Gas tankless · high draw
A. O. Smith ACT-199O-N 1
A. O. Smith ACT-199O-N 1 is a gas tankless water heater certified as a single rating set, 51 model numbers sold under six brand names, listed by A.O. Smith. Max flow 5.8 gal/min at a 67 °F rise from 199,000 Btu/h of gas input, UEF 0.95 at the high draw pattern. This set lists 5.8 gal/min, the fifth highest of the 40 gas tankless sets (shared by three sets). That UEF is 0.01 below the median of the 39 high-draw gas tankless sets (0.96).
Certified as of October 27, 2026 · EPA ENERGY STAR data, public domain · Compare in the Heater finder
Certified values
| Rating set | Gas input, Btu/h | Max flow, gal/min | Share of heat ceiling | UEF | UEF rank | Therms/yr |
|---|---|---|---|---|---|---|
| A. O. Smith ACT-199O-N 151 model numbers | 199,000 | 5.8 | 98% | 0.95 high | lowest of 39, 19 tied | 178 |
Share of heat ceiling: listed flow ÷ the flow the gas input could heat through 67 °F at 100% efficiency; test tolerances allow a little over 100%.
A. O. Smith ACT-199O-N 1: max flow against gas input
A. O. Smith ACT-199O-N 1Other gas tankless sets
Max flow at a 67 °F rise, gal/min
Where this set stands
| Measure | Value | Median of the group | Placing | Compared with |
|---|---|---|---|---|
| Max flow at a 67 °F rise | 5.8 gal/min | 5.2 gal/min | fifth highest of 40, three tied | 40 gas tankless sets |
| UEF | 0.95 | 0.96 | lowest of 39, 19 tied | 39 high-draw gas tankless sets |
| Rated gas use | 178 therms/yr | 176 therms/yr | highest of 39, 15 tied | 39 high-draw gas tankless sets |
| Gas input | 199,000 Btu/h | 180,000 Btu/h | highest of 40, 13 tied | 40 gas tankless sets |
Model numbers
The set covers 51 model numbers under six brand names, first certified 2023-02-23 and most recently 2026-07-15. An asterisk stands for a variant character in the model number.
| Model number | Brand | Vent | Fuel | Min flow | Certified |
|---|---|---|---|---|---|
| ACT-199O-N 1** | A. O. Smith | Power Vent | natural gas | 1.00 gal/min | 2023-02-23 |
| ACT-199O-P 1*1 | A. O. Smith | Power Vent | propane | 1.00 gal/min | 2023-02-23 |
| ACTH-199M-(N,P) 1** | A. O. Smith | Power Direct Vent,Power Vent | natural gas or propane | 1.00 gal/min | 2026-07-15 |
| ATO-540H 1** | A. O. Smith | Power Vent | natural gas | 1.00 gal/min | 2023-02-23 |
| ATO-540H 1*1 | A. O. Smith | Power Vent | propane | 1.00 gal/min | 2023-02-23 |
| ATO-540HX3-N 1** | A. O. Smith | Power Vent | natural gas | 1.00 gal/min | 2023-02-23 |
| ATO-540HX3-P 1*1 | A. O. Smith | Power Vent | propane | 1.00 gal/min | 2023-02-23 |
| ATO-540P-N 1** | A. O. Smith | Power Vent | natural gas | 1.00 gal/min | 2023-02-23 |
| ATO-540P-P 1*1 | A. O. Smith | Power Vent | propane | 1.00 gal/min | 2023-02-23 |
| GT15-540-NO 1** | A. O. Smith | Power Vent | natural gas | 1.00 gal/min | 2023-02-23 |
| GT-540-NEH 1** | American | Power Vent | natural gas | 1.00 gal/min | 2023-02-23 |
| GT-540-PEH 1*1 | American | Power Vent | propane | 1.00 gal/min | 2023-02-23 |
| GT-540P-NEH 1** | American | Power Vent | natural gas | 1.00 gal/min | 2023-02-23 |
| GT-540P-PEH 1*1 | American | Power Vent | propane | 1.00 gal/min | 2023-02-23 |
| GT-540X3-NEH 1** | American | Power Vent | natural gas | 1.00 gal/min | 2023-02-23 |
| GT-540X3-PEH 1*1 | American | Power Vent | propane | 1.00 gal/min | 2023-02-23 |
