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What are water heater tanks made of?

In this article, we’ll go in-depth into the most basic elements of a tank water heater: The material that they’re made of.

Picture this, you’re looking at the big, nondescript tube that is your water heater. Just looking at it it’s a bit hard to tell what it’s made out of. The outside is painted, the bottom’s got a burner, and intuitively you get the sense that the walls of the tank are thick, far too thick to be made of just metal. You’d be right.

In this article, we’ll go in-depth into the most basic elements of a tank water heater: The material that they’re made of.

Most home appliances are made from relatively thin metal like steel or copper. Think fuse boxes, washing machines, and plumbing pipes. Pipes are an exception because they’re either made from those metals or specialized plastics like PEX, PVC, or ABS plastic. Those materials all suit their relative appliances well enough, but water heaters are unlike all of these. Water heaters are made for two things: Heating water and keeping it hot. To do this well, it needs to last a long time and be a master of insulation. 

We’ll be going over three important materials in the construction of water heaters: Glass, magnesium, and steel.

Let’s start with the glass.

Vitreous Glass

Okay, let’s get technical. 

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You can think of your water heater tank’s walls as having layers, like a cake. The innermost layer of the tank is the vitreous glass lining. (Though thermoplastic is sometimes used instead) The glass isn’t the normal glass that you’d see in a window; this glass is called “vitreous glass” . Vitreous glass is almost more like enamel than traditional glass. It is applied by spraying powdered glass (called frit) onto the inside of the steel tank and pipes. Then, it’s all fired at incredibly high temperatures to bond the powdered glass together and to the steel. This process creates a cohesive coating that you can see in the image of the inside of a water heater from Bradford White. The blue is the VitraGlas lining. The layer of glass is used to prevent the extraordinarily corrosive hot water from destroying the insides of the tank wall. The glass can withstand the highly corrosive water much better than the steel would be able to. The vitreous glass lining is applied to the inside of the tank and any fixtures of the tank that hot water would touch. It’s also worth mentioning that some water does end up touching the walls of the tank and causes corrosion, but we’ll get to that later.

This is the powdered glass (called frit) used to coat the inside of the tank before it is fired and turned into a cohesive glass coating.

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the inside of a Bradford White water heater (the blue on the inside is the glass lining)

Vitreous glass (Or vitreous enamel) has a vast amount of applications in the world. Now that you know what it is, you may be able to think of all sorts of places where vitreous glass is used. Here’s a fun fact: this same kind of glass and application process is used for ornate dinnerware and other, similar styles of art. To do this they shape colored frit into intricate designs and then fire the dish, just like they do with water heaters. It’s an odd notion to think that the water heater in your basement has the same technology as the plate in your grandma’s china cabinet...


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Anode Rod

Inside the tank are all sorts of elements that help the water heater do its job, but one that’s critical to the structural integrity of the tank is the anode rod.

To further help the life of the water heater and prevent the steel from corroding, anode rods are placed right inside the water heater. The anode rod, often called a galvanic anode or even a sacrificial anode, sticks out from the tank and sits right within the water. They’re completely submerged within the water heater. The sole purpose of these rods is to sacrifice themselves to make sure the walls don’t get corroded. Seriously. They even need to be replaced once they have been worn out. You can see one that’s been used below. All the particles that make steel corrode are attracted to the anode rod - instead of your steel corroding, the anode rod does instead.

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An anode rod that badly needs to be replaced. You can see what it looks like once it’s been spent.

To understand how the anode works, you need to understand more generally how the process of corrosion happens. Corrosion is a chemical reaction between water and metal. Essentially, when the water comes into contact with the metal, it “steals” electrons from it which is what results in the formation of rust and loss of metal from the walls of the water heater.

The anode rod comes into play because it has a higher voltage than the metal on the insides of the tank wall. This higher voltage means that the corrosive water will prefer it to the metal of the tank walls. This ultimately makes it so that the water will end up corroding and “stealing” from the anode rod before it steals from the inside of the tank walls. If you’d like to know more, read this article from Bradford White about the chemistry anode rods.

Anode rods are used in places like boats, engines, and water heaters. In general, they can be made from magnesium, aluminum, and zinc. Anodes in water heaters are made from magnesium because it attracts the most electrons and has the best durability under the heat of hot water.

You may say “What about the vitreous glass lining?”. Well, it does a great deal to prevent corrosion, but ultimately it isn’t a perfect coating. It doesn’t prevent 100% of the steel corrosion and it’s critical to have another mechanism to prolong the life of your water heater

The anode rod is so effective that once it’s completely eaten up, the water heater will end up failing much more quickly. So make sure your anode rod is still good!


Okay, so now you know that inside the tank there’s an anode rod and the innermost layer of the water heater is vitreous glass. Well, what’s the next layer out? That would be the steel outer jacket. Frankly, there’s a lot less to say about the steel that makes up the walls of the water heater.

Steel is used because it’s strong and durable. There is no standard thickness for the steel within water heater walls. That being said, they are all safety-rated to withstand the amounts of pressure that will constantly be exerted on them. This is a bonus because it gives you a relative amount of certainty that your water heater won’t explode on you, which is nice. 

Another interesting aspect about the steel outer jacket is that sometimes there will be a foam (or even concrete) jacket installed right on the outside of the water heater. This foam jacket will help insulate the tank even more, increasing the efficiency of the water heater and saving energy. This isn’t necessarily installed on 100% of tanks and it needs to be installed correctly to ensure the longevity of the water heater. A poorly installed foam jacket can cause a buildup of mold between the tank and the foam or it could even destroy the tank walls due to rusting.


There’s probably more to water heaters than you originally thought. We didn’t even cover many of the other parts like flue vents and the heating elements. Hopefully, this in-depth guide to the basic elements of water heaters has helped you gain an appreciation and understanding of your own water heater. Water heaters are incredible. Well, at least we think so, it’s all we do. If you’re in the MN area and need a water heater repaired or replaced, give us a call at 612.712.1437 or fill our contact form We’ll be happy to help you out!

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