Navien NCB-300 Combi Boiler with Tankless Water Heater
Navien NCB-300 Combi Boiler with Tankless Water Heater Review
The Navien NCB-300 is the largest residential combi boiler in Navien's NCB line. 300K BTU providing both space heating and DHW for very large homes (3,500-5,000 sq ft).
Specs: 300,000 BTU input, AFUE 95% (condensing), 11.2 GPM DHW at a 70°F rise, fully modulating burner with 5:1 turndown. Built-in 0.5-gallon buffer with ComfortFlow recirculation. PVC condensing venting. 15-year warranty on heat exchanger.
The 300K BTU class is at the upper end of residential combi. Sized for very large homes with substantial space-heat demand combined with heavy DHW needs.
About $11,000-$15,000 installed depending on hydronic system modifications. Right buyer: very large home (3,500-5,000 sq ft) replacing aging boiler + water heater with single high-output combi unit.
- Natural gas combustion — faster recovery than electric resistance at comparable tank size
- UEF 0.95 — top-tier efficiency; qualifies for Inflation Reduction Act §25C residential credit
- 10-year warranty — solid above-average coverage for this class
- 7 GPM flow rate — sized for 2-bath simultaneous demand in warm-climate installs
- High 300K BTU input — rapid recovery for large-volume commercial-residential applications
- Tank standby heat loss — tankless or heat-pump alternatives eliminate this continuous efficiency gap