| MCT-199O-N 1** | American | Power Vent | natural gas | 1.00 gal/min | 2023-02-23 |
| MCT-199O-P 1*1 | American | Power Vent | propane | 1.00 gal/min | 2023-02-23 |
| MCTH-199M-(N,P) 1** | American | Power Direct Vent,Power Vent | natural gas or propane | 1.00 gal/min | 2026-07-15 |
| LCT-199O-N 1** | Lochinvar | Power Vent | natural gas | 1.00 gal/min | 2023-02-23 |
| LCT-199O-P 1*1 | Lochinvar | Power Vent | propane | 1.00 gal/min | 2023-02-23 |
| LCTH-199M-(N,P) 1** | Lochinvar | Power Direct Vent,Power Vent | natural gas or propane | 1.00 gal/min | 2026-07-15 |
| LTO-540H-N 1** | Lochinvar | Power Vent | natural gas | 1.00 gal/min | 2023-02-23 |
| LTO-540H-P 1*1 | Lochinvar | Power Vent | propane | 1.00 gal/min | 2023-02-23 |
| LTO-540HX3-N 1** | Lochinvar | Power Vent | natural gas | 1.00 gal/min | 2023-02-23 |
| LTO-540HX3-P 1*1 | Lochinvar | Power Vent | propane | 1.00 gal/min | 2023-02-23 |
| LTO-540P-N 1** | Lochinvar | Power Vent | natural gas | 1.00 gal/min | 2023-02-23 |
| LTO-540P-P 1*1 | Lochinvar | Power Vent | propane | 1.00 gal/min | 2023-02-23 |
| TS-540-GEH 1** | Reliance | Power Vent | natural gas | 1.00 gal/min | 2023-02-23 |
| TS-540-LEH 1*1 | Reliance | Power Vent | propane | 1.00 gal/min | 2023-02-23 |
| TS-540P-GEH 1** | Reliance | Power Vent | natural gas | 1.00 gal/min | 2023-02-23 |
| TS-540P-LEH 1*1 | Reliance | Power Vent | propane | 1.00 gal/min | 2023-02-23 |
| TS-540X3-GEH 1** | Reliance | Power Vent | natural gas | 1.00 gal/min | 2023-02-23 |
| TS-540X3-LEH 1*1 | Reliance | Power Vent | propane | 1.00 gal/min | 2023-02-23 |
| GTS-540-NEH 1** | State | Power Vent | natural gas | 1.00 gal/min | 2023-02-23 |
| GTS-540-PEH 1*1 | State | Power Vent | propane | 1.00 gal/min | 2023-02-23 |
| GTS-540P-NEH 1** | State | Power Vent | natural gas | 1.00 gal/min | 2023-02-23 |
| GTS-540P-PEH 1*1 | State | Power Vent | propane | 1.00 gal/min | 2023-02-23 |
| GTS-540X3-NEH 1** | State | Power Vent | natural gas | 1.00 gal/min | 2023-02-23 |
| GTS-540X3-PEH 1*1 | State | Power Vent | propane | 1.00 gal/min | 2023-02-23 |
| SCT-199O-N 1** | State | Power Vent | natural gas | 1.00 gal/min | 2023-02-23 |
| SCT-199O-P 1*1 | State | Power Vent | propane | 1.00 gal/min | 2023-02-23 |
| SCTH-199M-(N,P) 1** | State | Power Direct Vent,Power Vent | natural gas or propane | 1.00 gal/min | 2026-07-15 |
| T-H3-OS-N 1** | Takagi | Power Vent | natural gas | 1.00 gal/min | 2023-02-23 |
| T-H3-OS-P 1*1 | Takagi | Power Vent | propane | 1.00 gal/min | 2023-02-23 |
| TCT-199O-N 1** | Takagi | Power Vent | natural gas | 1.00 gal/min | 2023-02-23 |
| TCT-199O-P 1*1 | Takagi | Power Vent | propane | 1.00 gal/min | 2023-02-23 |
| TK-540P-NEH 1** | Takagi | Power Vent | natural gas | 1.00 gal/min | 2023-02-23 |
| TK-540P-PEH 1*1 | Takagi | Power Vent | propane | 1.00 gal/min | 2023-02-23 |
| TK-540X3-NEH 1** | Takagi | Power Vent | natural gas | 1.00 gal/min | 2023-12-18 |
| TK-540X3-PEH 1*1 | Takagi | Power Vent | propane | 1.00 gal/min | 2023-12-18 |
Nearest certified alternatives
| Rating set | Max flow | UEF | Gas input |
|---|---|---|---|
| Midea JSG52-32PCH-LP | −0.1 gal/min | ±0.00 | same Btu/h |
| A. O. Smith ATHR-199M 1 | +0.1 gal/min | ±0.00 | same Btu/h |
| A. O. Smith ATM-199M-N 1 | +0.2 gal/min | ±0.00 | same Btu/h |
| Navien NPE-240A2 | −0.2 gal/min | ±0.00 | same Btu/h |