Tankless Water Heater Not Heating: Causes and Repairs

What are the 5 causes of a tankless water heater not heating?

The 5 causes of a tankless water heater not heating are listed below.

• Scale on the heat exchanger blocks heat transfer in hard-water homes.
• A low flow rate stays below the minimum that fires the burner.
• An undersized gas line starves the burner.
• A blocked vent or air intake shuts the unit down.
• A clogged inlet filter cuts the water flow.

Is a tankless water heater error code serious?

It depends on the code: a scale or flow code needs cleaning, and an ignition or venting code needs a plumber.

What should you do when a tankless water heater stops heating?

The 4 things you should do when a tankless water heater stops heating are listed below.

1. Write down the error code on the display.
2. Open a hot tap fully to raise the flow rate.
3. Clean the inlet water filter.
4. Call a plumber when the code returns.

When should you call a plumber?

You should call a plumber in the 4 situations below.

• The error code returns after a reset.
• The unit has not been descaled in a year.
• The burner does not fire.
• Water leaks from the unit.

Related plumbing problems and next steps

How much does the plumbing repair cost?

The plumbing repair cost depends on the repair: water heater repair costs $222–$990, according to HomeAdvisor. Tankless water heater repair prices and local plumbers are on the tankless water heater repair in Lancaster, PA page.

What is a related plumbing problem?

A related plumbing problem is hot water runs out fast, and the hot water runs out fast guide lists its causes and first steps.

Written and fact-checked by the Lancaster PA Plumbers editorial team using HomeAdvisor, EPA WaterSense and City of Lancaster data. Last updated October 29, 2026.