents that might otherwise eventually cause rusting.

In larger boilers, generally with a capacity of more than 200 – 500 litres, an alternative is to use impressed current anodes, which counteract rust formation.
Products with special features in the area of steel supports and enamel coatings have been developed for enamelling boilers.
UNI standard 9905 “Vitreous enamel for water boilers – Prerequisites and tests” lists the prerequisites that an enamel must comply with to be considered rustproof.
Another standard applied by boilermakers is DIN 4753-3.

Enamel coating: surface appearance
In order to guarantee a good degree of rustproofing, the enamel used to protect the inside of a boiler must be between 150 and 500 microns thick, compact and uniform.

Rustproofing qualities of common finishes used to line boilers
Type of coating Guarantee in years
Vitreous enamel 5-10
Epoxy resin powders


Galvanising 1-2
Galvanising plus organic coating max 5 years
Surface appearance
Characteristic Benchmark standard
Thickness EN-ISO 2178
Uniformity Visual check
Compactness EN-ISO 8289

Enamel linings for boilers: functional characteristics
Characteristic Properties Benchmark standard
Hygiene Does not allow bacteria to proliferate Bacteriological study undertaken by the Modena Hospital Institutes
Physiological harmlessness Enamels used in boilers must contain no lead or cadmium

EN 12873-1
UNI 9905

Chemical and physical resistance    
Resistance to boiling water and to steam Boling water and steam do not rust enamel. UNI 6722 - ISO 2744
Resistance to acids Enamels used in boilers must be at least class A. EN14483-1
ISO 2722
Resistance to sudden temperature changes The sudden change of temperature from 150 to 15°C does not damage the enamelled surface. ISO 4530
UNI 9905
Mechanical resistance    
Adherence The coating must adhere to the support. UNI 8883
Resistance to abrasion The coating does not wear out with time. UNI 7233
Shock resistance Within certain limits, shocks that might be applied accidentally while the boiler is being transported and installed will not damage the coating. ISO 4532



The International Enamellers Institute
+39 02 3264283   +39 348 8003263
The International Enamellers Institute (IEI) Viale Vincenzo Lancetti, 43 20158 Milano - Italy
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