Gas tankless · high draw
A. O. Smith ATM-xxM-N 1
The A. O. Smith ATM-xxM-N 1 line holds two certified rating sets of gas tankless water heaters, 25 model numbers sold under five brand names, listed by A.O. Smith. Gas input runs from 140,000 to 199,000 Btu/h, and max flow at a 67 °F rise runs from 4.1 to 6.0 gal/min. A. O. Smith ATM-199M-N 1 lists 6.0 gal/min, the highest of the 40 gas tankless sets (shared by two sets). A. O. Smith ATM-140M-N 1 lists 140,000 Btu/h, the second lowest of the 40 gas tankless sets.
Certified as of October 27, 2026 · EPA ENERGY STAR data, public domain · Compare in the Heater finder
Size by size
| Rating set | Gas input, Btu/h | Max flow, gal/min | Share of heat ceiling | UEF | UEF rank | Therms/yr |
|---|---|---|---|---|---|---|
| A. O. Smith ATM-140M-N 111 model numbers | 140,000 | 4.1 | 98% | 0.95 high | lowest of 39, 19 tied | 178 |
| A. O. Smith ATM-199M-N 114 model numbers | 199,000 | 6.0 | 101% | 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 ATM-xxM-N 1: max flow against gas input
Max flow at a 67 °F rise, gal/min
Across the sizes
Moving from 140,000 Btu/h to 199,000 Btu/h of gas input lifts max flow from 4.1 gal/min to 6.0 gal/min, about 0.32 gal/min for each extra 10,000 Btu/h.
Strongest placings, size by size
| Measure | Value | Median of the group | Placing | Compared with |
|---|---|---|---|---|
| UEFA. O. Smith ATM-140M-N 1 | 0.95 | 0.96 | lowest of 39, 19 tied | 39 high-draw gas tankless sets |
| Rated gas useA. O. Smith ATM-140M-N 1 | 178 therms/yr | 176 therms/yr | highest of 39, 15 tied | 39 high-draw gas tankless sets |
| UEFA. O. Smith ATM-199M-N 1 | 0.95 | 0.96 | lowest of 39, 19 tied | 39 high-draw gas tankless sets |
| Gas inputA. O. Smith ATM-199M-N 1 | 199,000 Btu/h | 180,000 Btu/h | highest of 40, 13 tied | 40 gas tankless sets |
Model numbers
The line covers 25 model numbers under five brand names, first certified 2025-04-21 and most recently 2026-07-15. An asterisk stands for a variant character in the model number.
| Model number | Brand | Rating set | Vent | Fuel | Min flow | Certified |
|---|---|---|---|---|---|---|
| ATM-140M-N 1** | A. O. Smith | ATM-140M-N 1 | Power Direct Vent,Power Vent | natural gas | 1.00 gal/min | 2026-07-15 |
| ATM-140X3-N 1** | A. O. Smith | ATM-140M-N 1 | Power Direct Vent,Power Vent | natural gas | 1.00 gal/min | 2026-07-15 |
| GT15TM-140M-N 1** | A. O. Smith | ATM-140M-N 1 | Power Direct Vent,Power Vent | natural gas | 1.00 gal/min | 2026-07-15 |
| MTM-140M-N 1** | American | ATM-140M-N 1 | Power Direct Vent,Power Vent | natural gas | 1.00 gal/min | 2026-07-15 |
| MTM-140X3-N 1** | American | ATM-140M-N 1 | Power Direct Vent,Power Vent | natural gas | 1.00 gal/min | 2026-07-15 |
| LTM-140M-N 1** | Lochinvar | ATM-140M-N 1 | Power Direct Vent,Power Vent | natural gas | 1.00 gal/min | 2026-07-15 |
