The Navien LCB700 Blue Flame 21kW LCX is an external, floor-standing oil combi boiler, sited outside the building envelope in a weatherproof enclosure. Built around Blue Flame combustion technology and stainless steel heat exchangers, it was the first oil boiler on the UK market to offer an A+ energy rating when combined with three Smart Plus room thermostats, and achieves the highest 3-star DHW performance mark under EN Regulations.
14 l/min
DHW Max Flow Rate
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.
External, Floor-Standing Installation
This is an external model, designed for floor-standing siting outside the building in a suitable weatherproof location, not for internal cupboard installation. Confirm site access, oil supply routing, and flue clearances against the manufacturer's installation manual before ordering.
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.
Highest 3-Star DHW Rating
Achieves the highest 3-star mark for domestic hot water performance under EN13203-1, reflecting strong, consistent hot water delivery.
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 |
PLCB0021SD002 (21-LCX) |
| Boiler type |
External, floor-standing, room-sealed oil combi boiler |
| Fuel type |
Kerosene (28-second oil), HVO compatible |
| Dimensions (H x W x D) |
650 x 673 x 900 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 |
| Domestic Hot Water |
| Maximum flow rate (±15%) |
14 l/min |
| DHW control range |
30 – 60°C (default 50°C) |
| Max. hot water temp. rise, 90-litre draw-off (at max flow rate) |
40°C |
| EN13203-1 DHW performance rating |
★★★ (highest mark) |
| Performance & Emissions |
| ErP efficiency |
A (93%) — A+ (98%) with 3x Smart Plus controls |
| NOx emission rate |
57.2 mg/kWh |
| Lift weight |
91 kg |
| Flue |
| Flue diameter |
Coaxial plastic 60/100mm |
| Max. horizontal flue length |
20 m |
| Max. vertical flue length |
21 m |
Frequently Asked Questions
What's the difference between the internal and external LCB700 models?
The external (LCX) model is a floor-standing unit designed to be sited outside the building envelope, in a weatherproof location, whereas the internal (LC) model is designed for indoor installation, typically wall-mounted.
Does this boiler need a hot water cylinder?
No. As a combi boiler, the LCB700 Blue Flame LCX supplies domestic hot water directly, without a separate storage cylinder.
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%), as this was the first UK oil boiler range to achieve that rating.
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.
