Navien NCB-H Combi Boiler Not Heating: Troubleshooting Guide

Need a licensed gas fitter for your gas boiler? Call CANRO at (604) 359-1081 — residential natural-gas heating and hot water across Metro Vancouver since 1999.

Work out which job has failed

A combi boiler does two things with one burner: it heats the house and it makes hot water. Most no-heat calls start with a shivering household, and the first question we ask on the phone is whether the shower still gets hot. If it does, the burner, gas supply, fan and ignition are probably working, and the problem sits in the heating call, the pump or the loop. If neither works, the boiler itself has stopped or locked out, and the cause is more likely gas, ignition, condensate, venting or power.

The Navien NCB-H is listed by the manufacturer as a condensing combi boiler. Condensing combis are sensitive to a handful of things that homeowners can safely look at, and to a longer list that only a licensed gas fitter should touch. This guide separates the two.

Safe checks before you call

1. Confirm there is no smell of gas. If there is, leave the home, avoid switches and call FortisBC from outside.
2. Check the thermostat is set to heat, set above the room temperature, and has working batteries where it uses them.
3. Confirm the boiler has power: display lit, the switch at the boiler on, and the breaker or fuse feeding it not tripped. Reset a tripped breaker once only, and if it trips again, stop.
4. Look at the display and note what it says. Your manual explains what categories of message mean.
5. Read the pressure gauge. If it is well below the range marked on the gauge, do not refill it yourself; note the reading.
6. Check that the gas supply valve to the boiler has not been turned off by someone during renovations or cleaning.
7. Look outside at the vent termination for snow, ice, leaves or debris.
8. If the boiler stopped in cold weather, consider a frozen condensate line and mention it to the technician.

That is the end of the homeowner list. Opening the cabinet, adjusting gas valves, draining the system and pressing reset over and over are not safe or useful steps.

Hot water works, radiators are cold

This pattern points away from the burner and toward the heating side. Possible causes, from the most to the least common in our experience: a thermostat or zone control that is not calling; low system pressure that has stopped the heating circuit; air in the loop or in the circulator; a circulator that has failed or seized; a zone valve that is stuck closed; a diverter or valve inside the boiler that is not switching properly; or a blockage in the loop from sludge.

A technician tests each in turn, starting with what the thermostat is asking for and ending with flow through the loop. Measuring temperatures along the supply and return pipes quickly shows whether hot water is leaving the boiler and where it stops.

Heat works, hot water does not

This one is usually a domestic-side issue: a fouled plate heat exchanger, a blocked inlet strainer, a flow sensor or diverter that is not registering a tap opening, or a fault in the temperature sensing on the hot-water side. Because the heating side is working, the burner, gas and venting are typically fine. A technician tests flow and temperature at the boiler and at the fixtures.

Nothing works at all

A total shutdown suggests a safety lockout or a loss of power. Ignition faults, weak flame sensing, a blocked condensate path, a restricted vent and a gas supply issue can all cause it. In cold weather a frozen condensate line is a leading suspect. Because several of these involve combustion, the safe course is to leave the boiler off after one reasonable attempt and call. Repeated resets stress the components and, in the case of ignition faults, are not advisable.

Symptoms and likely causes at a glance

What the technician does

We work from the outside in. First the gas and power to the boiler, then the thermostat and controls, then what the display reports, then the physical path the boiler needs: venting, condensate and water. We watch a start-up sequence to see where it stops. Only then do we open the boiler to inspect the burner, ignition components and heat exchanger. We test each mode separately, since a combi's heating and hot-water paths share components but not all of them.

If the boiler needs a part, we quote it in writing before replacing it. If the problem points to something bigger, such as a compromised heat exchanger, we say so and discuss repair versus replacement, which we cover in a separate guide.

Local causes we see in the Lower Mainland

The wet, mild climate hides a cold-snap problem: frozen condensate lines. Older homes add sludge in cast-iron radiator loops. Renovations change things quietly, such as a valve closed by a plumber and never reopened, or a thermostat relocated to a spot that never calls. In the Fraser Valley, colder outflow winds and harder water add stress to the exterior components and the hot-water side. Basement mechanical rooms with a washer and dryer nearby can fill the boiler with lint, which affects combustion air.

How thermostats and zones cause no-heat calls

A surprising share of no-heat calls end at the thermostat, the wiring to it or the zone control behind it. A thermostat relocated during a renovation, a smart thermostat that lost its network settings and reverted, a schedule accidentally set to away, or a battery that finally died can all leave the boiler quietly waiting for a call that never comes. In homes with multiple zones, one zone valve that fails closed can leave a single floor cold while everything else works. The boiler is fine; the call never arrives or the heat has nowhere to go.

We always ask what the thermostat display shows and whether it is asking for heat. If you can tell us that on the phone, the technician arrives with the right tools and often the right part. Please do not rewire a thermostat or change its settings extensively to test a theory. Set it to heat, set it a few degrees above the room temperature, and wait a few minutes before concluding anything.

Why we ask whether other rooms are warm

The pattern of cold and warm rooms is a diagnostic map. If every room is cold, the boiler, the circulator or the pressure is the likely cause. If only the top floor is cold, air is high on the list. If only the rooms furthest from the boiler are cold, flow balance or a partly blocked loop is likely. If a single radiator is cold at the bottom and warm at the top, sludge has settled in it. Tell the technician which rooms are warm and which are not. It saves time, and it helps us bring the right parts and a flushing plan if one is needed.

When to call right away

• Any smell of gas: leave, then call FortisBC from outside.
• A carbon monoxide alarm: everyone out and call 911.
• Water leaking from the boiler or its piping.
• Soot, scorching or a burning smell around the boiler.
• A banging noise at ignition.
• A lockout that returns after one reset.
• A house that is getting dangerously cold, particularly with children, seniors or pets at home.

CANRO Heating & Cooling has serviced boilers across Metro Vancouver since 1999, and every gas job is done by a Technical Safety BC licensed gas fitter.

Frequently asked questions