Rheem RTEX-24 Electric Tankless Water Heater: Endless Hot Water, On Demand

For millennia, humanity has been locked in a relentless quest for a simple, profound luxury: hot water on demand. The ancient Romans, masters of engineering, came closest. They built the magnificent thermae, sprawling public bathhouses where a complex network of furnaces and lead pipes, the hypocaust system, delivered warmth to the fortunate citizens. It was a monumental undertaking, a testament to the deep-seated human craving for the comfort and cleanliness that hot water provides. It was civilization, measured in degrees.

Two thousand years later, have we truly mastered this ancient dream? For most of the last century, our solution has been the storage-tank water heater—a squat, cylindrical giant relegated to a dusty corner of the basement or garage. It’s a loyal, if deeply flawed, servant. We can think of it as a sleeping beast, constantly sipping energy to keep its 40- or 50-gallon belly full of hot water, just in case we need it. This continuous energy drain, known as standby heat loss, makes it one of the least efficient appliances in a modern home. And its loyalty is finite. We’ve all felt the sting of its betrayal: that moment in a long shower when the comforting warmth sputters and dies, leaving us in a sudden, shocking cold. The giant’s belly is empty.

This is the tyranny of the tank. It dictates the length of our showers, occupies precious square footage, and silently pads our energy bills. But what if we could escape it? What if, instead of storing heat, we could create it, brilliantly and instantly, in the exact moment we need it? This isn’t science fiction; it’s the philosophy behind a machine like the Rheem RTEX-24, and it represents a fundamental shift in our relationship with energy. It’s about moving from a state of passive storage to one of active, intelligent generation.

Unleashing the Electron Flood: Anatomy of an Instant

To understand how a tankless heater works, you must first appreciate the sheer physics of its task. Water is incredibly good at absorbing heat; it’s why we use it in our car’s cooling system. To raise the temperature of moving water significantly and instantaneously requires a colossal amount of energy. The RTEX-24 achieves this by becoming a controlled lightning storm inside a box.

When you turn on a hot water tap, a sensor detects the flow and awakens the beast. It unleashes 24,000 watts of power through its heating elements. To put that in perspective, a standard hairdryer runs on about 1,200 watts. The RTEX-24 is the equivalent of turning on twenty hairdryers at the same instant. This is the raw muscle.

The scientific principle at play is Joule heating, first described by James Prescott Joule in the 1840s. It states that the heat produced by an electrical current passing through a conductor is proportional to the square of the current. In simple terms: push enough electricity through a resistant material, and it gets incredibly hot, incredibly fast.

This immense power has an unavoidable consequence, one rooted in another cornerstone of physics: Ohm’s Law ($P = V \times I$). For the RTEX-24, the math is stark: 24,000 Watts divided by a standard 240-Volt supply equals a massive 100 Amperes of current. That is why, as its manual and any qualified electrician will attest, this device requires no less than three separate 40-Amp circuits, wired with thick 8-gauge copper. This isn’t an arbitrary inconvenience; it is a mandate from the laws of physics, enforced by the standards of the National Electrical Code (NEC) to ensure safety. You are, in effect, installing a small, dedicated power station on your wall.

But brute force alone is clumsy. The true genius of the system lies in its brain—the self-modulating technology. Imagine a driver who only knows how to floor the accelerator. That’s a “dumb” heater. The RTEX-24, however, is a virtuoso conductor. Its sensors act as the conductor’s ears, constantly listening to the incoming water temperature and the flow rate you’ve requested. The microprocessor then directs the precise amount of power—not a watt more or less than needed—to achieve the temperature you’ve set on the dial. Washing your hands might require just a whisper of power, while filling a tub on a frigid morning will call for the full orchestra. This intelligent control is what eliminates waste and achieves its remarkable energy efficiency.

The Real-World Gauntlet: Facing Cold and Hard Water

In a perfect lab setting, the science is clean. In our homes, it gets messy. Two major real-world factors challenge any tankless water heater: the temperature of the ground and the chemistry of the water itself.

First, thermodynamics. The heater’s job is to produce a “temperature rise.” It adds a fixed amount of energy to the water. This means its performance is entirely dependent on the starting temperature of the water entering your home. Let’s consider the geography of North America. If you live in Miami, your incoming groundwater might be a mild 70°F (21°C). To get to a 105°F (41°C) shower, the heater only needs to add 35 degrees of heat. But if you live in Montreal, where the current time reminds us that winters are no joke, your incoming water in January might be a bone-chilling 40°F (4°C). To reach that same 105°F shower, the heater must now add a whopping 65 degrees. For the same amount of energy, it can heat less water. As the RTEX-24’s own sizing chart shows, this means your maximum flow rate for a hot shower in a cold climate is significantly lower than in a warm one. This isn’t a flaw; it’s a non-negotiable law of nature.

The second challenge is the invisible enemy: water hardness. The “harder” your water, the more dissolved minerals like calcium and magnesium it contains. When heated, these minerals precipitate out and form limescale—a crusty, white deposit. This scale can build up on the heating elements, acting like a layer of insulation. The heater now has to work harder to push heat through this mineral blanket, reducing efficiency and potentially shortening its lifespan. This is why in regions with hard water, manufacturers and organizations like the Water Quality Association (WQA) strongly recommend regular descaling or the installation of a water softener to protect your investment.

Mastery of a Modern Ritual

To step from a world of storage tanks to on-demand hot water is to embrace a new philosophy. It’s an upgrade that touches on a desire as old as the Romans, but it requires an understanding of thoroughly modern science. The thrum of power, the silent intelligence of its modulation, the unforgiving realities of thermodynamics—it all converges at your faucet.

By understanding the “why” behind the strict installation requirements and the performance variables, we transform from passive consumers into informed masters of our domestic environment. We know that the torrent of warmth cascading into our bathtub is not magic; it is 24,000 watts of precisely controlled Joule heating, obeying the laws of physics and the standards of modern engineering. The flow is no longer just hot water. It’s a stream of comfort, earned through two millennia of ingenuity, and delivered, finally, on your own terms.