| LTM-140X3-N 1** | Lochinvar | ATM-140M-N 1 | Power Direct Vent,Power Vent | natural gas | 1.00 gal/min | 2026-07-15 |
| RTM-140M-N 1** | Reliance | ATM-140M-N 1 | Power Direct Vent,Power Vent | natural gas | 1.00 gal/min | 2026-07-15 |
| RTM-140X3-N 1** | Reliance | ATM-140M-N 1 | Power Direct Vent,Power Vent | natural gas | 1.00 gal/min | 2026-07-15 |
| STM-140M-N 1** | State | ATM-140M-N 1 | Power Direct Vent,Power Vent | natural gas | 1.00 gal/min | 2026-07-15 |
| STM-140X3-N 1** | State | ATM-140M-N 1 | Power Direct Vent,Power Vent | natural gas | 1.00 gal/min | 2026-07-15 |
| ATM-199M-N 1** | A. O. Smith | ATM-199M-N 1 | Power Vent,Power Direct Vent | natural gas | 1.00 gal/min | 2025-04-21 |
| ATM-199X3-N 1** | A. O. Smith | ATM-199M-N 1 | Power Vent,Power Direct Vent | natural gas | 1.00 gal/min | 2025-04-21 |
| GT15TM-199M-N | A. O. Smith | ATM-199M-N 1 | Power Vent,Power Direct Vent | natural gas | 1.00 gal/min | 2025-04-21 |
| GT15TM-199M-N 1** | A. O. Smith | ATM-199M-N 1 | Power Direct Vent,Power Vent | natural gas | 1.00 gal/min | 2026-05-27 |
| GT15TM-199X3-N | A. O. Smith | ATM-199M-N 1 | Power Vent,Power Direct Vent | natural gas | 1.00 gal/min | 2025-04-21 |
| GT15TM-199X3-N 1** | A. O. Smith | ATM-199M-N 1 | Power Direct Vent,Power Vent | natural gas | 1.00 gal/min | 2026-05-27 |
| MTM-199M-N 1** | American | ATM-199M-N 1 | Power Vent,Power Direct Vent | natural gas | 1.00 gal/min | 2025-04-21 |
| MTM-199X3-N 1** | American | ATM-199M-N 1 | Power Vent,Power Direct Vent | natural gas | 1.00 gal/min | 2025-04-21 |
| LTM-199M-N 1** | Lochinvar | ATM-199M-N 1 | Power Vent,Power Direct Vent | natural gas | 1.00 gal/min | 2025-04-21 |
| LTM-199X3-N 1** | Lochinvar | ATM-199M-N 1 | Power Vent,Power Direct Vent | natural gas | 1.00 gal/min | 2025-04-21 |
| RTM-199M-N 1** | Reliance | ATM-199M-N 1 | Power Vent,Power Direct Vent | natural gas | 1.00 gal/min | 2025-04-21 |
| RTM-199X3-N 1** | Reliance | ATM-199M-N 1 | Power Vent,Power Direct Vent | natural gas | 1.00 gal/min | 2025-04-21 |
| STM-199M-N 1** | State | ATM-199M-N 1 | Power Vent,Power Direct Vent | natural gas | 1.00 gal/min | 2025-04-21 |
| STM-199X3-N 1** | State | ATM-199M-N 1 | Power Vent,Power Direct Vent | natural gas | 1.00 gal/min | 2025-04-21 |
Nearest certified alternatives
| Rating set | Max flow | UEF | Gas input |
|---|---|---|---|
| A. O. Smith ATHR-199M 1compared with A. O. Smith ATM-199M-N 1 | −0.1 gal/min | ±0.00 | same Btu/h |
| A. O. Smith ACT-199O-N 1compared with A. O. Smith ATM-199M-N 1 | −0.2 gal/min | ±0.00 | same Btu/h |
| Midea JSG52-32PCH-LPcompared with A. O. Smith ATM-199M-N 1 | −0.3 gal/min | ±0.00 | same Btu/h |
| Navien NPE-240A2compared with A. O. Smith ATM-199M-N 1 | −0.4 gal/min | ±0.00 | same Btu/h |
| Navien NPE-180A2compared with A. O. Smith ATM-140M-N 1 | +0.2 gal/min | ±0.00 | +10,000 Btu/h |