Despite the higher price, stainless steel water heater tanks are generally more cost effective when comparing life cycle costs and should be presented as such.
Home water heaters are the real infantry of the mechanical world.They are often exposed to very harsh environments and their hard work is mostly ignored.On the water side of the heater, minerals, oxygen, chemicals and sediment are all attacked.When it comes to combustion, high temperatures, thermal stress, and flue gas condensate can all wreak havoc on materials.
When it comes to maintenance, domestic hot water (DHW) heaters are all but neglected.Most homeowners take their water heaters for granted and only notice them when they’re not working or leaking.Check the anode rod?Rinse off sediment?Is there a maintenance plan?Forget it, we don’t mind.No wonder most DHW equipment has a short lifespan.
Can this short lifespan be improved?Using DHW heaters made of stainless steel is one way to increase life expectancy.Stainless steel is a strong and durable material that provides better resistance to waterside and fireside attacks, giving the heater the opportunity to provide a long service life.The only real downside to stainless steel is the high cost of materials and fabrication.In the highly competitive DHW heater market, such high cost is a huge challenge to overcome.
Stainless steel is the generic name for ferrous alloys with a chromium content of at least 10.5%.Other elements such as nickel, molybdenum, titanium and carbon can also be added to provide corrosion resistance, strength and formability.There are many different combinations of these different metal alloys that produce specific “types” and “grades” of stainless steel.Just saying something is made of stainless steel doesn’t tell the whole story.
If someone said “give me some plastic pipes” what would you bring?PEX, CPVC, polyethylene?All of these are “plastic” pipes, but all have very different properties, strengths and applications.The same goes for stainless steel.There are over 150 grades of stainless steel, all with very different properties and applications.Stainless steels used in domestic water heaters are usually made from only a few stainless steels, usually types 304, 316L, 316Ti and 444.
The difference between these grades is the concentration of alloy in them.All “300″ grade stainless steels contain approximately 18% chromium and 10% nickel.The two 316 grades also contain 2% molybdenum, while the 316Ti grade has 1% titanium added to the mix.Compared to 304, molybdenum imparts better overall corrosion resistance to 316 grades, especially higher resistance to pitting and crevice corrosion in chloride environments.316Ti grade titanium gives it excellent formability and strength.Grade 444 does have chromium and molybdenum, but it doesn’t have any nickel.Generally speaking, the more nickel, molybdenum, and titanium in the mixture, the better the corrosion resistance and strength, but also the higher the cost.When someone says they have a “stainless steel” water heater, look carefully at the grades as they are not the same quality
Stainless steel is used in all different types of water heaters.It is most commonly used in indirect DHW heaters and condensing tankless water heaters.Indirect water heaters contain an internal heat transfer coil connected to the boiler or solar collector loop.They are more common in Europe than in Canada due to the dominance of European hydro and solar water heating systems.
Stainless steel construction constitutes a large part of these European indirect markets.In Canada, stainless steel and glass-lined steel indirect tanks are available, stainless steel tanks usually carry a higher price tag.In non-condensing tankless water heaters, the heat exchanger is usually made of copper.With the push for higher efficiency condensing units, the heat exchangers are either all stainless steel or a combination of primary copper and secondary stainless steel heat exchangers.Direct-fired tank water heaters remain the king of the Canadian water heater market.Carbon steel with glass lining dominates this segment.Stainless steel is commonly used in tankless or direct fired tank condensing water heaters.
To increase the efficiency of these devices, the flue gas must be cooled below the dew point to release the latent heat of the fuel.The resulting condensate is essentially distilled water vapor from the gaseous combustion products, which has a very low pH and high acidity.This acidic condensate must be piped into a drain for disposal, but the bigger problem is its corrosive effect on the water heater heat exchanger surfaces.
Heat exchangers made of ordinary steel or copper are difficult to withstand this flue gas condensate for long periods of time.Stainless steel is a good material choice because of its high corrosion resistance and flexibility, allowing it to form complex heat exchanger shapes.There are many brands of condensing tankless water heaters that use stainless steel heat exchangers.This allows them to encourage complete condensation of the flue gas in the heat exchanger and results in high EF ratings of up to 0.97.
