Navien NCB-240 Combi Boiler with Tankless Water Heater

The Navien NCB-240 is a combination boiler — a single appliance that provides both domestic hot water (the tankless water heating function) and hydronic space heat (for radiators, baseboard, or radiant floor systems). The "NCB" identifies it as a combi (combined heating + DHW), the 240 refers to the 240,000 BTU input rating. It is positioned for households retiring an aging boiler + separate water heater setup and consolidating into one unit.

Specs: 240,000 BTU input, UEF 0.95 (condensing operation), 6.6 GPM at a 70°F rise for domestic hot water, 199K BTU output dedicated to the space-heat loop. Built-in 0.5-gallon buffer for the DHW function. Stainless steel dual heat exchanger. 15-year warranty on the heat exchanger, 5 years on parts.

The combi advantage is space and capital efficiency: one appliance instead of two, one venting system, one gas connection, one electrical hookup. For homes converting from oil-fired boilers + tank-style water heaters, the combi consolidation often eliminates a separate fuel storage tank (the oil tank) entirely along with the floor space it occupied.

The combi limitation is sizing: a single combi unit must be sized for the larger of the two loads (typically space heat in cold climates), and the smaller load (DHW in summer) operates at low-modulation efficiency. For very large homes with high simultaneous DHW + space-heat demand, a separate boiler and tankless water heater can deliver more reliably; the combi is right-sized for medium-to-large homes (under 2,500 sq ft with moderate hot-water demand).

Install: requires PVC condensing venting, 1" gas line for full BTU output, dedicated electrical, and connection to the existing hydronic loop. Typical retrofit installed cost $7,000–$11,000 because the conversion involves both the combi unit and the hydronic-system rework. The right buyer is a homeowner doing an HVAC modernization that addresses both space heating and water heating simultaneously.