How Hot Water Heaters Work

gas water heater

Here on the Economy Plumbing Services blog we take a look at and talk about a lot about how hot water heaters can break, why they may be leaking, that sort of thing. But you know what we’ve never done? We’ve never explained how hot water heaters work. So if you’re curious how that big cylinder in your garage takes cold water and produces that relaxing, piping hot water we need!

Just about every home out there has a water heater, though they may take different shapes depending. For the part they all work the same way, at least broadly speaking. And that’s how we’re going to be talking abut it today, to keep it easily understandable. First step? Looking at the parts inside the water heater!


diagram of how hot water heaters workParts of a Water Heater

  • The Tank 

Inside that big cylinder is a heavy tank with a water protective liner. This holds between 40 to 60 gallons, usually. It keeps the water under 50-100 psi. The outside of the tank is covered in insulating materials to keep the heat in. Then there’s the outer shell of the heater.

  • The Dip Tube

The dip tube is the pipe that lets fresh water enter the water heater through the top of the tank. 

  • The Shut-Off Valve

The shut-off valve shuts off (no duh, right?) the water flowing into the heater. The shut-off valve is outside of the tank, usually above / along the line that feeds the water into the tank.

  • The Heat-Out Pipe 

This is the pipe that hot water travels out from the heater towards your home’s plumbing system and on to whichever appliance needs it.

  • The Thermostat 

The thermostat in a water heater works just like the one for your home. Whatever you set it to is the temperature your water heater is going to raise the water to. Some models of water heaters will have multiple thermostats.

  • The Heating Element 

There are two ways water heaters heat water. It could be electric or gas. Electric water heaters use a heating element, a bit of metal that gets crazy hot as electricity flows through it. Gas heaters will use a burner, igniting the gas and creating a flame to warm the tank of water.

  • The Drain and Pressure Relief Valves 

These two valves are responsible for the maintenance and safety of the water heater. The drain valve at the bottom of the heater allows for easy access to empty the tank, as well as replace heating elements, or flush the tank of sediment. The Pressure Relief valve helps keep the pressure at safe levels.

  • The Sacrificial Anode Rod 

This unique piece of the puzzle is suspended in the tank to help slow corrosion. It’s made of a steel core covered in either aluminum or magnesium. Basically, the corrosion will attack this rod before it does anything else – hence ‘sacrificial’.


Alright with those listed out you might already be getting a clearer idea of how it all works, but let’s lay it out here!

How it Heats the Water

Before anything else, the water heater’s thermostat is going to be set between 120 – 140° Fahrenheit. This makes it hot enough for most household uses and not too hot so as to avoid scalding etc. If you hover around the lower side of the dial, you can also save yourself some energy so be open to moving the dial if you like! With the thermostat set it’s the tank is ready to do its job.

First, the dip tube will take cold water from your freshwater lines to the bottom of the tank. At the bottom of the tank the water is warmed up via the heating element or burner until the water reaches the set temperature.

Hot water rises, so as the water heats, it moves to the top of the tank, and reaches the heat-out pipe. When an appliance draws from the water heater it will pull the hottest water first from the heat-out pipe, while the fresh cold water sinks to the bottom of the tank. When the hot water is used up, the tank will fill with cold water which will somewhat heat but instead the appliances, shower or washers, will draw warm or even cold water.

Simple thermodynamics… or something. It’s the principles of plumbing and it all works out! 

So – how does your hot water heater work? Does it deliver the hot water you need to get a good shower, run a load of dishes, or laundry? If not, you know who to call!

If Water Runs Through it Or To It, We Do It!

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