Released in 2007, the T-K3-OS is Takagi’s tankless water heater built only for outdoor installations. The T-K3-OS is one of Takagi’s smaller tankless water heaters. At 13.8 inches wide and weighing only 40 pounds, its ultra-compact design allows it to be mounted on a wall or installed between two wall studs using Takagi's TK-RBO2 recess box, so it's even more inconspicuous. Since it can only be installed outside, the T-K3-OS is ideal for warmer climates.

The T-K3-OS features a built-in power venting system, but unlike any of its predecessors, the new tankless water heater does not require any additional Category III venting or vent caps, making installation one step easier for its installers.

It can provide an unlimited supply of hot water continuously because there is no storage tank to heat and reheat. The T-K3-0S starts heating water at 0.5 gallons per minute (GPM), and can keep that endless supply of hot water running from as low as 0.4 GPM up to a maximum flow rate of 7.0 GPM.

Takagi's newest tankless water heater also features a thermal rating of 83% for both liquid propane and natural gas, so it is extremely efficient and environmentally friendly. Since tankless water heaters only burn gas when hot water is needed, installing this unit can help homeowners save up to 45% on their monthly gas bills.

The T-K3-OS also offers four separate temperature settings, ranging from 113 to 167 degrees, so it does not require the use of a separate remote control.