Takagi Tankless Leaking: Find the Source
The Short Answer: Water under a Takagi is not one failure. Cool acidic drip from the drain is condensate on T-H3-class condensing units. Hot water from the brass PRV is pressure. A weep at the filter cap or water-valve body is an O-ring. A drip from inside the jacket with those dry is often the heat exchanger — shut it down. Do not run a wet PCB.
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Search Intent & The Real Problem
You pulled the cover on a T-KJr2, T-K4, T-H3, T-D2, T-M32, or an AO Smith-badged twin and found a puddle. Indoor condensing units hide three wet systems in one jacket: potable water, condensate, and the T&P. Outdoor non-condensing units add rain and freeze-drain. Treating all of them as “the tankless is shot” is how people replace an exchanger because a filter cap was a quarter-turn off.
Codes, ignition, and scale live here: Takagi problems. Sister leaks: Navien from the bottom, Rinnai at the exhaust.
What other guides get wrong
They call condensate “a leak” and tape the drain. They skip temperature: condensate is cool; PRV discharge is house-temp to hot; an exchanger leak is usually hot and from inside the jacket. They acid-flush through the house plumbing. And they treat every outdoor drip as a cracked exchanger when the unit sat through a 15°F night with power off.
Step-by-Step Troubleshooting
Step 1: Identify the fluid
| Clue | Source |
|---|---|
| Cool, slightly acidic (pH ~3–5), only while the burner runs, from the PVC/PP drain | Condensate — normal if piped, a leak if the joint failed |
| From the brass PRV, pulses on a call for heat | Pressure / expansion — do not cap. Typical relief 150 psi / 210°F |
| Filter-cap threads, polymer water-valve body, drain-plug O-rings | Service fittings — often a save |
| Hot, from the exchanger fins or the bottom pan with fittings dry | Potable leak — shut down |
| Only after rain, outdoor non-condensing | Wind-driven rain or a missing cap |
| After a freeze, then a thaw | Split exchanger or freeze valve |
- What to look for: Dry it. Run one hot faucet. Wipe again. Temperature of the drip, and whether it is the drain, the PRV tube, the filter, or the pan.
- Required tools: Paper towel, flashlight. Optional pH strip.
Step 2: Condensate path (T-H3 and other condensing)
- What to look for: Trap full, drain line uphill, frozen termination, loose union at the cabinet, neutralized media blocking flow so it overflows inside. Indoor T-H3 / T-H3J / T-H3S will wet the pan and look like a bottom leak. A blocked collector can also shove droplets out the flue on post-purge — same physics as the Rinnai exhaust article.
- Required tools: Eyes on the drain path, wet/dry vac on the condensate drain only.
Non-condensing T-KJr2 / T-K4 should not be raining condensate into the cabinet. Water there is potable, rain, or freeze until proven otherwise. Clearing an external trap is DIY. Water behind the cover on the PCB is not.
Step 3: PRV versus house pressure
- What to look for: Relief on the hot outlet with a discharge pipe. Static pressure over 80 psi at a hose bib. Waterlogged expansion tank (Schrader spits water). PRV that dribbles after someone lifted the test lever. Same story as Rinnai T&P.
- Required tools: Hose-bib gauge, tire gauge on the expansion tank.
If that pipe is dry and the cabinet is wet, you are past “just a valve.” Never plug a relief.
Step 4: Filter housing, drain plugs, water-valve O-rings
- What to look for: Cold-inlet screen cap not seated, cracked polymer housing, isolation-valve stem packing, drain plugs at the bottom of the heat exchanger left weeping after a flush. Takagi water-flow / bypass servo bodies leak at O-rings and drip down the front into the pan — looks like “the exchanger.”
- Required tools: Isolation valves. Inlet-filter wrench or taped channel locks.
A new cap O-ring is a parts-store save. Close cold and hot isolation. If it still weeps from inside after the unit depressurizes, that water is the exchanger or a cracked internal waterway.
Step 5: Heat exchanger and freeze
- What to look for: White scale trails down the copper (non-condensing) or stainless secondary (condensing). Isolation valves closed, residual drained, drip continues from the fins or the bottom pan. Outdoor unit that iced with power off — freeze-split exchangers leak on the first thaw. Error history 14 / 16 (overheat) next to a wet jacket is scale plus a leak, not a reset loop — reset once after you fix the cause, not ten times through a flooded burner.
- Required tools: Isolation valves, freeze history. No epoxy.
Internal exchanger leaks are replacement of the exchanger or the unit. Do not silicone the cabinet floor.
When DIY Ends: Calling a Licensed Pro
Stop at a wet PCB, a stream from inside the jacket, gas smell, a freeze-split exchanger, or a PRV that is streaming. You may snug a condensate union, rinse the inlet filter, and photograph the drip. Heat-exchanger diagnosis, combustion, and replacement are licensed. Takagi / AO Smith will not love an exchanger that ran with a blocked condensate line or a unit that froze with the switch off.
Frequently Asked Questions
Q: Why is my Takagi tankless water heater leaking? A: Condensate overflow, a loose drain fitting, PRV discharge, a filter or water-valve O-ring, or a heat exchanger. Identify whether the water is cool condensate or hot potable before you buy parts.
Q: Can I keep using a Takagi that drips from the bottom? A: Not if the cabinet or board is wet, and not if the exchanger is the source. Condensate at a proper drain can wait a day. Internal potable leaks cannot.
Q: Is a Takagi heat-exchanger leak repairable? A: Internal exchanger leaks are replacement of the exchanger or the unit. External fittings, drain-plug O-rings, and the condensate trap are repairable. Do not epoxy the exchanger.