Tank water heaters with condensing technology are also now starting to be used more frequently, especially with some building code changes requiring higher water heater efficiency.There are two common building types in this market.Glass-lined tanks are building fully submerged secondary condensing heat exchangers.The outside (water side) and inside (fire side) of the heat exchanger coils are glass-lined, and the glass-lined inside prevents the condensation of flue gas.Tank models with all-stainless steel tank and coil construction are not common, but there are several such all-stainless steel constructions available.
The initial cost of a glass-lined tank is indeed lower, and only time will tell how resistant the heat exchanger will be in harsh condensing environments.These new condensate tank water heaters are able to achieve higher efficiencies than traditional direct fired water heaters, with thermal efficiencies ranging from 90% to 96%.As governments push water heater efficiency regulations higher and higher, we are sure to see more innovative high-efficiency tank water heaters enter the market.
Take a closer look at tank water heaters and you’ll find that most types of direct fired, indirect internal coil, and straight storage tanks have glass-lined and stainless steel construction.
So, what are the advantages of stainless steel over glass lined?How do you convince customers to invest more in stainless steel tanks?The biggest advantage of stainless steel is its natural resistance to freshwater corrosion, which increases service life.Due to its composition of corrosion-resistant metal alloys, stainless steel tanks are stronger and more durable than glass-lined tanks.Stainless steel tanks have a protective oxide barrier on the water side to naturally prevent corrosion.
Glass-lined tanks, on the other hand, rely on glass-lined to provide a barrier between carbon steel and water.Given the chance, the oxygen and chemicals in the water will attack the steel and rapidly corrode it.Since it is nearly impossible to apply any protective coating perfectly (no microscopic cracks or pinhole defects in the protective layer) glass-lined tanks include sacrificial anode rods mounted inside the tank.
The sacrificial anode rods will wear out over time, and when the process is complete, electrolysis will begin to erode the exposed steel areas inside the tank.The rate at which the anode is depleted depends on the water quality and the amount of water used.Sacrificial anodes typically last three to five years, and anodes can be replaced to prevent further damage.
In fact, regular inspection and replacement of anodes is often overlooked, and the tank leaks, causing the entire unit to be replaced.Unlike glass-lined tanks, stainless steel tanks do not require “sacrificial anodes” to prevent corrosion on their surfaces.This means there is no need to inspect or replace the anode, saving maintenance time and costs over the life of the water heater.
Because of this increased durability and corrosion resistance, you will often find stainless steel tanks have longer warranties, with some manufacturers offering lifetime warranties for tanks.
Stainless steel tanks also have the advantage of being lighter compared to glass-lined tanks, making them easier to transport, handle and install.The wall thickness of stainless steel used in tanks is usually much thinner than similar steel tanks with glass linings.Combined with the weight of the glass lined itself, glass lined jars are usually much heavier.
Unlike glass-lined jars, stainless steel jars require less attention when shipping, and the glass lining can be damaged during shipping.If the glass lining of the tank is damaged or cracked due to rough handling during shipping or installation, it will not be known until the tank fails prematurely.
Stainless steel tanks are generally able to withstand higher water temperatures than glass-lined tanks, and temperatures over 180F will not present any problems.Some glass-lined tanks are prone to stress at high temperatures, resulting in a higher risk of glass-lined damage.Temperatures above 160F can be a problem for some glass liners.Applications such as solar water heaters and some commercial industrial applications can see high temperature water storage requirements.
It is recommended to consult the glass-lined tank manufacturer for the recommended maximum operating temperature.Stainless steel tanks are usually a better choice for high temperature applications.
There is no doubt that the initial cost of a stainless steel tank is higher than that of a glass-lined tank.But for the reasons mentioned here, the life cycle cost of a glass-lined tank may become higher.When comparing these life cycle costs, stainless steel tanks are generally more cost effective in the long run and should be shown to customers.
Robert Waters is President of Solar Water Services Inc., which provides training, education and support services to the hydroelectric power industry.He is a Mechanical Engineering Technology graduate from Humber College with over 30 years experience in circulating water and solar water heating.He can be reached at solwatservices@gmail.com.
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