Rinnai · Tankless Water Heater Serious
10 — Air Supply or Exhaust Blockage
The unit detects insufficient combustion air or restricted venting. Safety lockout to prevent CO production.
- Keep using it?
- Limited use only
- DIY difficulty
- Moderate
- Verified against
- rinnai.us ↗
How to fix it
- Inspect intake and exhaust terminations outside — clear any blockage.
- Remove front cover and check fan blade for debris; spin freely by hand.
- Measure total vent length and elbow count against installation manual.
- Verify slope on horizontal runs (1/4 inch per foot toward unit or drain on condensing models).
- Check for sealed combustion: no other appliances pulling air from the same space.
- Replace fan motor if it doesn't spin smoothly or draws wrong amps.
Why it happens
Typical signs: Unit fires briefly then shuts off · Code 10 during cold weather · Reduced or no hot water
- Blocked intake or exhaust vent Bird nest, snow, ice, leaves, or wasp nest at termination.
- Vent length exceeds spec Too many elbows or runs too long for the model.
- Improper vent termination Concentric cap clogged or intake/exhaust too close together.
- Dirty fan or fan failure Combustion fan can't move required CFM.
- Negative pressure in mechanical room Other appliances starving the unit of air.
- Condensate blocking exhaust (condensing models) Sagging vent holding water.
Parts you may need
If diagnosis points to a failed part, these are the common replacements. As an Amazon Associate we earn from qualifying purchases.
More Rinnai tankless water heater codes
Always unplug an appliance before servicing it. This guide is for informational purposes and is not a substitute for a qualified technician. Verified against manufacturer documentation: rinnai.us.