Both are condensing tankless water heaters. Efficiency is nearly identical. The NPE-240S2 saves $3/year to operate. All figures from published specs and EIA energy prices.
Performance
| Spec | NPE-240S2 Navien · Condensing Tankless | T-H3J-OS-N-1 Takagi · Condensing Tankless |
|---|---|---|
| UEF | 0.96Difference: 0.01 | 0.95 |
| Max Flow Rate | 5.80 GPMDifference: 1.10 GPM | 4.70 GPM |
| BTU Input | 199,000Difference: 39,000 | 160,000 |
But does it fit? Sizing guide
Physical dimensions
| Dimension | NPE-240S2 | T-H3J-OS-N-1 |
|---|---|---|
| Height | 27.40" | 22.50"Difference: 4.90" |
| Width | 17.30"Difference: 0.45" | 17.75" |
| Depth | 13.20" | 10.75"Difference: 2.45" |
| Weight | 73 lbs | 58 lbsDifference: 15 lbs |
The NPE-240S2 is taller. Fit checked. Now the operating cost.
Operating cost
| Cost | NPE-240S2 | T-H3J-OS-N-1 |
|---|---|---|
| Annual Operating Cost | $239/yrDifference: $3/yr | $242/yr |
| 10-Year Operating Cost | $2,390Difference: $30 | $2,420 |
| Annual Energy | 176 thermsDifference: 2 therms | 178 therms |
10-year modeled difference is $30. At this scale, local energy rates, usage patterns, and install costs matter more than the efficiency delta.
Costs vary by state. The NPE-240S2 ranges from $116/yr (ID) to $878/yr (HI). The T-H3J-OS-N-1 ranges from $117/yr (ID) to $888/yr (HI).
$3/yr separates them. Install requirements determine the upfront gap. Installation accessories
Installation requirements
| Requirement | NPE-240S2 | T-H3J-OS-N-1 |
|---|---|---|
| Voltage | 120Videntical | |
| Amperage | 5Aidentical | |
| Breaker | 15A | — |
| Vent Type | Direct Vent | Power Vent |
| Vent Material | Category Iv Pvc Cpvcidentical | |
| Condensate Drain | Yesidentical | |
Install impact: different venting (Direct Vent vs Power Vent). Switching between these units may affect your utility room setup.
Warranty
| Coverage | NPE-240S2 | T-H3J-OS-N-1 |
|---|---|---|
| Heat Exchanger | 15 yridentical | |
| Parts | 5 yridentical | |
Features
| Feature | NPE-240S2 | T-H3J-OS-N-1 |
|---|---|---|
| Wi-Fi / Smart | Yes | — |
| Recirculation Ready | Yes | — |
Decision split
Pick this one if...
Navien NPE-240S2
- You want smart home / Wi-Fi control
- You need higher flow (5.80 GPM)
Pick this one if...
Takagi T-H3J-OS-N-1
- Your ceiling or space is under 27.40"
Neither if...
- You have no floor drain and cannot add condensate handling
Common accessories for this type of installation.
Common install considerations for gas tankless units
What is required depends on your existing installation, local code, and installer preference.
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Related
Sources. ENERGY STAR, manufacturer spec sheets, EIA average energy prices.Methodology