Takagi T-K4 Tankless Water Heaters Review and Buying Tips

A detailed review of Takagi tankless water heaters from the T-K4 series. What to look for when buying tankless, available models and top features. Buying tips. See where the T-K4 heaters can be used and how hard it is to install one.

Takagi manufacturer has been designing quality water heating devices for over 70 years. Most of the new models come with the condensing design, which uses the latest advanced technology, while others, which are also cost-effective, are non-condensing, still offering an energy-efficient solution and reliable operation.



Takagi T-K4 water heaters

Takagi T-K4 models are tankless and non-condensing water heaters designed for residential water heating in high-demand homes. They can also be used in recirculation systems, hydronic and radiant floor systems, and in combination with the water storage tanks, in direct or indirect setup. Some of them are using natural gas, some propane, and are installed either indoor or outdoor.

Takagi T-K4 series of tankless water heaters are built to provide great comfort to its users and with the provided remote controller – convenience, and at the same time save energy, floor space, and money. All the models are small (20" x 14" x 9") and lightweight (43-45 lbs.), making them easy to handle and hang on the wall.

Just by looking at how T-K4 works, you will see that savings are expected due to its great engineering. Water is heated on demand while passing through the heat exchanger, and hot water is supplied to the shower almost instantly, in endless supply and continuously. With such an innovative design, you will never run out of hot water. With the capacity to heat up to 3 showers simultaneously, there will be plenty of hot water for your large family. Even teenagers.

Moreover, with the advanced features, hot water is always hot, and delivery is uninterrupted. There will be much less temperature fluctuation, also known as "cold water sandwich," to deal with. Also, as there is no storage tank, water is not losing heat when not using it (no standby heat loss).

Model review

Takagi T-K4 water heaters are non-condensing which means that the units use one heat exchanger, and there is no water preheating. The energy efficiency is still high - it is around 0.80.

Takagi T-K4-IN (available on amazon.com) is a water heater designed for indoor installation only using a power vent or power direct vent for more flexibility in the installation. The flue pipe has to be per codes, Category III and made of stainless steel, with an exhaust vent diameter of 4", which can run horizontally through the wall or vertically through the roof, and up to 50 feet in length.

Takagi T-K4-OS (available on amazon.com) is the gas tankless water heater designed for outdoor installation only and 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.

The T-K4-OS model frees up an indoor space, and due to the outdoor location 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 it is protected from various weather conditions and abuse when the water heater is installed in it.

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 water heaters use tankless technology for on-demand water heating while providing a continuous and endless supply of hot water. A water heater adjusts its power during water heating 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 of 190,000 BTU, either using natural gas (N) or propane (P). In both cases, the energy factor is around 0.80, making these units - energy-efficient and green, with the reduced emission of greenhouse gases.

Easy temperature change

The remote control is included for 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 the factory preset value.

High capacity

Takagi T-K4 series is built to provide enough hot water to homes with 2.5-3.5 showers working simultaneously. The number of fixtures the unit is about to supply depends on the temperature rise, application, and peak flow rate requirements. So, when used in warmer regions, if, for example, the temperature rise is 30 F, the maximum flow can reach 8 GPM, while in colder regions where the temperature rise is 100 F, the maxim water flow can be up to 3.1 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 for 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, the red LED light will be blinking.


A water heater uses the electronic ignition to light the gas burner. Except for the igniter, the unit is also equipped with the flame sensor, right above the burner, to "inform" the electronic board if the flame is present or not and stop the gas delivery in case of an emergency.

Similar: Rinnai RL and Rheem RTG

Safety features

  • Internal freeze protection system with the provided electrical power. It activates when the surrounding temperature drops below 36.5 F.
  • A manual reset hi-limit switch ensures that water temperature does not exceed safe levels.
  • Internal freeze protection system with the provided electrical power. It activates when the surrounding temperature drops below 36.5 F.
  • Exhaust and water temperature safety control.
  • Overheat cutoff fuse to protect the heat exchanger from extreme temperatures.
  • The Air fuel ratio (AFR) sensor ensures proper gas combustion, reduced emission, and maximum efficiency.


Models/Specs Takagi T-K4
Power (BTU) 11,000-190,000
Water flow (GPM) 0.75-6.6
max # of baths 3.5
Water/gas connections (inch) 3/4
Temperature (F) 99-167
Energy factor 0.80
Warranty 15

According to the users' reviews, Takagi T-K4 water heaters, either indoor or outdoor, are good investments. They are affordable and provide comfortable and convenient water heating. The most praised features are the continuous and endless hot water, including small size, high efficiency, and higher savings than the old tank type (where replaced).

For those homeowners with basic plumbing knowledge, the manual and installation were straightforward. It was an easy DIY installation.

There were some comments that it takes longer than the tank-type to get hot water, about temperature fluctuation and lower power when more applications are used.

The produced hot water flow rate was good enough even for families of 5 and with 3 showers running at the same time, but the water flow and pressure were limited.


If you are looking for a small and compact tankless water heater with endless hot water and on-demand operation and need a cost-effective solution for a 2-3-bath house, Takagi T-K4 is the heater you might want to consider. High energy efficiency and reduced gas emission ensure savings and better environmental protection, and with the computerized safety system, your family and the household will be protected. 

The manufacturer provides an extended 15-year warranty on the heat exchanger, 5 years on other components, and 1 year on labor.


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