What Actually Happens Inside a Tankless Water Heater During a Flush

When people hear the word “flush,” they usually imagine water being run through a system for a few minutes. That’s not what proper tankless maintenance looks like.

A professional flush is a controlled cleaning process designed to remove mineral buildup from inside the heat exchanger, which is the heart of the system.

Why Tankless Systems Need to Be Flushed at All

Tankless water heaters heat water instantly as it passes through narrow internal channels. When hard water is heated, minerals separate and attach themselves to the hottest surfaces.

Over time, this buildup restricts flow, traps heat, and forces the system to work harder than it should.

Step 1: Isolating the System

The first step is shutting the unit off and isolating it from the home’s plumbing. This allows the cleaning solution to circulate only inside the heater without entering your household water supply.

This is a controlled, closed-loop process.

Step 2: Circulating the Descaling Solution

A pump circulates a manufacturer-approved descaling solution through the heat exchanger. This solution breaks down calcium and mineral deposits that have formed along the internal walls.

You can’t see this buildup from the outside, but it’s always there to some degree.

Step 3: Rinsing and Clearing the System

Once the minerals are dissolved, the system is thoroughly flushed with clean water to remove any remaining residue. This restores proper flow and heat transfer inside the unit.

Step 4: Filter Cleaning and System Check

Tankless systems also include inlet filters designed to catch debris before it enters the heater. These filters are removed, cleaned, and reinstalled during service.

Flow rate, temperature stability, and system operation are checked before returning the unit to service.

Why This Matters Long-Term

A flush is not just about today’s performance. It:

• Reduces internal stress on components

• Maintains efficiency

• Helps prevent premature failure

• Keeps the system operating within manufacturer specs

This is why annual service is recommended, especially in areas with mineral-heavy water.

Final Thought

Tankless water heaters are precision systems. Flushing isn’t optional maintenance, it’s preventive care that protects the most expensive parts of the unit.