Tankless Dead After Rain? Diagnose It Fast

The Short Answer: Rain knocks tankless heaters down three ways: water in the vent/intake, water on the control board, or a GFCI/breaker trip from a soaked outdoor unit. Kill power before you open the jacket. Dry vents and the receptacle first. If you see standing water on the PCB or a persistent 10/12/16 lockout, stop and hire a licensed tech.

Search Intent & The Real Problem

The storm passed. Showers are ice. The outdoor Rinnai, Noritz, Navien, or Rheem on the patio is either dark, clicking, or throwing an air-supply / flame-failure code. Indoor units do the same when a sidewall vent ate wind-driven rain or the condensate trap filled and the pressure switch opened.

This is not “the rain cooled the pipes.” Tankless heaters ignite on demand. If they will not fire after weather, combustion air or electronics got wet, or the unit is locked out to protect the exchanger.

What other guides get wrong

Generic troubleshooting pages often jump to a part or a reset before identifying the unit, fuel, and failure sequence. Start with the model label and the safest outside checks. If the fault involves gas, combustion, pressure, or 240 volts, stop at that boundary and call a licensed professional.

Step-by-Step Troubleshooting

Step 1: Confirm power actually survived the storm

• What to look for: Blank display, dead remote, or a GFCI receptacle with the button popped. Outdoor 120-volt tankless units must be on GFCI in most jurisdictions; rain finds a nicked whip and trips it.
• Required tools: Eyes, the outdoor disconnect, a known-good night-light or receptacle tester.

Reset a tripped GFCI once. If it trips again, there is a ground fault in the unit or the whip — that is not a reset race. Leave it off.

Step 2: Inspect the vent termination, not the burner

• What to look for: Leaves, a bird nest, or a downspout dumping into the intake. Concentric vents with the intake pointed into prevailing wind. Indoor units with a sidewall termination closer to grade than the manufacturer allows, so splash-back fills the pipe. Ice after a cold rain.
• Required tools: Flashlight, step ladder used with three points of contact.

Clear debris from the outside only. Do not shove a hose into the vent “to flush it.” You will flood the fan and the pressure switch.

Step 3: Read the lockout instead of guessing

• What to look for: Air/exhaust blockage (Rinnai 10, many brands similar), flame failure (11/12), overheat (16), or condensate / blocked drain codes. A 10 after rain is almost always water or debris in the intake/exhaust path. A 12 can be a wet flame rod or a drowned burner.
• Required tools: Owner’s manual or the sticker inside the jacket — read it from the front without pulling the gas train.

Power-cycle once after the rain stops and the termination is clear. Repeated resets while the exchanger is wet just add ignition attempts to a flooded burner.

Step 4: Condensate trap and drain — indoor and high-efficiency outdoor

• What to look for: Trap full of rainwater or sludge, a condensate line that runs uphill, a frozen condensate line at the wall, or a neutralized drain that overflowed back into the cabinet.
• Required tools: Wet/dry vac on the drain only, not on gas ports.

If the trap is serviceable from the exterior drain, clearing it is fair DIY. If the overflow is inside the cabinet, stop.

Step 5: Look for water on electronics — then stop

• What to look for: Mineral tracks on the PCB, rust at screw heads, a fogged display, or water behind the front cover. Boards that took rain often fail days later, not only during the storm.
• Required tools: None once you see moisture on a board.

A licensed tech will isolate power, dry or replace the board, check the flame rod, verify the fan, and combustion-test the unit. “Hair-dryer the board” is how you get a delayed short and a service call twice.

When DIY Ends: Calling a Licensed Pro

DIY ends at the cabinet boundary. You may reset one GFCI, clear an exterior termination, and read a code. You may not strip the front cover in the rain, dry a live PCB, or keep clicking ignition on a flooded burner. Wind-driven rain in a vent that was installed too close to a window or grade is an install defect; that is a licensed re-pipe of the vent, not a new heater by default. If the unit sat in standing water, treat it like any flooded gas appliance: do not restart it.

Frequently Asked Questions

Q: Why does my tankless water heater stop working when it rains? A: Water in the intake/exhaust, a tripped GFCI, or rain on the control board. Outdoor units and poorly terminated sidewall vents are the usual victims.

Q: Can I run my outdoor tankless during a storm? A: A correctly installed, rain-capped unit can. If yours dies every storm, the termination, clearances, or electrical whip is wrong — hire a licensed installer to correct it.

Q: What error code means rain got in the vent? A: Air-supply / exhaust blockage codes (often 10) and flame-failure codes (11/12) after weather. Confirm against your brand’s chart; then clear the termination or call a tech if it returns.