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Comparison
Verdict in brief
The two smallest Navien S2 condensing tankless models differ by 30,000 BTU/h of maximum input and, unusually, by efficiency in the larger unit's favour: the NPE-180S2 is rated 10,000–150,000 BTU/h at UEF 0.96, the NPE-150S2 18,000–120,000 BTU/h at UEF 0.93. Published flow at a 67 °F rise, the winter-class figure in Metro Vancouver, is 4.3 GPM for the 180S2 against 3.4 GPM for the 150S2, so both are one-fixture-at-a-time units in January and the honest option for two simultaneous showers is a larger S2 model. Both need a sealed vent route, a condensate drain and 120 V power.
Published manufacturer figures with units. Storage and flow figures are different measures and are shown on their own rows; blanks mean not applicable or not published.
| Specification | Navien NPE-150S2Gas tankless | Navien NPE-180S2Gas tankless |
|---|---|---|
| Minimum gas input | 18,000 BTU/h | 10,000 BTU/h |
| Maximum gas input | 120,000 BTU/h | 150,000 BTU/h |
| Flow at a 35 °F (19.4 °C) rise | 6.8 GPM (25.7 L/min) at 35 °F (19.4 °C) rise | 8.4 GPM (31.8 L/min) at 35 °F (19.4 °C) rise |
| Flow at a 45 °F (25 °C) rise | 5.3 GPM (20.1 L/min) at 45 °F (25 °C) rise | 6.5 GPM (24.6 L/min) at 45 °F (25 °C) rise |
| Flow at a 67 °F (37.2 °C) rise | 3.4 GPM (12.9 L/min) at 67 °F (37.2 °C) rise | 4.3 GPM (16.3 L/min) at 67 °F (37.2 °C) rise |
| Minimum flow rate | 0.5 GPM (1.9 L/min) | 0.5 GPM (1.9 L/min) |
| Uniform Energy Factor (UEF) | 0.93 | 0.96 |
| Vent type | 2 in or 3 in PVC, CPVC, approved polypropylene or stainless steel; special gas vent type BH (Class II A/B/C) | 2 in or 3 in PVC, CPVC, approved polypropylene or stainless steel; special gas vent type BH (Class II A/B/C) |
| Gas connection | 3/4 in | 3/4 in |
| Electrical supply | 120 V AC, 60 Hz; up to 200 W (2 A), 350 W (4 A) with external pump | 120 V AC, 60 Hz |
| Height | 27.4 in (696 mm) | 27.4 in (696 mm) |
| Width | 17.3 in (439 mm) | 17.3 in (439 mm) |
| Depth | 13.2 in (335 mm) | 13.2 in (335 mm) |
| Weight | 62 lb (28.1 kg) | 68 lb (30.8 kg) |
| Maximum altitude | 10,100 ft for natural gas and propane | 10,100 ft for natural gas and propane |
“—” means not applicable to that system type or not published by the manufacturer. Flow figures are only comparable at the same temperature rise; UEF values are only comparable within the same draw pattern.
Both units share the S2 cabinet, 27.4 in high, 17.3 in wide and 13.2 in deep, the 0.5 GPM minimum activation flow, a 3/4 in gas connection, 3/4 in cold inlet and hot outlet, 120 V AC operation and a maximum altitude of 10,100 ft. The 150S2 is the lighter of the two at 62 lb (28.1 kg) against the 180S2's 68 lb (30.8 kg). The differences are the input range, the published flow and the efficiency, and they all point the same way.
Efficiency runs counter to the usual expectation. The smaller NPE-150S2 is rated UEF 0.93 against 0.96 for the larger NPE-180S2, so the higher-input unit is the more efficient one, not the smaller. Both are high-draw-class Uniform Energy Factors. Navien's page marks the NPE-150S2 not ENERGY STAR certified; a buyer comparing them on running cost should note the 180S2's higher UEF rather than assume the smaller unit uses less gas.
The published flow table is where the real choice sits. At a 35 °F (19.4 °C) rise Navien rates the 150S2 at 6.8 GPM (25.7 L/min) and the 180S2 at 8.4 GPM (31.8 L/min), but that is not winter delivery. Metro Vancouver winter incoming water can sit below 8 °C, so heating to a 49 °C shower is roughly a 40 °C (72 °F) rise, and at the 67 °F published rise the figures are 3.4 GPM (12.9 L/min) for the 150S2 and 4.3 GPM (16.3 L/min) for the 180S2. Neither can serve two shower-class fixtures at once in January; the 35 °F headline is the marketing number, and the manual's flow-at-rise table governs sizing.
Both are conversions rather than like-for-like swaps: each needs a sealed 2 in or 3 in PVC, CPVC, polypropylene or stainless vent terminated as a forced-draft direct vent, a condensate drain, a 120 V receptacle and a gas supply sized for the input. That is 120,000 BTU/h for the 150S2 and 150,000 BTU/h for the 180S2, both far above a storage tank's 40,000 BTU/h, so the meter and line are checked at assessment. Neither makes hot water during a power outage, and Navien's published resale policy voids warranty for internet or e-commerce purchases, so supply runs through contractors.
Navien · Gas tankless
The Navien NPE-150S2 is the smallest of Navien's current S2 condensing tankless models: 18,000–120,000 BTU/h on natural gas or propane, UEF 0.93, and published flows of 6.8 GPM at a 35 °F rise and 3.4 GPM at a 67 °F rise.
Navien · Gas tankless
The Navien NPE-180S2 is the smallest of Navien's current S2 condensing tankless models: 10,000–150,000 BTU/h on natural gas or propane, UEF 0.96, and published flows of 8.4 GPM at a 35 °F rise and 4.3 GPM at a 67 °F rise.
The NPE-180S2. Navien rates it at 4.3 GPM (16.3 L/min) at a 67 °F rise against 3.4 GPM (12.9 L/min) for the NPE-150S2, and 8.4 GPM against 6.8 GPM at a 35 °F rise. The 67 °F figure is the winter-class number for Metro Vancouver; the 35 °F figure is not winter delivery.
No. Navien rates the NPE-150S2 at UEF 0.93 against UEF 0.96 for the NPE-180S2, so the smaller unit is the less efficient of the two under the same high-draw class. The 150S2 is also marked not ENERGY STAR certified on Navien's page.
At a 67 °F (37 °C) rise Navien rates the 150S2 at 3.4 GPM and the 180S2 at 4.3 GPM, so neither serves two shower-class fixtures simultaneously in January. For that demand the larger S2 sizes or a hot-water demand assessment is the honest answer.
A consultation includes a hot-water demand review so capacity is right for your winter, not the brochure's.
On-site service in Metro Vancouver, BC. Call to check availability.
Sources and verification
Facts checked September 21, 2026. Last updated September 21, 2026. Technical reviewer: not yet assigned.