Tankless Leaking After a Freeze

The Short Answer: A tankless that leaks after a freeze is a split heat exchanger, a cracked fitting, or a freeze-drain valve that will not reseat — not a “drip until spring.” Freeze protection is electric. Power off in 15°F weather is how outdoor and garage units burst. Shut the cold isolation valve, kill 120 V and gas, then hire a licensed plumber. Do not fire a wet cabinet.

Search Intent & The Real Problem

The polar night passed. You have a puddle. People search this and get vinegar-flush articles. Rinnai, Navien, Noritz, Rheem, Takagi — same physics: thin waterways, no 50-gallon buffer, freeze heaters on 120 V. Board dead = protection dead. First thaw opens the split. Designed freeze-drain ports on some outdoor units dump on purpose around the mid-30s °F; that is not the same as a cracked exchanger. Brand leak maps: Rinnai from the bottom, Navien from the bottom, Noritz from the bottom, Takagi leaking, Rheem leaking inside. Outage the same night: leaking after a power outage, Rinnai after an outage. Constant dump with no freeze: drains excessively.

What other guides get wrong

They say tankless “cannot freeze because there is no tank.” There is a heat exchanger full of water. They Flex Seal a split outdoor R53 and call it a repair. They tell you freeze protection “runs on gas.” The ceramic heaters and the board are 120 V. They confuse condensate, T&P, designed freeze-drain ports, and a burst waterway — four fluids, four jobs. And they have you fire the burner “to thaw it.” That is how you flood the PCB and dump unburned gas.

Step-by-Step Troubleshooting

Step 1: Name the wet part — freeze dump vs a split

• What to look for: One dry paper towel per fitting. Temperature of the drip. Photo of the cabinet before you wipe everything into one story.
• Required tools: Paper towel, flashlight.

Can you still run cold elsewhere? Isolate the heater either way — using water while it leaks is a house decision, not a reason to keep the unit live.

Step 2: Kill water and energy the right way

• What to look for: Cold isolation closed. Hot isolation closed if you have service valves. Appliance switch off, then the disconnect or GFCI. Gas cock off. A unit with no isolation kit is a house shutoff plus a wet floor — hire it. Do not open the front cover on a live, dripping PCB.
• Required tools: Isolation handles, eyes on the GFCI. Bucket.

If the GFCI will not hold, leave it off. That is water on 120 V, not a nuisance.

Step 3: Ice plug vs burst — do not apply a torch

• What to look for: Outdoor unit, power was off, still frozen, little or no puddle yet. Ice can block flow so the board never fires — that looks like no water flow until it thaws and then leaks. Warm the room, not the copper, with space heat that is not a torch or a heat gun on plastic. Indoor garage that hit 15°F with no freeze kit is the same split as an outdoor patio unit. Recirc lines in an attic freeze first and dump into the cabinet later.
• Required tools: Freeze history (when power died, overnight low). No open flame on the exchanger.

A freeze-split then a shut isolation is how “no flow” hides a leak. Open a faucet only after you are ready to catch water.

Step 4: T&P and house pressure — the thaw that is not a split

• What to look for: Static over ~80 psi at a hose bib. Waterlogged expansion tank. First recovery after the restore lifts a healthy 150 psi / 210°F relief. That is Rinnai T&P physics, not a cracked plate. A stream from the relief is shut down and a licensed call. Never plug it. Condensate that overflowed because the pump sat dead during the outage is cool and acidic, not potable.
• Required tools: Hose-bib gauge. Tire gauge on an expansion tank if you have one.

Rain the same week: dead after rain can wet the cap without splitting the exchanger.

Step 5: When the exchanger is actually dead

• What to look for: Water returning inside the jacket with isolation open and T&P dry. Cracked polymer fittings at the top. Filter housing split. Board wet. Warranty on freeze is almost always void if power was off — still a licensed diagnosis so you are not sold a $1,800 exchanger for a $12 drain-port valve. Flex Seal, epoxy, and “tank repair kits” on a pressure vessel are delays until the ceiling fails. Sister tank-type freeze after an outage is a different machine; this page is tankless waterways.
• Required tools: None for a DIY “repair.” Photo for the warranty claim. Licensed combustion after any exchanger swap.

Manifold gas pressure (NG ~3.5–7 in. w.c. / LP ~10–11 in. w.c. — confirm the plate) is irrelevant until the waterway holds water. Do not purge a gas line because it leaked.

When DIY Ends: Calling a Licensed Pro

You may isolate the cold, kill power and gas, name the wet fitting, and photograph the cabinet. Stop at a stream from inside the jacket, a wet PCB, gas smell, a freeze-drain valve that will not reseat, static over 80 psi, and any thought of torching ice. Heat-exchanger replacement, freeze-kit add-ons, and PRV/expansion work are licensed. Same-day isolation beats a collapsed ceiling. Running the burner “to thaw it” is how boards die and warranties void.

Frequently Asked Questions

Q: Why is my tankless water heater leaking after a freeze? A: A split heat exchanger or fitting after freeze protection lost 120 V, or a freeze-drain port that opened and will not reseat. Condensate and T&P can also wet the floor on the first thaw. Name the fluid. Hire a licensed plumber.

Q: Can I run a tankless that froze if I just reset it? A: Not if it leaked. Isolate water, keep power off, and have the waterway inspected. Ice in the exchanger that has not split yet still is not a torch job. Freeze protection needs power.

Q: Does tankless freeze protection work during a power outage? A: No. The heaters and the board are electric. Gas will not save an outdoor or garage unit at 15°F with the outlet dead. That is why outages in a cold snap become exchanger jobs.