Bosch Greentherm 5300 5.3 GPM Tankless Water Heater

The Bosch Greentherm 5300 5.3 GPM Tankless Water Heater (WTG-5300-NG) is Bosch's compact condensing tankless for 2–3 person households or single-bathroom applications. 5.3 GPM peak flow, 120,000 BTU condensing burner, 0.95 UEF, 12-year warranty. The smaller-capacity Bosch — right pick for households where the full 9 GPM premium tier is overkill.

Headline specifications

• Type: indoor condensing tankless, natural gas (LP-convertible)
• Maximum flow rate: 5.3 GPM at 35°F rise
• BTU input: 12,000–120,000 BTU/h (modulating)
• Energy Factor (UEF): 0.95
• Min activation flow: 0.4 GPM
• Temperature range: 100°F – 140°F
• Warranty: 12-year heat exchanger, 5-year parts
• Venting: PVC/CPVC up to 60 ft
• Dimensions: 25" H × 14" W × 10" D
• Lower gas-line requirement: 1/2" or 3/4"

Who this model is for

1–3 person households with single bathroom or low concurrent demand, ADUs / in-law suites / garage apartments with their own tankless circuit, existing natural gas service with 1/2" gas line (the 5300's 120K BTU input is the rare condensing tankless that runs on 1/2" gas at typical run lengths). Compact form factor fits where the 9 GPM-class units are too large.

For 3+ person households or 2 simultaneous demand points, step up to the Greentherm 9000. For single point-of-use only (one sink), the Bosch Tronic mini-tanks are cheaper.

Where the Greentherm 5300 beats the alternatives

Vs Rinnai RU98iN 5.4 GPM ($1,199): nearly identical spec. Same UEF, same flow class. Rinnai wins on US installer network depth. Bosch wins on compact form factor (25" vs 26" height — small but real).

Vs Greentherm 9000 ($1,649): 5300 saves ~$400 for half the flow. For 1–3 person single-bathroom households, the 5300 is correct sizing. The 9000 just runs at lower modulation for the same demand pattern — paying for capacity you don't use.

Vs gas tank (e.g. Bradford White Compact 30G): compact tank is cheaper upfront ($700 vs $1,249). Tankless wins on energy efficiency ($90/year less in operating cost), space savings, endless hot water. Payback on tankless premium: ~6 years.

Where it falls short

5.3 GPM is the limit. Concurrent shower + dishwasher will exceed capacity in cold-water months. Strictly single-demand-at-a-time household.

Smaller Bosch US installer network. Verify dealer presence in your area.

No built-in WiFi.

12-year warranty (vs 15-year on Greentherm 9000) — Bosch warranty scales with capacity tier.

3/4" gas line preferred even though 1/2" is technically acceptable at short runs. Installer will assess gas-line capacity case by case.

Install considerations

Verify gas line capacity. PVC/CPVC schedule 40 vent up to 60 ft (shorter than 9000's 100 ft). 120V outlet near install. Service valves for descaling. Permit typical.

Install cost: $900–$1,700 like-for-like tankless swap; $2,500–$4,000 tank-to-tankless.

Maintenance

• Annual descaling
• Inlet filter clean annually
• Burner inspection every 3 years

Bottom line

The Bosch Greentherm 5300 is the compact condensing tankless for 1–3 person single-bathroom households. 5.3 GPM, 0.95 UEF, 12-year warranty. Right pick when full 9 GPM capacity is overkill and the compact form factor matters. For larger households step up to Greentherm 9000. For broader installer support at similar spec, Rinnai RU98iN is the alternative.