It’s a familiar scene of domestic frustration. You’re tackling a sink full of greasy cookware, relying on a torrent of hot water to cut through the grime, when the stream suddenly turns lukewarm. Or perhaps you’ve noticed the shower head, once vigorous, is now sputtering, its nozzles choked by a chalky, white residue. We often blame the faucet or the pipes, but the true culprit may lie silent and hidden: a high-performance machine slowly being suffocated from the inside.
Many have turned to the promise of electric tankless water heaters, like the powerful Rheem RTEX-36, for an endless supply of hot water. It’s an enticing vision—a compact, wall-mounted unit delivering relentless heat on demand. Yet, this endless stream is not guaranteed. It is conditional upon understanding a fundamental battle happening within its copper pathways: a constant war between thermal physics and water chemistry. The secret to truly clean, efficient, and unending hot water isn’t just in the machine you buy, but in understanding the science of how to keep it healthy.
The Heart of Instant Heat: A Symphony of Power and Precision
To appreciate the challenge of keeping a tankless heater clean, we must first marvel at its sheer power. At its core, a unit like the Rheem RTEX-36 is a masterclass in controlled energy conversion, governed by one of physics’ most fundamental principles: Joule’s First Law. When you turn on a hot water tap, a sensor detects the flow and signals the unit to awaken. In that instant, it draws an immense 36,000 watts of electrical power.
To put that in perspective, 36 kilowatts is the equivalent of turning on over six hundred 60-watt incandescent light bulbs simultaneously. It is an enormous amount of energy, channeled through robust copper and brass heating elements. These metals are chosen for their superb thermal conductivity; they are designed to transfer that raw electrical energy into heat and infuse it into the passing water with breathtaking speed and an efficiency rating of over 99%. A sophisticated electronic brain, using a technique called power modulation, acts like a conductor, constantly adjusting the energy flow to ensure the water exiting the unit is at the precise temperature you’ve set, whether you’re running a trickle of water or a full-force shower. This is the science that delivers the promise of instant, unwavering heat.
The Geography of Performance: Why Your Location Dictates Your Flow
While the heater’s power is a constant, the performance you experience is profoundly influenced by your environment. The single most important concept to understand is Temperature Rise—the number of degrees the heater must raise the water’s temperature from the moment it enters your home to the moment it leaves the tap.
A 36kW heater has a finite amount of energy. This energy can either be used to heat a large volume of water by a small amount, or a smaller volume of water by a large amount. This creates an inverse relationship between the flow rate (Gallons Per Minute, or GPM) and the required temperature rise.
Consider the tale of two homes. In Miami, Florida, the groundwater entering a house in winter might be a mild 65°F (18°C). To reach a comfortable shower temperature of 105°F (40°C), the heater only needs to achieve a 40°F temperature rise. At this demand, the RTEX-36 can comfortably supply a high flow rate, easily supporting multiple fixtures at once.
Now, transport that same unit to a home in Chicago, Illinois, where the winter ground water can be a frigid 40°F (4°C). To reach the same 105°F shower, the heater must now work much harder, achieving a staggering 65°F temperature rise. The laws of physics dictate that to achieve this greater heating feat, the flow of water must be slower. The 6 GPM maximum rating is a theoretical ceiling; your home’s real-world performance is written by its geography. This isn’t a flaw in the design; it’s a fundamental principle of thermodynamics.
The Unseen Threat: Hard Water’s Chemical Assault
Herein lies the critical link to cleanliness and maintenance. The very process of rapidly heating water, especially in certain regions, triggers a chemical reaction that can cripple the system. Much of North America is supplied with “hard water,” which is rich in dissolved minerals like calcium (Ca^{2+}) and magnesium (Mg^{2+}). In cold water, these minerals remain harmlessly dissolved.
However, when water is heated, its chemistry changes. The dissolved minerals precipitate out of the solution, forming solid calcium carbonate (CaCO_3) and magnesium carbonate (MgCO_3)—the compounds that make up limescale. Think of it as the plaque that builds up inside the arteries of your home. This rock-hard scale clings tenaciously to the hottest surfaces it can find: the very heating elements designed to warm your water.
This buildup is a silent killer of performance. Limescale is an excellent insulator. A thin layer coating the heating elements forces the unit to work harder and consume more electricity to transfer the same amount of heat to the water. As the buildup thickens, it restricts water flow, leading to the sputtering shower heads and reduced pressure you notice at the tap. In the worst-case scenario, the scale can cause the elements to overheat and fail completely, turning your source of endless hot water into a silent, useless box on the wall.
The Essential Ritual: Scientific Maintenance for Peak Performance
This is why, for a high-performance tankless water heater, maintenance isn’t optional; it’s an essential ritual rooted in chemistry. The goal is to periodically dissolve and flush out the mineral buildup before it causes permanent damage. This is where the “Frequently Bought Together” item often seen with these heaters—a set of service valves—becomes critically important.
These valves allow you to isolate the heater from your home’s plumbing and circulate a cleaning solution through it. The process, known as descaling, typically involves using a mild, food-grade acid like white vinegar (acetic acid) or a citric acid solution. The acid reacts with the alkaline mineral deposits of the limescale, dissolving them into a solution that can be safely flushed away.
How often should this be done? The answer, again, lies in geography. By consulting a resource like the U.S. Geological Survey’s water hardness map, you can determine the mineral content of your local water supply. In regions with very hard water, descaling may be a necessary annual ritual. In areas with naturally soft water, you may only need to do it every few years. This proactive cleaning doesn’t just prolong the life of the heater; it restores its original efficiency, ensuring you aren’t paying extra on your utility bills to heat water through a layer of stone.
The Non-Negotiable Prerequisite: Taming an Electrical Leviathan
Before any of this can be considered, there is one final, unyielding law to obey: the law of electrical capacity. A 36kW, 240V appliance is an electrical leviathan. It demands a staggering 150 amps of dedicated electrical service.
To be clear, this is not a device you can simply plug in. For most homes, installing this unit is a major electrical project. It typically requires four separate 40-amp double-pole breakers in your home’s main electrical panel, which itself must have enough surplus capacity—usually a 200-amp main service or greater—to handle such a massive load.
This work is unequivocally not a DIY project. It requires a licensed and insured electrician who understands local building codes and the precise wiring requirements needed to safely power the unit. Attempting to run this heater on an inadequate circuit is not only a fire hazard but will also cause the unit to perform poorly, if at all. Viewing this requirement not as an inconvenience but as a non-negotiable prerequisite for safety and performance is the first, most critical step in considering such a powerful upgrade for your home.
A New Relationship with Water
An electric tankless water heater represents a shift in our relationship with a fundamental home utility. It asks us to move beyond the “fit and forget” mentality of the old, inefficient storage tank. It offers incredible performance, but in return, it demands respect for the laws of physics, an understanding of your local chemistry, and a commitment to its well-being.
The path to truly clean, pure, and unending hot water flows not just from the tap, but from the knowledge we apply to the machines that serve us. By understanding the science within its walls, we empower ourselves to maintain the health of our home’s arteries, ensuring a powerful and efficient flow for years to come.
