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12

Rinnai Water Heater

Severity: Moderate

Source: OnlyErrorCodes.com — Every error code. One place.

Rinnai Tankless Water Heater Code 12 — Flame Loss Fix

What Does This Error Mean?

Code 12 is the mid-cycle flame loss code — ignition succeeded, the unit was running, then the flame went out.
This is usually a gas pressure or venting issue, not an igniter issue.
Check that no other large gas appliance was starting at the same time (water heater plus dryer plus furnace pulls more gas than the line can deliver in some homes).
If isolated incidents, monitor.
If frequent, call for service.

Affected Models

  • Rinnai RU series
  • Rinnai V series
  • Rinnai R75LSi
  • Rinnai RU199iN
  • Rinnai RUR series

Common Causes

  • Low gas pressure during simultaneous appliance use
  • Vent partially blocked causing combustion air starvation
  • Flame sensor partially fouled with carbon
  • Gas valve modulating incorrectly under load
  • Wind-induced downdraft pushing flame off the burner

How to Fix It

  1. Note when Code 12 happens.

    Was anything else gas-using running at the same time?
    Furnace, dryer, stove, fireplace?
    Multiple high-demand appliances together can drop gas line pressure below what the heater needs.
    If Code 12 only happens during those moments, gas line sizing is the issue.

  2. Check the vent termination outside.

    Walk outside and inspect both vent openings.
    Even a partial blockage (50% leaves, half a bird nest) can cause Code 12 — combustion is partially restricted, flame becomes unstable, sensor sees it drop out.
    Clear any debris.

  3. Check propane tank level (propane only).

    On propane installations, low tank pressure causes flame instability.
    If the tank is below 25%, that's likely the cause.
    Refill before continuing.
    Cold weather drops effective tank pressure further — a 30% tank in winter may behave like a 10% tank in summer.

  4. Reset and test.

    Press on/off, wait 30 seconds, press again.
    Run a single hot tap with no other gas appliances running.
    If the heater runs cleanly under those conditions, the issue is multi-appliance gas supply.
    If it fails even alone, the issue is internal.

  5. Call a licensed installer if frequent.

    Occasional Code 12 (once a month) you can monitor.
    Frequent Code 12 (multiple times a day) needs service.
    Common fixes: clean flame sensor, adjust gas pressure regulator, increase gas line size, install a vent extension.
    Diagnosis takes about an hour on site.

Frequently Asked Questions

Why does Code 12 happen more in winter?

Winter combines several factors that stress the heater: more cold air to heat (longer run times), more competing gas demand (furnace running), lower propane vapor pressure if propane-fueled, and sometimes downdrafts from wind.
Code 12 clusters in winter for these reasons.
An annual pre-winter service prevents most of it.

Should I worry about CO from Code 12?

Code 12 itself is the heater's protection working — flame loss triggers shutdown to prevent unburned gas escaping.
That said, repeated Code 12 events suggest combustion is unstable, and unstable combustion can produce more CO than normal.
Have a working CO detector near the heater regardless, and address frequent Code 12 promptly.