How Tankless Water Heaters Work: A Look Inside the Smart Technology

A tankless water heater appears to perform magic. You turn on a tap, and seconds later, perfectly hot water flows out at a seemingly unwavering temperature. It’s a far cry from the fluctuating temperatures of a traditional tank that’s slowly running out of hot water. This isn’t magic; it’s a high-speed conversation between a few sophisticated electronic components.

Let’s pop the hood and see how this digital ballet creates your perfect shower. At the heart of it are three key players: the flow sensor, the thermistors, and the modulating gas valve.

1. The Trigger: The Water Flow Sensor

This is the gatekeeper. It’s a small turbine or magnetic sensor located in the cold water inlet pipe. When you turn on a hot water tap, the moving water causes the sensor to spin or trip. This sends an electrical signal to the unit’s “brain”—the printed circuit board (PCB)—that says one simple thing: “Wake up! Someone wants hot water, and they want it now.”

This is also why there’s a slight delay (or “cold water sandwich”) when you first turn on the tap. The heater doesn’t know you need hot water until this sensor detects flow.

2. The Eyes: The Inlet and Outlet Thermistors

Thermistors are simply temperature sensors. Your tankless heater has at least two of them, and they act as the unit’s eyes, constantly gathering critical data.

• The Inlet Thermistor: This measures the temperature of the cold water coming into the unit. This is crucial data, as we learned in our sizing guide. The PCB needs to know if it’s starting with 70°F water from a Florida summer or 38°F water from a Minnesota winter.
• The Outlet Thermistor: This measures the temperature of the water as it leaves the heat exchanger. Its job is to report back to the PCB: “Are we hitting our target temperature?”

3. The Muscle: The Modulating Gas Valve

This is where the real intelligence lies. Unlike a simple on/off valve in an old furnace, the modulating gas valve is like a precision dimmer switch for the gas burners. Based on the constant stream of data from the flow sensor and thermistors, the PCB performs thousands of calculations per second.

The conversation goes something like this: * PCB: “Flow sensor reports 2.5 gallons per minute. Inlet thermistor reads 55°F. The user wants 120°F water. Calculate the required heat output.” * PCB to Gas Valve: “Open to 75% power to achieve a 65°F temperature rise at this flow rate.” * Outlet Thermistor: “Reporting 121°F… a little too hot.” * PCB to Gas Valve: “Modulate down to 73% power.” * Outlet Thermistor: “Reporting 120°F. Holding steady.”

Now, imagine someone else flushes a toilet, causing a drop in water pressure. * Flow Sensor: “Flow has dropped to 2.2 GPM!” * PCB: “Recalculating! To maintain 120°F at this new, lower flow rate, we need less heat.” * PCB to Gas Valve: “Modulate down to 65% power, now!”

This entire feedback loop happens in milliseconds. It’s this constant, high-speed adjustment that allows a quality tankless heater, like the Camplux WA528 which advertises ±1°F temperature stability, to deliver a consistent stream of hot water, even when water pressure and flow rates are changing.

So, the next time you step into a perfectly hot, stable shower, you can thank that tiny, silent conversation happening between a sensor, a chip, and a valve—working in perfect harmony to turn a cold start into pure comfort.