Takagi Tankless Not Heating? Problems and Error Codes

The Short Answer: A Takagi that will not heat either never starts (power, low flow, a clogged inlet filter), tries and shows a code (111 ignition failure, 121 loss of flame, 101 or 991 combustion), or runs but delivers lukewarm water (demand, scale, or a plumbing crossover). Watch the heater while a hot tap runs, read any code, check flow, power, and gas supply, and call a technician if a code returns.

Find your model and code style

The rating plate on the side of the cabinet shows the model, such as T-KJr3, T-H3P, T-H3, or T-D3U. Current Takagi models display three-digit codes on the remote or the built-in controller. Some older models used two-digit codes; those are not covered by the table below, so use the manual for your model.

Takagi three-digit codes

These meanings appear in Takagi’s maintenance sheets for the T-H3P (opens in a new tab), T-KJr3 (opens in a new tab), and T-D3U (opens in a new tab). Not every code applies to every model.

Code 991 and its warning 101 relate to combustion quality. Takagi’s checks include gas type, vent blockage and length, the distance from other exhaust terminals, burner and fan dirt, and lint in the heat exchanger. Treat them as a reason to have the heater inspected rather than reset repeatedly.

Not heating: which of three cases?

Watch and listen at the heater while someone opens a hot tap fully. A Takagi that will not heat is usually in one of three states, and each has different checks.

It does nothing: no fan, no code

The heater either has no power or does not see enough flow.

• Power. Check that the controller or remote is lit. If not, check the outlet or circuit breaker that feeds the heater. Reset a tripped breaker or GFCI once; if it trips again, leave it off.
• Flow. A tankless heater needs a minimum flow to start. Open a tub spout fully on hot. If the heater starts there but not at a sink or shower, that fixture’s flow is below the heater’s minimum; a clogged aerator, a low-flow head, or a mixing valve set mostly to cold can cause it.
• Isolation valves. Make sure the cold and hot valves under the heater are fully open, especially after a flush or plumbing work.
• Inlet filter. Takagi’s T-H3P sheet (opens in a new tab) lists clogged cold-inlet (and return) filters among the causes of no hot water and long waits. Your owner’s manual says whether you may clean it yourself; if it requires removing the cover, have it done for you.

It tries, then shows a code

You hear the fan, maybe clicking, and the controller shows a code, most often 111 or 121. Owner checks are limited to the gas supply: the gas valve to the heater is open, other gas appliances work, and a propane tank has fuel and is not iced. After fixing an obvious cause, turn the tap off and on once. A returning code needs a technician. For 101 or 991, look at the outside vent terminal for leaves, snow, or a nest, and have the heater inspected even if it seems to work afterward.

It runs, but the water is lukewarm

Takagi’s sheets also mention scale locking the motor drive of the bypass or flow-adjustment valve. Descaling through isolation valves with a pump and an approved solution is part of normal maintenance in hard water; follow your manual or have it done. Do not raise the set temperature to cover a fault. It increases scald risk and does not fix scale or a crossover.

What a technician will check

For a heater that does nothing, a technician checks the flow sensor and power supply. Otherwise they will use the code and Takagi’s service appendices to check gas pressure, the igniter and gas valve, flame rod current, thermistor resistance, fan voltage, the hi-limit switch, and the board, and look for plumbing crossovers when the water is lukewarm. They will also review venting and the gas line sizing, since an undersized connector or gas line is a common cause of ignition and combustion codes at full firing rate. Most of these are part repairs. A leaking heat exchanger usually means replacing the exchanger or the whole unit, depending on age.

For leaks, see Takagi tankless leaking; for noise, see Takagi vibration.

Frequently Asked Questions

Q: My Takagi has no display. How do I see the code? A: Takagi’s sheets refer to codes shown on the remote or built-in controller and on an LED on the circuit board inside the cabinet. If your unit has no remote, a technician can read the board LED with the cover off. Note the time and what you were doing when the heater stopped.

Q: How do I clear a Takagi code? A: Several codes clear when water flow stops, and others need the power turned off and on. Do it once after fixing an obvious cause. A code that comes back needs diagnosis; see how to reset a Takagi tankless water heater.

Q: Should I flush my Takagi if it is not heating? A: Only if it runs and the water is lukewarm in a hard-water area. Flushing does not fix a dead controller, a gas supply problem, or an ignition code.

Q: Why is it fine in summer but weak in winter? A: Incoming water is colder in winter, so the heater must raise the temperature more at the same flow. If it now struggles at flows that used to work, scale may also be reducing its output.