Gas Water Heater Not Igniting: 8 Causes and How to Diagnose Each
A gas water heater fails to ignite for one of eight reasons: dead ignition batteries, closed or empty gas supply, insufficient water flow, low water pressure, a blocked or misaligned igniter, a dirty flame sensor, blocked burner ports, or a tripped safety cut-off. Work through them in the order below — it puts the checks requiring no tools and no disassembly first, so the causes a user can resolve unaided are eliminated before a technician is called.
This guide is written from a manufacturer's perspective. We build these units, and we see what actually comes back from the field.
Before You Start: Two Safety Rules
If you smell gas, stop. Close the gas valve, ventilate the room, and leave. Do not operate any electrical switch. Ignition troubleshooting is only appropriate once you have confirmed there is no gas odour.
Never bypass a safety device to force ignition. Every cut-off in a gas water heater exists because a specific failure mode can be fatal. A unit that refuses to light is often a unit protecting you.
The checks below are diagnostic — they identify where the fault sits. Anything involving the gas valve, the burner assembly, or internal wiring belongs to a licensed gas technician.
Cause 1: Ignition Batteries Are Depleted
Battery-ignition water heaters stop sparking when cell voltage drops below the threshold the igniter needs, which happens well before the batteries are fully flat. It is worth checking first because it costs nothing and takes a minute, not because it is necessarily the most likely cause.
How to identify it
Turn on the hot tap and listen at the unit. A healthy igniter produces a rapid, even clicking. If the clicking is slow, weak, irregular, or absent entirely, suspect the batteries first.
What to do
Replace both cells with fresh alkaline batteries — not partially used ones, and never mix an old cell with a new one. Confirm correct polarity when reseating. Also inspect the battery contacts: corrosion or green residue on the terminals blocks current even when the cells themselves are good. Clean contacts with a dry cloth.
Why this matters in outage-prone markets
Hobun flue type units run on DC 3V battery ignition specifically so they work with no mains electricity. That independence is the reason the configuration is specified in regions with unstable grids — but it does transfer one maintenance duty to the end user. Distributors serving these markets should state battery replacement intervals plainly in the local-language manual, because the alternative is a service call for a fault that costs almost nothing to fix.
Cause 2: Gas Supply Is Closed, Empty, or Not Yet Purged
No gas at the burner means no ignition, and the igniter will keep clicking without producing flame. Clicking with no flame is the classic signature of a gas-side problem rather than an ignition-side one.
How to identify it
Check whether other gas appliances in the building work. If nothing gas-fired operates, the problem is upstream of the water heater entirely.
Then verify:
- The gas shut-off valve at the unit is fully open, handle aligned with the pipe
- The LPG cylinder is not empty — lift it, or check the regulator gauge
- The regulator is properly seated on the cylinder
The one most people miss
After changing an LPG cylinder, the regulator and hose contain a small amount of air that must clear before gas reaches the burner. The unit may fail to light on the first attempt or two. Reset and retry — but stop after three or four attempts, ventilate the area, and wait several minutes before trying again. Repeated ignition attempts on an unlit appliance release unburnt gas into the room, and that risk outweighs the inconvenience of waiting.
If the unit still will not light after the line has clearly had time to clear, the cause is elsewhere. Move on to the next check rather than continuing to cycle it.
Cause 3: Water Flow Is Below the Activation Threshold
Tankless gas water heaters only fire when a flow sensor detects sufficient water movement — below that minimum, the unit will not ignite by design. It is not broken. It has not been asked to run.
How to identify it
Open the hot tap fully. If the unit fires at full flow but not at partial flow, the flow rate at partial opening sits under the activation threshold.
Common causes of restricted flow:
- Clogged aerator or showerhead — scale buildup is the usual culprit
- Partially closed inlet valve
- Blocked inlet water filter screen at the unit
- Sediment in the pipework after mains work
What to do
Clean the aerator and the inlet filter screen first — both are user-serviceable and both are frequent offenders in hard-water regions. If the unit fires normally at the kitchen tap but not at the shower, the restriction is at the shower fitting, not in the heater.
