The T-H1, Takagi's greenest model, is a condensing unit with an average efficiency of 92% with natural gas and 95% with propane. At a ∆T of 60ºF in warmer areas - where the winter ground water is 60ºF and with the output set at 120ºF - the unit will provide 6.1 GPM, or 366 GPH. In older areas where the input temperature may be 40ºF or less, you can expect a flow rate of about 4.3 GPM or 258 GPH. The T-H1 can provide at least three showers' worth of water simultaneously along with one appliance at input temperature of 60ºF or greater.

The T-H1 is also the industry's most efficient. This state-of-the-art tankless water heater uses not one, but two heat exchangers to squeeze additional energy from the water, resulting in the higher-than-average efficiencies stated above. Takagi has added a stainless steel secondary heat exchanger to recover waste heat from the vent stack before it leaves the unit to preheat the incoming ground water. This second heat exchanger and the resulting condensation that forms from the high efficiency operation, classify it as a "Condensing" Water Heater. The condensate that these types of heaters produce can be slightly acidic, so it's collected and sent through a built-in neutralizer, allowing for disposal into standard drainpipe.

With a variable gas input up to 199,000 BTU per hour and its high efficiency, the T-H1 can deliver more than 10 gallons of hot water per minute under ideal conditions. As a result, the unit will meet the needs of most large homes, as well as single heater-usage commercial and industrial applications. Able to deliver water up to 182ºF, the T-H1 is appropriate for sanitary applications needing 180ºF water to meet code requirements.

The four preset temperatures of the unit are: 113ºF, 122ºF, 140ºF, and 182ºF. To achieve any temperature other than those presets, the user will require the TK-RE10 temperature controller.