Tankless Water Heater Repair

Many tankless problems come from scale buildup, a clogged inlet filter, too little water flow or gas and venting issues, and the unit's error code usually points to which. Minor repairs cost about $50–$300, bigger repairs $300–$1,300, and professional descaling about $150–$350.

Fix this weekDIY: Moderate1–2 hoursPro: $300–$1,300

Last reviewed September 2026

Troubleshoot it now

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Check 1 of 13Testing: Tankless heater

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Is there an error code?

Look at the display and check the code in the manual. It usually names the problem.

Signs it's this problem

An error code on the display
Water turns cold during a shower ('cold water sandwich')
Hot water only at high flow, not at a trickle
The unit fires and shuts off repeatedly

Why it happens

Tankless units heat water instantly as it flows through a small heat exchanger. Hard water leaves mineral scale that restricts flow and triggers overheating errors, and the unit won't fire if flow is too low or combustion air and venting are blocked.

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How to fix it, step by step

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1.Look up the error code in the manual; it usually names the problem.
2.Open a hot tap wider; many units need a minimum flow to fire.
3.Turn off the water and clean the cold-water inlet filter screen.
4.Descale the heat exchanger using the service valves, a small pump and vinegar or a descaling solution, following the manual.
5.Check that air intake and exhaust vents outside are clear of debris, snow or nests.
6.If error codes point to gas supply, ignition or the flame sensor, call a qualified technician.

Safety: Turn off power and gas before opening the unit. Gas and venting work should be done by a qualified technician.

Tankless Water Heater Repair cost

US national ranges for professional work, including labor and materials
Job	Typical cost
Minor tankless repair	$50–$300
Major tankless repair	$300–$1,300
Professional descaling / flush	$150–$350
Replace with a gas tankless heater	$3,000–$5,500

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Repair typeQuote amount (USD)

Typical professional price for this job: $300–$1,300

Common questions

How often should a tankless water heater be flushed?

At least once a year, and more often with hard water. Regular descaling prevents many of the most common error codes and extends the unit's life.

Why does my tankless water heater go cold in the shower?

Often it's low flow (a water-saving showerhead below the unit's minimum), scale buildup, or a clogged inlet filter. Descaling and cleaning the filter fix most cases.