Cause 4: Inlet Water Pressure Is Too Low
Distinct from flow rate: pressure is the force in the line, and if it falls below the unit's minimum start-up pressure, the water valve will not open far enough to trigger ignition. A unit can have adequate flow at the tap and still sit below its pressure threshold.
How to identify it
Suspect low pressure when the unit fails to ignite only at certain times of day (peak municipal demand), only on upper floors, or only when another outlet is running simultaneously.
The specification that matters when sourcing
Minimum start-up pressure is one of the most important — and most overlooked — specifications for buildings served by rooftop gravity tanks or weak municipal supplies.
The figure is set by the design of the water-gas linkage valve and the flow sensor, so it varies by model rather than by venting type. Do not assume a category-wide rule: ask the supplier for the minimum start-up pressure of the exact SKU before ordering. It is a hard physical limit, not a tuning parameter — no amount of installation adjustment will overcome it.
If you are sourcing for a market with known low-pressure infrastructure, make this figure a written line item in the specification, alongside the water source type (mains, rooftop tank, or pumped) it has to work with.
Cause 5: The Igniter Is Blocked, Fouled, or Misaligned
A spark must jump a precise gap at a precise position to light the gas — carbon deposits, corrosion, or a bent electrode will prevent ignition even with correct gas and water supply.
How to identify it
You hear clicking, gas is confirmed present, water flow is adequate, batteries are new — but there is no flame, or flame appears only after several attempts.
What a technician will check
- Carbon fouling on the electrode tip
- Electrode gap outside specification
- Physical misalignment relative to the burner port
- Cracked ceramic insulator, which leaks the spark to earth before it reaches the gap
- Damaged or loose ignition lead
This is a technician-level task. The electrode sits inside the combustion chamber, and both the gap and the position are specified values, not approximations.
Cause 6: The Flame Sensor Is Dirty
The flame sensor confirms combustion actually started; if it cannot detect flame, the control board shuts the gas valve within seconds as a safety response. This produces a distinctive symptom.
The signature
The unit lights, runs for two to five seconds, then shuts down. Repeatedly. If you see ignition followed by immediate shutdown, look at flame sensing before anything else — the gas and ignition systems are demonstrably working.
Why it happens
The sensor is a thin rod sitting directly in the flame path. Over time it accumulates a fine oxide coating. That coating is electrically insulating, so the sensor stops registering the microamp signal that proves flame presence — even though the flame is right there in front of it.
What to do
A technician cleans the sensor rod with fine abrasive and verifies the microamp reading against specification. It is a routine service item, not a component failure. In dusty environments or where combustion air quality is poor, it recurs on a predictable cycle — worth building into a preventive maintenance schedule for commercial installations.
Cause 7: Burner Ports Are Blocked
Blocked burner ports prevent gas from reaching the ignition point in the correct pattern, so the flame either fails to establish or lights only partially across the burner.
How to identify it
Partial ignition is the telltale sign: flame on one section of the burner and not others, or a flame that lights then collapses. In units that do run, blocked ports often show up first as uneven heating or a yellow-tipped flame.
Why ports block
Dust and lint drawn in with combustion air, spider webs and insect nests in units that sat idle, corrosion debris shed from the flue, and combustion residue from prolonged operation on poorly regulated gas.
Units that have been out of service for a season are particularly prone. Insect nesting inside the burner assembly and venturi is common in warm climates and is a genuine, documented cause of ignition failure — not folklore.
What to do
Burner cleaning requires partial disassembly of the combustion chamber and must be done by a qualified technician, who should also inspect the venturi and the air inlet path while access is open.
Cause 8: A Safety Device Has Tripped
Gas water heaters contain multiple independent cut-offs, and any one of them will block ignition until the underlying condition is cleared and the unit is reset.
Common trips:
| Device | What triggers it | What it indicates |
|---|---|---|
| Overheat / ECO cut-off | Outlet temperature exceeded limit | Scale in heat exchanger, or restricted flow |
| Flame failure device (FFD) | No flame detected after ignition attempt | Gas supply, igniter, or sensor fault |
| Exhaust blockage sensor | Restricted flue or fan failure | Blocked vent terminal — a genuine CO risk |
| Dry burn protection | Combustion with insufficient water | Empty pipework or blocked inlet |
Anti-freeze protection is a separate case. On most models it actively protects the heat exchanger rather than locking out ignition, so it does not normally present as a no-start fault. Check your model's manual for how it behaves.
