Navien's NPE-2 series is one of the most popular lines of condensing gas tankless water heaters in the US, and the 180, 210 and 240 sizes each come in two versions: A2 and S2 (the smallest size, the 150, is S2 only). The two versions share the same cabinet size, the same dual stainless steel heat exchangers and the same venting options. The difference is what is inside the cabinet for recirculation. This guide explains what the A2 adds, how the specs compare model by model, and which one makes sense for your home.
The short answer
- NPE-A2 (ComfortFlow): has a built-in recirculation pump and a small buffer tank. Choose it if you want recirculation for faster hot water at distant fixtures, either with a dedicated return line or with a crossover valve at the farthest fixture.
- NPE-S2: no internal pump or buffer tank. It carries a slightly higher efficiency rating and weighs less. Choose it if you do not need recirculation, or you plan to use an external recirculation pump.
What the A2 adds: ComfortFlow
Navien calls the A2's built-in recirculation system ComfortFlow. It has two parts:
- Internal recirculation pump: keeps hot water moving through the hot line so it is closer to the fixtures when you open a tap. It can run on a schedule or other modes set in the unit's controls, as described in the Navien manual.
- Buffer tank: a small tank inside the unit that holds heated water. It helps reduce the "cold water sandwich," the short burst of cooler water you can get when hot water is turned off and back on quickly.
The A2 can recirculate through a dedicated hot water return line or, in homes without one, through a crossover valve installed under the farthest sink, which returns water through the cold line. Follow Navien's piping diagrams for the setup you choose. With a Navien recirculation setup, the A2 models also support a low-flow activation option, listed at under 0.01 GPM.
A2 vs S2: model-by-model specs
The table below uses the published specs on our NPE-2 product pages. Flow rates are the maximum at each temperature rise.
| Model | Gas input (BTU/h) | GPM at 35°F rise | GPM at 67°F rise | UEF | Weight |
|---|---|---|---|---|---|
| NPE-150S2 | 18,000 to 120,000 | 6.8 | 3.4 | 0.93 | 62 lb |
| NPE-180S2 | 10,000 to 150,000 | 8.4 | 4.3 | 0.96 | 68 lb |
| NPE-180A2 | 10,000 to 150,000 | 8.4 | 4.3 | 0.95 | 73 lb |
| NPE-210S2 | 12,000 to 180,000 | 10.1 | 5.3 | 0.96 | 73 lb |
| NPE-210A2 | 12,000 to 180,000 | 10.1 | 5.1 | 0.95 | 77 lb |
| NPE-240S2 | 13,300 to 199,900 | 11.2 | 5.8 | 0.96 | 73 lb |
| NPE-240A2 | 13,300 to 199,900 | 11.2 | 5.6 | 0.95 | 77 lb |
A few things stand out:
- The 150 size is S2 only. There is no NPE-150A2. Our listing also notes that the NPE-150S2 is not ENERGY STAR certified, while the 180, 210 and 240 sizes in both versions are.
- Efficiency: the S2 rates 0.96 UEF in the 180, 210 and 240 sizes, compared with 0.95 for the A2.
- Cold-weather flow: at a 67°F rise, the 210 and 240 S2 models are rated slightly higher than their A2 counterparts.
- Weight: the A2 is heavier because of the pump and buffer tank.
What is the same on both
- Condensing design with primary and secondary stainless steel heat exchangers.
- Cabinet size: 17.3" W x 27.4" H x 13.2" D.
- 3/4" water and gas connections.
- 120 V AC power.
- Venting with 2" or 3" PVC, CPVC, polypropylene or stainless steel, as the manual allows, with 0" clearance to combustibles for the vent.
- Natural gas and propane certification, indoor or outdoor installation, and altitude approval up to 10,100 feet (see the installation manual for details).
- Minimum activation flow of 0.5 GPM.
- SCAQMD Rule 1146.2 low-NOx compliance.
Which one should you choose?
Choose an A2 if
- You wait a long time for hot water at a far bathroom or kitchen.
- You have a dedicated return line, or you can install a crossover valve at the farthest fixture.
- You want to reduce the cold water sandwich between showers.
- You prefer an all-in-one unit instead of adding and wiring a separate pump.
Choose an S2 if
- Your fixtures are close to the water heater and wait time is not a problem.
- You want the highest efficiency rating in the line and the lightest unit.
- You already have, or plan to install, an external recirculation pump. Check Navien's manual for the supported piping and settings.
- You want the smallest size, the NPE-150S2, for a smaller home or a warm climate.
Sizing between 180, 210 and 240
The model number tells you the size, not the version. Choose the size based on how many fixtures run at the same time and how cold your incoming water gets in winter. For example, the NPE-240A2 and NPE-240S2 are both rated for 5.6 to 5.8 GPM at a 67°F rise, which is roughly two 2.5 GPM showers at once on a cold winter day. Our guide What Size Tankless Water Heater Do I Need? walks through the math.
Installation notes
Both versions need a gas line sized for the full input, a condensate drain (usually through a neutralizer), and venting to the Navien manual and local code. Gas piping, venting and combustion air work should be done by a licensed plumber or gas fitter. For a full comparison of Navien water heaters and boilers, see our Navien model comparison. If an NPE unit shows a code on the display, see Navien Tankless Error Codes: What They Mean and What to Check.
Shop the Navien NPE-240A2, the Navien NPE-240S2, or every size in our Navien tankless water heaters collection.
FAQ
What does the A in NPE-240A2 stand for?
Navien uses the A version for models with the built-in ComfortFlow recirculation pump and buffer tank. The S version has neither.
Can I add recirculation to an NPE-S2 later?
The S2 has no internal pump, but recirculation can be added with an external pump. Follow the Navien installation manual for supported piping and settings.
Is the A2 less efficient than the S2?
Slightly, on paper. In the 180, 210 and 240 sizes, the A2 is rated 0.95 UEF and the S2 is rated 0.96 UEF.
Do I need a return line for the A2?
No. The A2 can use a dedicated return line or a crossover valve at the farthest fixture in homes without one.