Takagi Water Heater Leaking

Common Symptoms

• Water dripping from the condensate drain port at the bottom of the unit (condensing models)
• T&P (temperature and pressure) relief valve dripping onto the floor or into the drain line
• Water seeping from the inlet or outlet union fitting connections
• Water dripping from the flow sensor body or surrounding plumbing connections
• Water appearing around the unit after a hard freeze
• Visible water stains on the heat exchanger body — possible scale perforation
• Error 90 error code alongside a visible heat exchanger leak

Most Likely Causes

1. 1

   Condensate Drain Line Leak (TK-CB1 Condensate Neutralizer, Error 90 Heat Exchanger Perforation)

   Takagi condensing models (T-KJr2, T-H3, T-D2) extract additional heat from exhaust gases, which causes water vapor to condense inside the heat exchanger — this condensate is acidic (pH 3–4) and must drain through the condensate drain port at the base of the unit. Two failure modes: (1) Condensate drain line blockage — if the condensate drain line becomes clogged with debris or mineral deposits, condensate backs up inside the unit and can overflow through gaps in the cabinet. Clear the line and install or replace the Takagi TK-CB1 condensate neutralizer kit, which raises the pH of the condensate to an acceptable level before it enters the drain system. (2) Heat exchanger perforation from scale — if the primary heat exchanger wall has been perforated by scale-induced corrosion (indicated by Error 90 history followed by a new water leak from the heat exchanger body), the condensate leak originates from inside the heat exchanger, not the drain line. This requires a descale-vs-replace decision (see final cause below).

2. 2

   T&P Relief Valve Dripping (Expansion Tank, Watts 100XL Replacement)

   The temperature and pressure relief valve on Takagi tankless units is a safety device that opens when water temperature exceeds 210°F or system pressure exceeds 150 psi (exact rating printed on the valve body). A dripping T&P valve in a system that is not obviously overheating is almost always caused by thermal expansion pressure in a closed plumbing system — the supply side has a backflow preventer, pressure reducing valve, or check valve that prevents expanded hot water from relieving back into the street main. As water heats, pressure rises and the T&P valve drips to relieve it. Fix: install a thermal expansion tank on the cold water supply line upstream of the Takagi unit. If the valve continues dripping after an expansion tank is installed, test the valve by carefully lifting the test lever — if water flows freely and stops cleanly when released, the valve is functioning normally but the expansion tank needs adjustment or replacement. If the valve leaks continuously even when the system is cold, replace it with a Watts 100XL or equivalent rated for the Takagi's BTU output and working pressure.

3. 3

   Inlet/Outlet Union Fitting Leak (Teflon Tape + Pipe Dope, Takagi Isolator Valve Kit)

   The threaded union fittings at the cold water inlet and hot water outlet connections are the most common external leak points on Takagi tankless units, especially in the first few years of operation or after any service work. Union fittings use both Teflon thread tape and pipe joint compound (pipe dope) applied to the male threads for a watertight seal. If only Teflon tape was used (common in residential rough-in), minor leaks can develop as the fitting cycles through thermal expansion. Repair: shut off the cold water supply and drain the unit, remove and clean the union fitting threads, apply 3–4 wraps of Teflon tape followed by a thin coat of pipe dope on top of the tape, and reinstall hand-tight plus one to two turns with a wrench. Takagi recommends installing isolator valve kits at both the inlet and outlet connections — these provide service port valves for future descale procedures and are supplied as a kit (Takagi isolator valve kit) with the correct thread specifications for T-KJr2, T-H3, and T-D2 models.

4. 4

   Flow Sensor O-Ring Leak

   The flow sensor on Takagi tankless units is threaded into the cold water inlet manifold with an O-ring seal. Over time, the O-ring can harden, crack, or deform from heat cycling, causing a slow drip from the flow sensor body. This leak is typically a few drops per minute visible directly at the flow sensor housing. Repair: shut off the cold water supply, remove the flow sensor (typically 3/4-inch thread), replace the O-ring with a new one of the correct durometer (standard EPDM plumbing O-ring; confirm diameter from your Takagi model service manual), apply a thin film of silicone plumber's grease to the new O-ring, and reinstall. This repair takes approximately 30 minutes and requires no soldering.

5. 5

   Freeze Damage (Built-In Freeze Protection, Drain Procedure, Foam Insulation on Supply Lines)

   Takagi tankless units have built-in freeze protection that operates a small electric heating element inside the unit when temperatures approach freezing — but this only protects the internal components when the unit is plugged in. The supply and return lines running to the unit are not protected by the internal freeze protection system. In freeze-prone climates, Takagi recommends installing foam pipe insulation on all supply lines entering the unit. If the unit was unpowered during a hard freeze (power outage, trip), or the supply lines froze, the expanding ice can crack the heat exchanger, the flow sensor body, or the inlet/outlet connections. After a hard freeze, inspect all connections carefully — cracks in the heat exchanger body require unit replacement. Drain procedure for winterization: shut off the cold water supply, open a hot water faucet to relieve pressure, use compressed air to blow out the cold supply line and hot outlet line from the service ports, confirm no standing water remains in the heat exchanger, and unplug the unit.

6. 6

   Scale-Perforated Primary Heat Exchanger (Descale vs. Replace Decision)

   When scale buildup inside the primary heat exchanger is severe enough to perforate the heat exchanger wall (indicated by Error 90 history plus a new water leak from the heat exchanger body, not from external fittings), a descale-vs-replace decision is required. Decision criteria: (1) If the unit shows Error 90 but no visible leak from the heat exchanger body, always attempt descale first — the perforation may not have occurred yet. (2) If there is a visible water drip from the heat exchanger body and the unit is under 8 years old, obtain a Takagi service estimate — heat exchanger replacement may be covered under extended warranty or be cost-effective. (3) If the unit is over 10–12 years old with confirmed heat exchanger perforation, unit replacement is typically more economical than heat exchanger replacement, which can cost $400–$800 in parts alone. Note: Takagi (A.O. Smith / Bradford White group) heat exchanger parts sometimes cross-reference across brands.

