How Water Heaters Work
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How Water Heaters Work

I enjoy hot showers, but since the water traveling into my house is only at ground temperature (read: cold), it needs to be heated up. This isn’t done at the water plant—it’s actually coming from inside the house, thanks to a water heater. Here’s how they work.

There are two main types of water heaters: tank and tankless. The former uses … well … a tank, and is much cheaper. The latter is more expensive, but much more efficient. We’ll be focusing on tank water heaters, as they’re the most common type you’ll find in most households.

The Dip Tube Circulates Incoming Cold Water and Helps Evenly Heat It

A cutaway illustration of a gas water heater
Image from US Department of Energy/Wikimedia Commons

When water comes into your house from the city’s water plant (or a well), a soon-to-be-hot water line branches off and makes a stop at your water heater before it travels through the rest of your house. Your water heater contains two holes at the top: a cold water “inlet” and a hot water “outlet.” But how do you prevent the incoming cold water from just directly exiting the outlet? A dip tube!

Incoming cold water gets directly channeled to the bottom of the tank via a dip tube, which is usually made from a thermoplastic polymer (i.e. a heavy-duty plastic). The purpose of this is twofold: it forces the already-hot water to the top where the outlet is, and it ensures that incoming cold water is heated thoroughly before it eventually exits out the top.

Dip tubes are fairly cheap and easy to replace. They also don’t require much maintenance, if ever. The only time you’d likely need to replace a dip tube is if the one that came with your water heater was somehow defective and prematurely failed.

The Gas Burner (or Electric Coils) Provide the Heat

Water heater gas valve
Image from Scott Akerman/Flickr

Most water heaters are powered by either natural gas or electricity. There are other methods as well (like heat pump and solar power), but gas and electricity are the most common.

Gas water heaters are powered by a burner at the bottom of the tank, not unlike the burner on a typical gas stove top. The exhaust gases produced by the burner get expelled through a vent pipe that runs up the center of the tank and out through a chimney at the top.

Electric water heaters are powered by two or three electric coils that are inserted into the side of the tank at varying heights. A cutaway from a This Old House video shows these coils inside the tank.

To keep the water consistently hot, the gas burner or electric coils will periodically fire up for a few minutes at a time. A layer of insulation built around the tank keeps the hot water from rapidly cooling down and preventing the heating elements from turning on more than they need to.

The Anode Rod Attracts Metal-Corroding Elements to Prevent Rusting

Corrosion on the inside of a water heater
The inside of this water heater has been subjected to years of corrosion. Screenshot from This Old House/YouTube

Water and metal don’t mix very well, and since water heaters are made out of steel, there needs to be protocols in place to prevent rusting. The inside of the tank is lined with a protective coating, but water naturally still wants to attack it, and eventually the lining will get eaten away and the tank will begin to rust. To prevent that, a sacrificial anode rod is installed.

The rod is made out of a less noble metal than steel (aluminium, magnesium, or zinc), which results in the water attacking the anode rod first, leaving the inner lining alone.

The “sacrificial” part is important to know, however, because the anode rod needs to be replaced every few years, just like the battery in your smartphone. Eventually, the anode rod will begin to disintegrate, and if you don’t replace it, your water will turn to corrode the inner wall of your water heater instead, eventually causing a major leak (probably at the most inopportune time).

Luckily, replacing an anode rod is super easy, and replacement anode rods are really cheap.

The Pressure Relief Valve Prevents Your Water Heater from Exploding

The pressure relief valve on a water heater
Screenshot from Repair Clinic/YouTube

When water heats up, it expands and creates pressure. There are all sorts of ways water heaters deal with this, and there are multiple safety mechanisms inside to prevent over-pressurization, but the last line of defense is the pressure relief valve.

The normal operation of the water heater’s thermostat will prevent water from overheating and creating too much pressure. If that fails and the heating elements just keep heating and heating the water, with nowhere for the excess pressure to go, the pressure relief valve automatically opens up to relieve that excess pressure. Without this, the water heater would turn into a gigantic hydro-bomb in your basement.

But these valves can fail too, which is why they need to be maintained and replaced when needed. Mineral and calcium buildup can seize the valve shut, preventing it from opening when it needs to. To counter this, it’s recommended to manually open and close the valve about once a year to prevent buildup—And make sure there’s a discharge pipe connected to it.

Are you about to embark on a DIY water heater repair journey? Document it and share it with the world by creating a repair guide on iFixit! Our water heater category is somewhat lacking, but you can help improve it.

Title image by Daniel Crookston/Flickr