Bosch Greentherm T 9900 SE Indoor Tankless Water Heater

The Bosch Greentherm T 9900 SE Indoor Tankless Water Heater is Bosch's commercial-grade condensing tankless for high-demand residential and light-commercial applications. 11.0 GPM peak flow, 199,000 BTU condensing burner with extreme 18:1 modulation turndown, 0.96 UEF, 15-year heat exchanger warranty, and the SE designation refers to commercial-tier internal hydronics.

Headline specifications

• Type: indoor condensing tankless, commercial-tier residential, natural gas (LP-convertible)
• Maximum flow rate: 11.0 GPM at 35°F rise
• BTU input: 11,000–199,000 BTU/h (18:1 modulation)
• Energy Factor (UEF): 0.96
• Min activation flow: 0.3 GPM
• Temperature range: 100°F – 160°F (commercial-tier high-temp option)
• Warranty: 15-year heat exchanger, 5-year parts
• Venting: PVC/CPVC up to 100 ft, with concentric or two-pipe options
• Built-in cascading support for multi-unit installs
• Dimensions: 30" H × 18" W × 12" D

Who this model is for

4–6 person households with 3+ bathrooms, light-commercial residential (B&Bs, group homes, small daycares), homes with hot-tub or whirlpool tub fill loads, and installs that may scale to dual-unit cascade for very high demand. The 18:1 modulation turndown is the differentiator — at very low demand (single bathroom sink), the burner modulates down to 11K BTU and stays efficient; at peak demand it runs the full 199K. Lower-tier tankless typically maxes out at 12:1 turndown.

For typical 3–4 person households without commercial-tier demand patterns, the standard Greentherm 9000 at $400 less is the right pick. The 9900 SE's commercial-tier features add cost without consumer-relevant benefit at typical residential demand.

Where the Greentherm T 9900 SE beats the alternatives

Vs Greentherm 9000 ($1,649): 9900 SE adds 18:1 modulation (vs 12:1), high-temp option to 160°F, built-in cascading support. ~$400 premium. Worth it only for commercial-light or true high-demand residential.

Vs Rinnai RU199iN ($1,645): Rinnai 11.1 GPM, similar warranty class, stronger US installer network, built-in Control-R 2.0 WiFi. Bosch 9900 SE 11.0 GPM, 18:1 modulation (vs Rinnai's 12:1), no built-in WiFi. Bosch wins on modulation depth and commercial-tier features; Rinnai wins on installer network and WiFi.

Vs Navien NPE-240A2 ($1,499): Navien wins on ComfortFlow buffer and NaviLink WiFi at lower price. Bosch wins on modulation depth. For most buyers Navien is the better-value flagship tankless; Bosch is the niche pick for modulation-priority commercial-light applications.

Where it falls short

$2,000+ MSRP territory is premium. Most residential buyers will find equivalent capability at lower cost from Rinnai or Navien flagships.

No built-in WiFi.

Bosch US installer network — smaller than Rinnai/Navien. Verify dealer and service support before commiting to a premium-tier unit.

Cold-water sandwich without buffer tank. The high modulation helps (burner stays warm during low-flow events) but doesn't eliminate the issue.

3/4" gas line, PVC venting, annual descaling.

Install considerations

Verify 3/4" gas line capacity to 199,000 BTU. PVC/CPVC venting up to 100 ft. 120V outlet. Service valves for descaling. Cascading installs need additional plumbing and control wiring. Permit required.

Install cost: $1,400–$2,400 like-for-like; $3,200–$5,500 tank-to-tankless with full gas/vent/electrical upgrades.

Maintenance

• Annual descaling required
• Inlet filter clean annually
• Burner inspection every 3 years
• Modulation calibration check annually for commercial-tier applications

Bottom line

The Bosch Greentherm T 9900 SE is the commercial-tier residential tankless — 11 GPM, 0.96 UEF, 18:1 modulation, 15-year warranty, cascading-ready. Right pick for true high-demand residential, light-commercial, and B&B applications. For typical residential, the Greentherm 9000 at $400 less is correct. For most buyers, Rinnai RU199iN or Navien NPE-240A2 offer equivalent or better feature sets at lower price.