Navien boilers
Navien is part of KD Navien, a South Korean heating firm founded in 1978. Navien says it’s the fourth largest gas boiler maker in the world, and the market leader in South Korea. Its boilers are made at one large factory in South Korea. Navien says it builds 2 million units a year there.
Navien came to the UK in 2014 with its NCB-CE gas combi. It also makes the LCB700, an oil boiler. In 2022 it launched the NCB700 ON. Navien says it was the first combi able to run three showers at once. The NCB500 ON is the smaller model in the same range. Navien calls both of them “Combi Crossover” boilers. Navien also sells system boilers. Its boilers come with a warranty of up to 12 years.
Navien fault codes
Navien fault codes start with E and a three-digit number. The one you’re most likely to see is E302. It means the pressure in your heating system is too low.
Navien’s help pages also explain the codes and symbols for each model. There’s a separate answer for the NCB-CE, the NCB300, and the NCB500 and NCB700. So check which model you have first. The name is on the front of the boiler.
How to top up a Navien boiler
The steps depend on your model. These come from Navien’s own help pages.
NCB500 and NCB700: these have a green lever. Pull it down slowly, and you’ll hear water filling the system. Watch the gauge or the pressure on the screen. Let go of the lever when it reads between 1 and 1.2 bar.
NCB-CE and NCB300: these have two small taps, one black and one blue. Open the black one a quarter of a turn, then slowly open the blue one. Close both when the pressure reads between 1 and 1.2 bar.
On every model, E302 should clear once the pressure is right. If it doesn’t, add a little more water. Navien says not to go over 1.5 bar. If you do, bleed a radiator until it drops below 1.2 bar. Our guide on how to top up low boiler pressure explains why pressure drops.
Before you call an engineer
- Check the pressure gauge, and top up if it’s low.
- Make sure your other gas appliances work, and check any prepayment meter.
- In freezing weather, look at the condensate pipe outside. See how to thaw a frozen condensate pipe.
If the boiler still won’t work, book a repair through Navien or call a Gas Safe engineer. Have the model name and the code ready. It helps them bring the right parts.
Navien gas boiler ranges
These are the gas boiler ranges on the Navien website as of 21/10/2026.
- NCB700 ON Combi Crossover Combi
- NCB500 ON Combi Crossover Combi
Older Navien ranges
You may still find these in homes. They are no longer sold new, but many are still working.
- NCB-CE
- NCB300
Common Navien fault codes
Codes can differ between models and ages of boiler. Always check the user manual for your exact model.
| Code | What it means | What you can do |
|---|---|---|
E302 | The system pressure is too low, under 1 bar. | Top up until the gauge or screen reads 1 to 1.2 bar. Don't go over 1.5 bar. |
Navien manuals
Navien puts its user guides and installation manuals on its own website. You can find Navien manuals here. You'll need your model name. The GC number helps too. Our guide shows you how to find your boiler's model and GC number.
Stay safe. You can reset a boiler, top up its pressure and thaw a frozen condensate pipe yourself. Anything inside the case is a job for a Gas Safe registered engineer.
If you smell gas, leave the building and call 0800 111 999.