Rinnai 10: Air Supply or Exhaust Blockage

Winter mornings are the classic trigger for this one: the display reads 10 and the water heater won't fire, usually because something outside — snow, ice, a nest — is blocking the intake or exhaust vent the unit breathes through.

Applies to: Rinnai tankless and condensing water heaters.

What it means

Air Supply or Exhaust Blockage — the unit isn't getting enough combustion air or the exhaust path is restricted. Because combustion needs a clear path for both intake air and exhaust gas, the control board shuts the burner down rather than run with a restricted vent.

Most common causes

The blockage is usually at one of these points:

Blocked or obstructed intake or exhaust vent terminal
Improper venting materials, length, or connections
Combustion fan blockage or failure
DIP switches not set correctly for the venting configuration
Scale/debris buildup in the heat exchanger fins

What you can safely check yourself

There's genuinely useful homeowner troubleshooting here, since the vent terminal is outside and visible:

Check the outdoor vent intake and exhaust terminals for snow, ice, leaves, insect nests, or other debris and clear anything you can safely reach
Confirm nothing was recently placed near the vent terminal (stored items, new landscaping)

When to call a professional

The vent terminals look clear but the code persists, the vent is not easily/safely accessible, or you suspect the combustion fan itself has failed.

Venting work beyond clearing visible outdoor debris, and anything involving the gas supply, should be left to a licensed HVAC technician or plumber.

What the fix costs

Clearing a blocked vent terminal yourself costs nothing but a few minutes. If a technician needs to inspect or clear venting that isn't safely reachable, expect a standard diagnostic or service call in the $150–$600 range; a failed combustion fan pushes the repair toward $200–$500 installed once parts and labor are included (Angi, HomeGuide contractor rate guides). These are national ranges — get a written quote locally before agreeing to a repair.

Related codes

All Rinnai codes

Every Rinnai water heater fault code — the full list on one page.

Safety. Restricted venting can push combustion exhaust — including carbon monoxide — somewhere it shouldn't go. If the vent terminal isn't safely reachable, is buried under snow you'd need a ladder for, or the code keeps returning after you've cleared it, stop and call a licensed technician instead of running the unit past this fault.

Sources

Rinnai tankless water heater installation and troubleshooting manual (manufacturer documentation)

This site is an independent reference, not a substitute for a licensed plumber or electrician. Gas water heaters can leak carbon monoxide, leak gas, or catch fire if serviced incorrectly. Anything involving the gas line, gas control valve, venting, or main electrical wiring should be handled by a licensed professional. Pages are written with AI assistance from the sources listed on each page and checked against them before publishing.