Takagi T-K4 Tankless Water Heaters Review

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The detailed review of Takagi T-K4 tankless water heaters. T-K4 units are designed for residential water heating in high demanding homes, using propane and natural gas, and for outdoor and indoor installation.

Takagi T-K4 models

There are two gas models from T-K4 series and two from T-K4U (Ultra Low NOx models), all designed as the non-condensing units:

  • Takagi T-K4-OS is the gas tankless water heater for the outdoor installation only, that is using the built-in power vent to move the products of combustion out. Since it is installed outside, it continuously uses the fresh air for gas combustion. T-K4-OS model frees up an indoor space, eliminates the danger if the gas leaks and saves money since it does not use any vent pipes. Takagi also offers the recess box, TK-RB02, so when the water heater is installed in it, it is protected from various weather conditions and abuse.
  • Takagi T-K4-IN is a gas water heater for the indoor use only, which can use a direct vent (DV) kit for more flexibility in the installation. The flue pipe has to be per codes, Category III and made of stainless steel, which can run horizontally through the wall or vertically through the roof, and up to 50 feet in length. Both indoor and outdoor units are small and lighter than the water tank heaters, so wall-hanging design allows you to free up the floor space.

Takagi T-K4 features


Takagi TK4Takagi T-K4 water heaters are using the tankless technologyfor the on-demand water heating while providing a continuous and endless supply of hot water. During the water heating, water heater adjusts its power by using the fully modulating gas valve, for the optimized operation.

The modulation allows water heaters to work in the range from the minimum 11,000 BTU to the maximum 190,000 BTU, either using the propane or natural gas. In both cases, energy factor is 0.82 or higher, which makes these units energy efficient and also green, with the reduced emission of greenhouse gases.

Temperature change

A remote control is included for the convenient use - the temperature of the hot water can be set in the range from 99 F to 167 F (16 options), also to monitor the heating operation and notify when something is wrong with the unit. If the remote control is not used, you can select one of four temperatures by changing the dip-switch settings on the computer board: 113, 122, 131 and 140 F where the 122 F is factory preset value.


The flow rate through the water heater is limited to 8 gallons per minute, which can supply up to four showers at the same time (calculated household flow rate for the shower is 2 GPM). The minimum flow rate, which activates the water heater, is 0.75 GPM while 0.4 GPM is the minimum flow that keeps these gas water heaters running.


T-K4 water heaters are self-diagnostic. The diagnostic system allows easy troubleshooting and repair. If something is wrong with the unit, and you have a remote control, TK-RE02, installed, the error code will display, or if the controller is not installed, red LED will be blinking.

Safety features

  • Internal freeze protection system with the provided electrical power. It activates when the surrounding temperature drops below 36.5 F.
  • Manual reset hi-limit switch
  • Exhaust and water temperature safety control
  • Overheat cutoff fuse
  • Air fuel ratio (AFR) sensor


If you are looking for a tankless water heater with an endless hot water, with the on-demand operation, that is compact and has space saving design; and you live in the house with the high demand for hot water, Takagi T-K4 is a good choice for you. By being the energy efficient, these water heaters save the energy and with the computerized safety system, protect you and your home. The warranty is 15 years on the heat exchanger, 5 on other components and 1 on labor, which is very similar to most of the manufacturers in North America.

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Pictures are courtesy of Takagi WH company

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