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State GTS-510C-NI (Takagi TK-510C, AO Smith ATI-510C, Reliance TS-510C)

Original price $997.52 - Original price $1,017.47
Original price
$997.52 - $1,017.47
Current price $997.52


Tankless Non-condensing Water Heater Concentric Vent 199,000 BTU Natural Gas/Propane

The 110C, 310C and 510C series tankless water heater are fully modulating, gas fired, with sealed combustion and power vented flue. It is optimized for residential hot water systems. Either directly or indirectly, this heater can be used with water storage tanks, recirculation systems, or combined domestic hot water and space heating applications.

The GTS-510C-NI tankless water heater delivers a maximum flow rate of 10 GPM. However, please refer to the Flow Rate vs. Temp. Rise graph below to determine how much hot water you will get. It may not be 10 GPM under all circumstances.

This heater uses up to 199,000 BTUs of natural gas or liquid propane. It is non-condensing with an Energy Factor of 0.82. The venting design is concentric and meets California and Texas Ultra-Low NOx emission requirements. It features a long, 15-year limited warranty on the heat exchanger for residential installations, and a 10-year limited warranty with commercial installations. 5-year limited warranty on all parts.

Product features

• Easily converted from natural gas to propane with included kit
• Copper heat exchanger provides 25 times more heat transfer than stainless steel. This makes the outgoing water temperature more consistent, and gets you the correct water temperature faster. It also reduces pressure drop through the heat exchanger.

Safety features

• Built-in protection from freezing
• Overheat cutoff fuse
• Manual high limit reset
• Inlet and outlet thermistors provide constant temperature monitoring for water temperature you expect in your shower and everywhere

State/A.O. Smith/Takagi/American/Reliance brands Tankless Non-Condensing Water Heaters

With innovative designs and stronger components, these non-condensing water heaters are more durable and longer-lasting. Count on AO Smith's brands of non-condensing tankless water heaters for expertise and innovation.


Certified by CSA according to ANSI Z21.10.3. Minimum energy factor of 0.82. Complies with the energy efficiency requirements of the US Department of Energy, ASHRAE 90.1, SCAQMD Rule 1146.2 and some other air quality districts with contiguous requirements for low NOx emissions (14 ng/J or 20 ppm). Certified to NSF 5 standards.

Specifications of Indoor Models



Gas Consumption Input

Inlet Gas

Factor (EF)


Hot/Cold Water


in inches

Min. BTU/H Max. BTU/H Min. in.
Max. in.










3/4" NPT

3/4" NPT 20.5 13.75 10.5 51









3/4" NPT

3/4" NPT 20.5 13.75 10.5 51
GTS-510C-NI3 Natural 15,000 199,000 5.0 10.5
0.82 10 3/4" NPT
3/4" NPT 20.5 13.75 10.5 51

Operating water pressure range is 15 to 150 psi. 40 psi (and above) is recommended to achieve the maximum flow rate.

1For Propane, the minimum inlet gas pressure 8.0 inches of W.C. and maximum of 14.0 inches of W.C.
2Current numbers based on factory tests. 0.5 GPM is required for heating activation. 0.4 GPM is required to maintain flame after initial ignition.
3Suitable for commercial applications.

Electrical: For flame ignition, the heater uses a 120 V/60 Hz power source with a standard appliance power cord. Max amps including freeze protection mode: 0.83 Amps

Certified from sea level up to 10,100 ft. elevations.

Complies with lead-free standards.

Venting and combustion

This heater uses the Takagi Proprietary APNC35 Concentric Vent Design which allows for an extremely long maximum venting length of 43 feet and up to 4 elbows. This power direct vent system can be installed with vertical or horizontal vent termination installation. Venting instructions:

Tankless Non-Condensing Concentric Vent Venting Instructions


Spec Sheet
Owners Manual

More Technical Details

This is a fully modulating, non-condensing, on-demand gas fired tankless water heater. It features a maximum input rating of 199,000 Btu/h, and uses either natural gas (NG) or propane (LP). For LPG, the heater can be changed in the field using the included conversion kit. The heater has an integrated temperature controller that provides diagnostic information, allows the user to set an output temperature, and shows fault history.

This heater uses a commercial-grade copper alloy, a quick release bronze or brass waterways, and a fin tube heat exchanger. The heater is controlled by a solid-state printed circuit board with the following:

• Thermistors that monitor water temperature
• A flow sensor that measures flow rate
• A flame sensor that monitors combustion
• An air-fuel ratio rod that measures and adjusts the air input and fuel use. This helps maintain the highest combustion efficiency.
• For electrical surge protection, inline surge and fusing absorbers
• An electronic spark igniter
• High-temperature limit switch that monitors water temperature
• Aluminized stainless steel burners
• Overheat cutoff fuse
• Modulating gas valve
• Dual freeze protection that incorporates both heating blocks and automatic flame ignition to protect the heat exchanger.

For additional water heating capacity, using the Easy-Link system up to four heaters can be connected. The Easy-Link controls can be built into circuit board to eliminate the need for external controls. The linking control wire is provided with the heater. The MultiUnit Controller (part number 008300005) can manifold together 5 to 20 heaters. Both the Easy-Link and Multi-Unit Controller will modulate the system for the highest performance. It rotates the priority heater every 12 hours or 100 starts to balance the duty and cycle operation.

State GTS-510C-NI (Takagi TK-510C, AO Smith ATI-510C, Reliance TS-510C)

— indicates a set temperature of 125°F (52oC) or lower
---- indicates a set temperature 130°F (55oC) or higher

FAQ (Frequently Asked Question)

Q: Why does this product title say "State GTS-510C-NI (Takagi TK-510C, AO Smith ATI-510C, Reliance TS-510C)"?
A: A. O. Smith owns all the following brands: State Water Heater, Takagi, Reliance (sold in hardware stores such as True Value, Ace, and Do It Best), American Heater (sold in Lowe's). The model numbers in the title are all for the identical heater. The only difference is the label on the front of the heater, the model numbers, and the brands referred to in literature. The heater you receive may have any of those brands depending upon quickest shipment to you.

Optional Accessories

• Isolation Valve Kit (item number 9007604005)
• Remote Control (item number 9009069005)
• Product Preserver (item number 9008871005)
• Multi-Unit Controller (item number 9008300005)
Also see the blue Related Gadgets tab above.