Two wall-mounted tankless water heaters with expansion tanks on an exterior wall
8 components covered

Field manual · Water Heater

Takagi tankless (T-KJr2, T-H3 family): quick sheet

Covers: Takagi residential gas tankless heaters: T-KJr2 (non-condensing, indoor and outdoor) and the T-H3 condensing family (e.g. T-H3-DV-N). Takagi is an A.O. Smith brand; retailers report the Takagi brand was discontinued at the end of 2025, so this sheet is mainly for servicing installed units. Error codes are in the Takagi / A.O. Smith tankless code list in the library.

Single sourceBrands: Takagi, A.O. Smith8 components

Takagi units use three-digit error codes (111 ignition, 121 flame loss, 311/321/331 thermistors, 441 flow sensor, etc.). The T-KJr2 is a compact 140,000 BTU/h non-condensing unit; the T-H3 is a 199,000 BTU/h condensing unit with PVC/CPVC venting. Typical calls: clogged inlet filter, scale, low gas supply on undersized lines, and cross-flow at mixing faucets causing temperature swings.

Safety first

  • GAS: no work on the gas valve, burner, gas pressure or venting; refer to a licensed plumber or gas technician
  • Unplug the unit before opening it
  • Do not install outdoor models where Takagi says freezing exposure is not allowed
  • Scald risk at outlet

How it works

A flow sensor in the cold inlet tells the board when water is moving. On the T-KJr2 Takagi lists about 0.5 gpm to start the burner and about 0.4 gpm to keep it running. The board then runs the fan, ignites electronically and modulates gas (T-KJr2 19,500-140,000 BTU/h; T-H3-DV-N 15,000-199,000 BTU/h).

The T-KJr2 offers preset temperatures (113, 122, 131 and 140°F on the unit; other values need the optional remote). Takagi's support page calls the 122°F default right for most homes and notes that setting it too high can cause temperature fluctuation.

Non-condensing T-KJr2 units vent with Category III stainless vent; condensing T-H3 units may use PVC/CPVC/PP (Category IV) and produce condensate. Neither may be common-vented with other appliances.

Water quality matters: Takagi recommends a softener or scale-control device at about 7+ grains per gallon and says reverse-osmosis water must not be used because it damages the heat exchanger.

Components & tests

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Official manufacturer documentation

Always confirm values against the tech sheet for the exact model.

OnCall Tech is a reference tool for trained technicians and can make mistakes. Always disconnect power and confirm with the manufacturer's documentation before you work. © 2026 OnCall Tech. Original content; not a reproduction of any manufacturer document.Generic part photos (not of this brand): Two wall-mounted tankless water heaters with expansion tanks on an exterior wall by Downtowngal, CC BY-SA 4.0

Diagnostic guides for this appliance

Guide · Water Heater

Heat-pump (hybrid) water heater: slow recovery, noise, errors

Hybrids need enough room air and a clean air filter, a working condensate drain and the right mode. Many complaints are mode settings or a dirty filter. Refrigerant faults are sealed-system work.

5 steps ~60 minmoderate
Guide · Water Heater

Tankless water heater descaling (flush)

Scale insulates the heat exchanger, causing overheat codes and fluctuating temperature. Flush with a pump and descaling solution through the isolation valves.

5 steps ~75 mineasy
Guide · Water Heater

Hot water recirculation pump problems

Check timer/settings, check valves, air in the loop and the pump itself. On tankless units with built-in pumps, a recirculation code may point to air or a leak in the loop.

5 steps ~60 minmoderate

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A.O. Smith ProLine / Signature and State atmospheric gas tanks: quick sheet (diagnostic and electrical side)

Residential atmospheric-vent, standing-pilot FVIR gas tanks: A.O. Smith ProLine (plumber channel, e.g. GCR and GCG families), A.O. Smith Signature (sold at Lowe's) and comparable State models, with a thermopile-powered electronic gas control and status LED (Honeywell-type control on most current units). Power-vent, direct-vent and condensing models are excluded.

8 components7 common failuresA.O. Smith, State
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Rinnai V-series non-condensing tankless (V53DeN, V65, V75, V94): quick sheet

Rinnai Value (V) series non-condensing gas tankless heaters: V53DeN (outdoor), V65eN/V65iN, V75eN/V75iN, V94eN/V94iN and their propane (P) versions. Water side, electrical side and diagnostics only; gas, burner and venting are referral items.

9 components7 common failuresRinnai
Takagi tankless (T-KJr2, T-H3 family): quick sheet · OnCall Tech