Tankless Water Heater Leaking After a Freeze
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The Short Answer: If a tankless heater starts leaking after a hard freeze, water probably froze inside it and split the heat exchanger, a water valve, or a fitting. Built-in freeze protection needs electric power, so an outage during a cold snap is the usual trigger. Close the water supply to the heater, turn off its power and gas, and have it inspected before using it again.
Why freezing breaks tankless heaters
A tankless heater holds only a small amount of water in narrow passages, but that water can freeze solid in an unheated garage, attic, or outdoor installation. Most units have small electric heaters that warm the water passages in cold weather. They only work while the heater has power. If the power was out, the heater was unplugged, or the temperature dropped below what the protection is designed for (wind chill on outdoor units can make a difference), the water can freeze. Ice expands and splits copper, brass, or plastic parts. The leak often appears when the ice thaws, sometimes days later.
Some outdoor models also have drain-down or freeze-relief valves that release water on purpose to protect the heater. A drip from one of those during cold weather can be normal; your manual identifies them.
Shut it down
- Close the cold-water inlet valve to the heater (usually at the bottom of the unit, often blue). If there is no valve, close the home’s main water valve.
- Turn off the heater’s power switch or breaker if you can reach it from a dry spot.
- For gas models, close the gas valve to the heater.
- Open a hot tap to relieve pressure and catch any water left in the line.
Where is the water coming from?
| Location | Likely cause | What it means |
|---|---|---|
| Inside the cabinet, bottom of the unit | Split heat exchanger or internal water valve | Technician; often a major repair |
| Isolation valves or their service-port caps | Cracked valve body or cap | Usually a minor repair |
| Pipes near the heater | Split pipe or fitting | Plumbing repair |
| Relief valve discharge | Pressure while thawing, or a damaged valve | Replace the valve if it keeps leaking |
| Designated freeze-relief or drain valve on an outdoor unit | Designed release in cold weather | Normal if it stops when temperatures rise |
| Condensate line on a condensing model | Frozen or split drain line | Clear or replace the line |
If the heater still contains ice, wait for it to thaw naturally by warming the space. Do not use a torch or heat gun on the cabinet.
What the technician will check
A plumber or tankless technician will pressurize the heater with the cover off and look for leaks at the heat exchanger, flow sensor, water valves, and internal connections. They will check whether the circuit board got wet, and inspect the condensate system on condensing models. They will also look at why freeze protection failed: power supply, a tripped GFCI, a failed freeze-protection heater, or exposure beyond the unit’s rating.
Repair or replace: cracked isolation valves, fittings, and drain lines are inexpensive. A split heat exchanger is a major repair, and on an older heater, replacement is often more sensible. Many manufacturers exclude freeze damage from warranty coverage, so check yours before approving work.
Preventing the next one
- Keep the heater plugged in and powered all winter, including when you travel.
- If a long outage is expected during freezing weather, drain the heater following its manual.
- Insulate exposed water lines near the heater. For outdoor units in very cold climates, ask about the manufacturer’s freeze-protection accessories.
- For a leak that happens during an outage rather than after a freeze, see water heater leaking after a power outage. For leaks without a freeze, see tankless water heater leaking from the bottom.
Frequently Asked Questions
Q: Can freeze protection work without power? A: No. The built-in heaters and controls need electricity. Gas alone does not protect the unit when it is not firing.
Q: The heater froze but is not leaking. Can I use it? A: Let it thaw completely, then run water while watching for drips inside and below the cabinet. If you see any leak, an error code, or reduced flow, stop and have it inspected.
Manuals and supporting references
These links distinguish general safety information from model-specific documentation. A manual directory helps you locate your unit’s instructions; it does not establish that every model uses the same parts, codes, or test values.
- A. O. Smith technical bulletin 51: puddles around a heater (opens in a new tab)Common water-leak sources and the distinction between fittings and a failed tank.
- PG&E: recognizing and responding to a gas leak (opens in a new tab)Emergency actions for suspected fuel-gas leaks.
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