The critical point
A tripped safety device is a symptom, not the disease. Resetting the unit without finding out why the device tripped means it will trip again — and the second time, the underlying condition will have progressed.
Exhaust blockage in particular must never be reset-and-ignored. A restricted flue on a room-sealed appliance is one thing; on a flue type unit that draws combustion air from the room, it is a carbon monoxide hazard. Find the blockage.
Diagnostic Sequence
Work in this order. It is arranged cheapest-and-most-likely first:
- Listen for clicking. No clicking → batteries or ignition circuit. Clicking → move on.
- Confirm gas. Test another gas appliance. Check the valve and cylinder. Purge after a cylinder change.
- Open the hot tap fully. Ignition at full flow only → flow restriction, clean aerator and inlet filter.
- Note when it fails. Only at peak hours or on upper floors → low inlet pressure.
- Watch what happens after ignition. Lights then dies in seconds → flame sensor.
- Look at the flame pattern. Partial or uneven → burner ports.
- Check for error codes. Consult the model manual and match against the code table.
- Call a licensed technician for anything involving the igniter, burner, gas valve, or a repeated safety trip.
What Distributors and Installers Should Take From This
Many ignition complaints turn out not to be product defects at all. Depleted batteries, air still in the LPG line after a cylinder change, clogged aerators, and low supply pressure are all site conditions — and all of them arrive as warranty claims.
Three actions measurably reduce service cost:
Localise the manual properly. Battery replacement and post-cylinder-change purging must be stated in the end user's language, in plain terms. A translated spec sheet is not a manual.
Match the specification to local infrastructure. If the destination market has unstable power, specify battery ignition. If it has weak water pressure, verify minimum start-up pressure before you order. Specification errors surface as warranty claims months later, in a different country, at your cost.
Give installers a one-page diagnostic card. Most of the sequence above can be worked through before anyone opens the casing. A field-service card in the box pays for itself on the first avoided callout.
Frequently Asked Questions
My gas water heater clicks but won't light. What does that mean? The igniter is working and the fault is on the gas or water side. Confirm the gas valve is open and the cylinder is not empty, purge the line if you recently changed cylinders, then verify water flow is above the activation threshold.
Why does my water heater light and then immediately shut off? That pattern points to the flame sensor. The unit ignites successfully, fails to confirm flame, and closes the gas valve as a safety response. The sensor rod usually needs cleaning.
Can low water pressure stop a gas water heater from igniting? Yes. Every unit has a minimum start-up pressure below which the water valve will not open far enough to trigger ignition. The threshold varies by model, so check it against the specification for your exact unit.
Do I need mains power for a gas water heater to ignite? It depends on configuration. Battery-ignition flue type units need no mains power at all. Forced exhaust and balanced flue units require mains power for the fan and will not ignite during an outage.
How often should ignition batteries be replaced? It varies with usage frequency and battery quality. Replace them as soon as clicking becomes weak or irregular, rather than waiting for total failure — and always replace both cells together.
Is it safe to keep retrying ignition? A few attempts are fine. Repeated attempts are not: unburnt gas can accumulate. If the unit fails to light after three or four tries, stop, ventilate the area, and diagnose before continuing.
When the Problem Is the Product
Most ignition failures trace to site conditions or maintenance. But some trace to build quality — an igniter positioned outside tolerance, a flame sensor that fouls abnormally fast, a flow sensor with a drifting threshold.
Hobun has manufactured gas water heaters since 1997. Our production process includes incoming material inspection, in-process monitoring, air-tightness and water-tightness testing, and finished product inspection before units leave the line.
If you are evaluating suppliers and want to understand how ignition systems are specified and tested, send us your market's power and water-pressure conditions and we will tell you which configuration fits.
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Author block By [Tong Li] | [Senior Gas Appliance Engineer] | 10 years in gas appliance design Technically reviewed: 26.8.2026 Zhongshan Hobun Electric & Gas Appliances Co., Ltd. — manufacturing gas and electric water heaters since 1997.
