What Really Happens If You Never Flush a Tankless Water Heater

One of the most common things I hear is:

“It’s been working fine, so I didn’t think it needed anything.”

“What about your filter?” “Did a water line get broken during the build process of your home? Are you sure the filter is clean?”

And for a while, that’s true. A tankless water heater can run for years without obvious problems — even when it’s never been flushed. But that doesn’t mean nothing is happening inside the system.

Here’s a clear, honest breakdown of what actually happens when tankless maintenance is skipped.

The First 1–2 Years: No Symptoms (But Scale Starts Forming)

In the early years, everything feels normal.

• Water heats quickly
• Pressure seems fine
• No error codes
• No noise

But internally, minerals from the water begin coating the heat exchanger. You won’t feel it yet — but it’s starting.

Years 3–5: Subtle Performance Changes

This is when most homeowners start noticing small things:

• Hot water takes longer to arrive
• Temperature fluctuates slightly
• The unit sounds louder than it used to
• Flow on the hot side feels weaker

At this stage, scale has narrowed the internal water pathways. The system is working harder to keep up.

Years 6–8: Noticeable Problems

If the unit still hasn’t been flushed, problems become harder to ignore:

• Consistent temperature swings
• Reduced hot water output
• Rumbling or vibrating noises
• Burner cycling more aggressively
• Error codes related to flow or ignition

The heater isn’t failing — it’s fighting mineral buildup.

Years 8+: Expensive Consequences

This is where skipped maintenance gets costly.

Heavy scale can cause:

• Overheating of the heat exchanger
• Sensor failures
• Premature component wear
• Complete system shutdown
• Warranty issues if maintenance was required

At this point, flushing may help — but some damage can be permanent.

Why a Flush Works (Even After Years)

A professional flush:

• Dissolves mineral buildup inside the heat exchanger
• Restores proper flow
• Reduces burner strain
• Improves temperature stability
• Makes the system run quieter

Many homeowners are shocked by how much better the system performs after its first-ever flush.

The Bottom Line

A tankless water heater doesn’t fail suddenly — it slowly loses efficiency over time when maintenance is skipped.

Annual flushing is simple, affordable, and one of the easiest ways to protect your investment and avoid expensive problems later.