Quick DIY Checks

1. 1Step 1 — Identify the leak source precisely: dry the area around the Takagi unit completely with towels. Run the unit through a full hot water draw cycle (2–3 minutes). Watch closely to identify exactly where water first appears — condensate drain port at the base, T&P valve discharge pipe, a union fitting connection, the flow sensor body, or the heat exchanger casing itself. Take a photo of the leak location before beginning any repairs. The repair procedure differs completely depending on the source.
2. 2Step 2 — Condensate drain line (condensing T-KJr2, T-H3, T-D2 models): if water drips from the condensate drain port or overflows at the base during operation, locate the condensate drain line at the bottom of the unit and confirm it exits to a proper drain or the Takagi TK-CB1 condensate neutralizer. Clear any blockage in the drain line (a wet/dry shop vac at the exit end of the line works well). Install or replace the TK-CB1 condensate neutralizer kit if it is absent, clogged, or more than 3 years old. If the drain is clear but the heat exchanger body itself is visibly dripping water (not from the drain port), this indicates heat exchanger perforation — proceed to the descale-vs-replace evaluation in Step 7.
3. 3Step 3 — T&P valve drip (expansion tank and valve test): locate the T&P relief valve on the Takagi unit — it will have a discharge pipe running to the floor or a drain. Check if there is a thermal expansion tank installed on the cold water supply line upstream of the Takagi. If no expansion tank is present, install one (3-gallon minimum for residential; confirm pre-charge pressure equals static supply pressure). After installing the expansion tank, monitor the T&P valve over 24 hours. If the T&P valve still drips, carefully lift the test lever — if water flows and stops cleanly, the valve is working correctly and the expansion tank pressure needs adjustment. If the valve leaks continuously when the system is cold, replace with a Watts 100XL or equivalent rated for your Takagi model's specifications.

1. 4Step 4 — Inlet/outlet union fitting leak: confirm the leak is originating at a union fitting, not from deeper inside the unit. Shut off the cold water supply and drain the Takagi by opening a hot water faucet. Disassemble the leaking union fitting, clean all thread surfaces, apply 3–4 wraps of Teflon tape to the male threads, apply a thin coat of pipe joint compound (pipe dope) on top of the Teflon tape, and reassemble hand-tight plus one to two turns. Restore water supply and check for leaks. If the fitting connection is original and the unit is several years old, consider replacing the standard fitting with the Takagi isolator valve kit — this provides service port access for future descale procedures.
2. 5Step 5 — Flow sensor O-ring replacement: confirm the drip is from the flow sensor housing (a small cylindrical sensor threaded into the cold water inlet manifold with a wire harness). Shut off the cold water supply and drain the unit. Remove the flow sensor using an adjustable wrench or pliers. Inspect the O-ring — if it is hardened, cracked, or deformed, replace it with a new EPDM O-ring of the correct size (consult the Takagi parts diagram for your model number). Apply a thin film of silicone plumber's grease to the new O-ring before reinstalling. Torque the sensor snugly — do not overtighten, as this can crack the sensor housing. Restore water supply and confirm the drip has stopped.
3. 6Step 6 — Freeze damage inspection: after any hard-freeze event, inspect all external connections (inlet union, outlet union, T&P valve, flow sensor, condensate drain) for cracks or splits. Fill the unit with water and check for drips from the heat exchanger body with the unit powered but not firing. A drip from the heat exchanger body after a freeze event indicates internal heat exchanger cracking — this requires unit replacement. If all external connections look intact, restore normal operation and monitor for 24 hours before concluding no freeze damage occurred.
4. 7Step 7 — Scale-perforated heat exchanger evaluation (Error 90 + heat exchanger body leak): if the leak is from the heat exchanger body (not from external fittings) and Error 90 has appeared recently, attempt a full descale procedure first (see Takagi water heater not heating article). After descaling, if the leak from the heat exchanger body stops, the perforation may have been minor scale damage that sealed during descaling — monitor closely. If the leak continues or grows after descaling, heat exchanger replacement or unit replacement is required. Units under 8 years old may be eligible for extended warranty coverage — contact Takagi / A.O. Smith technical support at 1-888-882-5244.

Repair vs Replace

Recommended Tools & Parts

• Takagi TK-CB1 Condensate Neutralizer Kit

  Takagi-specific condensate neutralizer kit for T-KJr2, T-H3, and T-D2 condensing models. Raises condensate pH from acidic (pH 3–4) to neutral before entering the household drain system.

  $30–$60

  Buy on Amazon →

• Watts 100XL T&P Relief Valve

  Replacement temperature and pressure relief valve. Match the BTU and pressure rating stamped on the original valve — the Watts 100XL is a widely compatible replacement for Takagi residential models.

  $15–$35

  Buy on Amazon →

• Thermal Expansion Tank (3-gallon)

  Install on the cold water supply line upstream of the Takagi to absorb thermal expansion in a closed plumbing system. Eliminates T&P valve dripping caused by pressure rise during heating cycles.

  $25–$50

  Buy on Amazon →

• Takagi Isolator Valve Kit

  Service isolation valve kit for Takagi cold inlet and hot outlet connections. Replaces standard union fittings with service port valves for future descale access.

  $30–$60

  Buy on Amazon →

• EPDM O-Ring Set (Flow Sensor)

  Standard EPDM plumbing O-rings for flow sensor replacement. Confirm the correct diameter from the Takagi service manual for your model number.

  $5–$15

  Buy on Amazon →

Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.

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