A lot of people picture a tankless water heater as a small tank hidden inside a box, a little reservoir that stays warm and waits for a tap to open. That isn’t how it works. There is no stored hot water in the unit at all. Everything happens in the seconds between turning the handle and feeling warmth arrive, and that short window is where all the mechanics live. To understand which part a technician is replacing when something fails, it helps to follow a single stream of water from the moment it enters the unit to the moment it leaves as hot.
How the Flow Sensor Knows You Opened a Tap
Nothing fires up until water actually moves. At the cold inlet sits a flow sensor, usually a small turbine or paddle wheel that spins when water passes through it. As it turns, it generates a pulsing electrical signal, and the speed of those pulses tells the system how fast water is flowing. This is the trigger for the entire sequence. If the flow is below a minimum threshold, often around half a gallon per minute, the heater stays dormant to avoid igniting for a trickle.
Because the sensor is the first thing to react, problems here show up as a unit that won’t start even though water runs fine, or one that cycles on and off at low flow. Mineral buildup on the turbine or a worn bearing can keep it from spinning freely, and the control system reads that as no demand.
The Heat Exchanger and Where the Warming Actually Happens
Once flow is confirmed, water is routed through the heat exchanger, and this is the heart of the machine. In a gas unit, the exchanger is a dense coil or bank of finned tubing sitting directly above a burner. The flame heats the metal, and the water winding through those passages absorbs that heat in a single pass. In an electric unit, the water flows past heating elements instead, but the principle is the same: a large surface area transfers energy into fast-moving water.
The engineering challenge is enormous. The system has to dump a great deal of heat into water that is only in contact with the metal for a moment, and it has to hold the outlet temperature steady even as flow changes when someone opens a second faucet. That is why the exchanger is built from copper or stainless alloys that conduct heat quickly and tolerate repeated thermal stress. It is also why scale is such an enemy here: a thin crust of hardened minerals on the inner walls acts as insulation, forcing the burner to work harder and overheating the metal until it cracks.
What the Control Board Coordinates Behind the Scenes
Between the sensor reading and the hot water leaving the outlet, a control board is doing continuous math. It reads the inlet temperature, the flow rate, and the outlet temperature, then adjusts how much gas the valve delivers or how much power the elements draw so the water hits your set point. When flow rises, it calls for more heat; when you ease off the tap, it backs the burner down within a fraction of a second.
The board also runs safety logic. It watches a flame sensor to confirm ignition, monitors for overheating, and shuts the unit down with an error code if any reading falls outside safe limits. Many of the cryptic fault numbers homeowners see are the board reporting exactly which of these checks failed, which makes it a valuable diagnostic tool rather than just a brain.
The Parts Most Likely to Fail and What a Leak Repair Involves
Knowing the flow of water through the unit makes the common failures predictable. Igniters and flame sensors wear out, flow sensors clog, and the heat exchanger is the component most likely to develop a true leak after years of scale and thermal cycling. A leak can also come from gaskets, fittings, or the isolation valves, so the first job is tracing water back to its source rather than assuming the worst.
When a cracked exchanger or a failed seal is confirmed, the fix means draining and depressurizing the unit, removing panels, and either resealing a joint or swapping the component outright, work that a shop like Leak Repair in the Waco area handles as a routine call. A quick patch rarely holds on a part under constant pressure and heat, which is why proper repair follows the water path back to the exact failure point.
Most of these failures trace back to scale, and scale is preventable. Flushing the unit with a descaling solution once a year, more often in hard-water homes, keeps the exchanger clean, the flow sensor spinning, and the whole on-demand sequence working the way it was designed to.
