The Navien LCB700 Blue Flame 21kW LSX is an external, floor-standing oil system boiler, sited outside the building envelope and designed to work with a separate hot water cylinder. Built around Blue Flame combustion technology and a stainless steel heat exchanger, it was the first oil boiler on the UK market to offer an A+ energy rating when combined with three Smart Plus room thermostats.
20m / 21m
Max Horiz / Vert Flue
Blue Flame
Combustion Technology
10 yrs
Parts & Labour Warranty
OFTEC Registration Required
Installation, commissioning and servicing of this oil-fired boiler must be carried out by an OFTEC registered technician. A completed commissioning checklist must be left with the end user, and the flue and oil supply pipework must be installed in accordance with the manufacturer's installation and operation manual and current OFTEC/Building Regulations requirements.
System Boiler, External Installation — Cylinder Required Separately
This is a system boiler, not a combi, and requires a separate unvented or vented hot water cylinder, which is not included in this listing. It is also an external model, designed for floor-standing siting outside the building in a suitable weatherproof location, not for internal cupboard installation.
Why This Boiler
Blue Flame Combustion Technology
Delivers superior combustion efficiency and ultra-low NOx emissions compared with conventional yellow-flame oil burners, while maintaining strong reliability.
HVO Compatible
The boiler is compatible with Hydrotreated Vegetable Oil (HVO), a renewable drop-in alternative to kerosene, supporting a lower-carbon transition for off-grid properties.
Long Flue Runs
Supports up to 20m horizontal or 21m vertical flue lengths, giving flexibility when siting an external unit away from a convenient wall penetration point.
Combined PRV/Condensate Drain
A single combined pressure relief valve and condensate drain simplifies pipework and speeds up installation compared with boilers requiring separate connections.
Stainless Steel Heat Exchanger
Built for optimum durability and long-term efficiency, resisting the corrosive effects of acidic condensate over the working life of the boiler.
10-Year Warranty as Standard
Navien back the LCB700 Blue Flame range with a 10-year parts and labour warranty included as standard, without needing separate registration steps.
Technical Specification
| General |
| Article number |
PLCB0021SH002 (21-LSX) |
| Boiler type |
External, floor-standing, room-sealed oil system boiler |
| Fuel type |
Kerosene (28-second oil), HVO compatible |
| Dimensions |
470 x 600 x 860 mm |
| Guarantee |
10 years parts and labour |
| Heating Performance |
| Heat output (80/60°C) |
16 – 21 kW |
| Heat output (50/30°C, condensing) |
23.0 kW |
| Performance & Emissions |
| ErP efficiency |
A (93%) — A+ (98%) with 3x Smart Plus controls |
| NOx emission rate |
57.2 mg/kWh |
| Lift weight |
72 kg |
| Flue |
| Flue diameter |
Coaxial plastic 60/100mm |
| Max. horizontal flue length |
20 m |
| Max. vertical flue length |
21 m |
Frequently Asked Questions
Does this boiler include a hot water cylinder?
No. As a system boiler, the LCB700 Blue Flame 21kW LSX requires a separate hot water cylinder to store and deliver domestic hot water. Cylinders are sold separately.
What's the difference between the internal and external LCB700 models?
The external (LSX) model is a floor-standing unit designed to be sited outside the building envelope in a weatherproof location, whereas the internal (LS) model is designed for indoor installation.
Can this boiler run on HVO instead of kerosene?
Yes, the LCB700 Blue Flame range is HVO compatible, allowing use of Hydrotreated Vegetable Oil as a renewable drop-in alternative to standard kerosene, subject to local fuel availability.
How do I reach the A+ ErP rating on this boiler?
The boiler achieves ErP A (93%) as standard. Fitting three Smart Plus room thermostats raises the overall system rating to ErP A+ (98%).
Can I install this boiler myself?
No. UK regulations require that oil-fired boiler installation, commissioning and servicing work is carried out by an OFTEC registered technician.
