The Magic of Hot Water 

We take for granted many of the comforts of modern life. If an apocalyptic event neutered our system of specialists and severed our ability to access information instantaneously, how many of us could recreate most of the mechanical wonders in our homes? We’re surrounded by wires, fiber optics, and pipes, but largely ignorant of their schematic webs. This observation is not intended to critique modern society. We’re (largely) more educated than ever before. We rely on the totality of human knowledge to propel experts into innovation and maintenance.

Yet it might not hurt to be curious about the guts of our society, just in case zombies or chupacabras ever rise up and seize control.

I recently realized I didn’t really understand how hot water arrives throughout my abode. I turn on the shower and choose my desired temperature. I knew it came from a hot water heater, but that gadget seemed like a magic box. Who knows what wizardry happens in there?

It’s time to rectify my ignorance!

A typical hot water heater

Advances have brought tankless water heaters to reality but, for most of the lifespan of this technology, we’ve relied on heaters with giant vats.

The functionality of traditional hot water tanks is a fantastic mixture of science, simplicity, and ingenuity. We warm water for our homes thanks to a variety of sources: electricity, natural gas, or solar. When a tank utilizes natural gas, for example, a flame burns on the bottom of the tank to heat the water. These tanks can be very large – 40, 60, or 80 gallons – and it’s difficult to keep that much water consistently at the same temperature. How does the tank know which water is hot enough to send throughout the home?

Essentially, the tank doesn’t need to know. It relies on the physical laws of the universe. Typically, fluids (both air and water are fluids) become less dense as they are heated. The old adage “hot air rises” is a result of this fact. Gravity rides everything, but it rides denser things with more oomph. If there’s a gradient in temperature/density within a fluid, the warmer parts tend to rise. This magic happens inside a hot water tank, too.

The basic anatomy of a tanked hot water heater - graphic by US Department of Energy

The image above might seem a bit complex, but it’s actually rather straightforward.

In this type of water heater, a pipe sits in the center of the contraption. At the bottom is a burner, which works on natural gas. The toxic fumes from burning natural gas rise through the flue and exit the home. Surrounding this vertical metal vent is a tank that contains water. Metals are wonderful conductors. As the fumes rise, they also warm the metal, which transfers the energy to the water. Cold water enters the tank at the top but is piped to the bottom, above the flame. As the water heats, it rises, where another pipe waits to disperse to the rest of your house’s system. Want hotter water? Burn more gas.

That’s the simple part and it’s rather clever to harness physics so efficiently.

What’s the ingenious part? Most hot water tanks are composed of metal. If metals are so good at conducting heat, what stops the hotness from escaping to the air around the tank? Engineers needed to design a device that simultaneously transfers heat easily but also acts as a massive insulator.

The answer once again arose from the air. Our atmosphere is a lot worse at transferring heat than metal, so the tanks are ringed with insulation filled with a multitude of air pockets. As a result, the tanks can keep water relatively warm for long periods.

OK, so how does the water get from the heater to the various sinks and showers around the house?

The pipes into a house are under pressure, which keeps the flow going in the correct direction. The handles on your faucets are mini dams that keep back jets of water. When you turn a faucet, the dams open and water spurts out. Turn it all the way open for a refreshing stream; barely rotate for a drip.

Engineers developed another magic trick at the faucet level. It would be extremely impractical to summon water at a specific temperature throughout households at will. However, with just a turn of one or two handles, we can tune the temperature to what we desire. How does this wonder happen?


As you can see in the diagram above, modern water lines feature two separate sources, one hot and one cold. The cold comes from the outside; the hot comes from your heater. Inside a faucet, the handles work on ratios. Open the hot line and you get water as warm as it comes from your heater. Open the cold line and you receive a chilly rivulet. The faucets can mix the lines in the spigot, though. Need lukewarm water? Maybe a ratio of one-quarter hot to three-quarters cold will work. Need a nearly scalding shower? Flip the proportions.

This shrewd method means we only need two temperatures at any moment, not a wide spectrum.

Of course, hot water heaters have additional features, but the simplicity of the preceding explanation is the bulk of the process.

Is knowing this procedure enough to get the world back into hot water if everything falls apart? Probably not, but taking a peek behind the wizard’s curtain is always beneficial. Then again, G.I. Joe taught us that knowing is half the battle. Perhaps we could rebuild society starting with the hot water heater.

Further Reading and Exploration

How A Water Heater Works – Landmark Home Warranty

How Do Faucets Regulate Water Temperature? – Wonderopolis

What makes heat rise? – West Texas A&M

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