The Navien NCB700-2S+/42K is a wall-hung, room-sealed gas condensing Combi Crossover boiler rated at 42kW DHW output — engineered to deliver hot water to two to three bathrooms simultaneously without the flow or temperature drop-off typical of a conventional combi. Navien's ON technology maximises recirculation of domestic hot water around the property's pipework, combining the constant, on-demand flow of a system boiler with the space-saving convenience of a combi — removing the need for a hot water cylinder even in larger homes.
17.2 l/min
DHW Flow (ΔT35°C)
12 yrs
Parts & Labour Warranty
Gas Safe Registration Required
Installation, commissioning and servicing of this boiler must be carried out by a Gas Safe registered engineer. A Benchmark commissioning checklist must be completed and left with the end user, and the flue must be positioned in accordance with the clearances set out in the manufacturer's installation and operation manual.
LPG Conversion & Sizing
The boiler is shipped ready to fire on natural gas, with an LPG conversion kit included in the packaging for field conversion by a suitably qualified Gas Safe engineer. Confirm mains water flow rate and pressure are sufficient to make full use of its 17.2 l/min hot water delivery before ordering.
Why This Boiler
Navien ON Recirculation Technology
Built-in DHW recirculation keeps hot water primed at outlets around the property, removing the wait and the fluctuation between hot and cold that affects conventional combi boilers under heavy simultaneous demand.
ASME-Certified Stainless Steel Heat Exchangers
Dual ASME-certified stainless steel heat exchangers deliver optimum durability and long-term efficiency, resisting the corrosion that shortens the life of standard combi heat exchangers.
Industry-Leading 12:1 Turndown
A 12:1 modulation ratio allows the burner to idle at a very low minimum output, improving part-load efficiency and comfort across a wide range of demand.
NaviCirc™ Hot Water Return
NaviCirc™ simplifies the setup of a hot water return system using existing pipework, supporting fast, consistent delivery to outlets throughout the property via the boiler's built-in recirculation pump.
Hydrogen-Ready (20% Blend)
Tested and approved for use with up to a 20% hydrogen blend in the gas supply, helping future-proof the installation against the UK's net zero transition.
12-Year Warranty
Navien back the NCB700 ON with a 12-year parts and labour warranty as standard — the longest in the current Navien gas range.
Technical Specification
General
Article number
PNCB7000042L001 (NCB700-2S+/42K)
Boiler type
Wall-hung, room-sealed, fan-assisted condensing Combi Crossover
Fuel type
Natural gas (LPG conversion kit included, field conversion required)
Dimensions (W x D x H)
440 x 335 x 695 mm
Guarantee
12 years parts and labour
Shower rating
Two to three showers
Heating Performance
Heat output range (80/60°C)
3.4 – 34.0 kW
Modulation (max/min)
12:1
Domestic Hot Water
DHW output range
3.5 – 42.0 kW
DHW flow rate (ΔT = 25°C)
24.1 l/min
DHW flow rate (ΔT = 35°C)
17.2 l/min
DHW flow rate, kitchen tap (ΔT = 45°C)
13.4 l/min
Performance & Emissions
ErP efficiency
A (94%) — A+ (98%) with single Smart Plus control
NOx classification
Class 6
Flue
Flue diameter
Coaxial 60/100mm
Max. horizontal coaxial flue length
20 m
Max. vertical coaxial flue length
21 m
Frequently Asked Questions
What does ON technology actually do?
It maximises recirculation of domestic hot water around the property's pipework, so hot water is already primed at outlets rather than needing to travel the full length of pipe from the boiler on every draw-off — reducing water wastage and the wait for hot water.
Can this 42kW model really run two or three bathrooms?
Yes — the NCB700 ON range was designed specifically for large-property hot water demand. The 42kW model delivers 17.2 l/min at a 35°C temperature rise, comfortably covering two bathrooms with strong simultaneous performance; for guaranteed three-bathroom, high-flow demand the larger 54kW NCB700-3S/54K model is available.
Do I still need a hot water cylinder with this boiler?
No. As a Combi Crossover model, the NCB700 ON supplies domestic hot water directly, without needing a separate storage cylinder, despite its system-boiler-level performance.
Can I run this boiler on LPG?
An LPG conversion kit is included in the boiler's packaging as standard, but the boiler ships configured for natural gas. Conversion must be carried out by a Gas Safe registered engineer holding the appropriate LPG qualification.
Can I install this boiler myself?
No. UK law requires that all gas boiler installation, commissioning and servicing work is carried out by a Gas Safe registered engineer.
