Fast Plumbing Answers: Tankless Water Heater Won't Ignite

You can’t get your gas tankless hot water heater to ignite. Most articles and online resources tell you it’s a blocked filter- but yours seems to be clear. You need other solutions, fast- before you have to take another cold shower.

Don’t worry: we’ll answer all your questions in 2 minutes or less.

We’re Mother, a Dallas-Fort Worth plumbing company focused on tech-driven, modern plumbing solutions with the homeowner in mind. In this article, our plumbing expert walks you through the three things every natural gas tankless water heater needs to ignite, plus five common culprits you can identify easily.

If you need more help with tankless water heater repair in the Dallas area, call us 7 days a week. We’ll send a licensed plumber within 24 hours for a free estimate and lasting solutions.

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3 Things You Need to Ignite Your Tankless Water Heater

First things first- we’re talking about natural gas tankless water heaters. Electric units don’t have igniters. 

“Tankless units need three things in order to light properly,” says our Journeyman Plumber Dillon Ashton. “Gas, air, and water flow.”

Without a combination of all 3 of these ingredients, your on-demand water heater will fail to ignite. In this case, 2 out of 3 is bad.

How your gas tankless water heater ignites

The ignition process of your tankless on-demand heater requires four steps:

1. You turn on a hot water tap. Internal sensors in your system pick up the flow of water and signal your tankless water heater ignition system.
2. Activation. Your igniter is activated by the unit’s flow sensor. This is either a hot surface ignitor (ceramic or silicon carbide), or an electric spark ignition device. 
3. Gas entry. While the igniter activates, your gas control valve opens to introduce a proper amount of natural gas to the burner. 
4. Ignition complete: The heat or spark created in the activation process ignites the gas. A flame is produced to heat the water in your tankless system.

5 Reasons Your Tankless Water Heater Won’t Ignite

Dillon outlines the five primary reasons that ignition failure occurs in gas tankless water heaters:

1. A clogged water filter: Low water flow means your unit may not activate the ignition process.
2. Limited gas supply: Like water, a low gas flow stops the activation process.
3. Lack of air: A blocked vent or intake prevents enough oxygen for the burner to light.
4. Ignition system issues: Issues with electrodes and sensors within the system are common in older, damaged tankless units.
5. Electrical problems: If all else fails, a tripped breaker or power loss may be to blame.

Safety tip: “Don’t keep forcing resets,” Dillon advises. “If it’s a bigger issue like gas delivery or failed ignition component, you don’t want to risk further damage or safety problems.”

Clogged water filters

“If the filter’s clogged with debris, the unit won’t sense enough flow to fire up,” Dillon explains. “This is especially true with the filter’s inlet screen.”

Advice: Regular cleaning of your tankless unit’s water filter is a good first step. Be sure to shut off the power and cold water supply to your system before beginning. Then, close the hot water valve and flush any remaining water out of the system.

Remove the filter before cleaning debris- if you attempt to clear it while it’s installed, that debris will just end up in other components of your on-demand heater. Strong water pressure and a metal wire brush should be sufficient.

Reinstall the filter completely before turning on the water supply and power.

Gas supply limitations

“Low gas pressure, an undersized gas line, or a closed valve- these can all prevent ignition,” advises Dillon. 

Advice: Check the gas intake valve. If it’s closed, move it to the “open” position.

If it’s not your valve, call a licensed plumber. Whether it’s a gas pressure issue or an improperly sized natural gas line, these aren’t DIY fixes. 

Fix your igniter and get back to saving money! Our DFW guide to home tankless water heater installation reveals how much money your household could save.

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Insufficient air supply

Every fire needs oxygen. Your igniter is no exception. Dillon tells us the most common cause of insufficient air flow into your tankless water heater’s ignition system is a blocked vent.

“Blocked venting or intake absolutely keep the burner from lighting,” he says. “I’ve seen a number of causes for this- anything from a bad vent installation to a bird’s nest.”

Advice: Be sure to turn off your on-demand unit and the power to the unit before attempting to clear or clean your air vent. Most tankless air filters can be accessed by removing the cover screws on the front panel.

An air compressor or shop vacuum is enough to clear dust, lint and sediment particles from the vent. If you’re dealing with a larger issue, or a blockage on your roof, call a licensed plumber to address the situation. This isn’t just a safety precaution- using the wrong cleaning supplies can void your roof and shingle warranties.

Faulty ignition components

Dillon outlines the two ignition components most likely to fail in most tankless systems. Both are a part of spark electrodes, which produce a reliable flame for your hot water supply:

1. Spark probes: The part of your spark electrode that creates the flame.
2. Flame sensor: The part of your spark electrode system that monitors the flame and maintains its consistency.

These two components operate in tandem to produce the flame- it’s difficult for the average homeowner to distinguish between them when symptoms arise.

Advice: Call a licensed plumber to evaluate any damaged or broken components within your tankless water heater’s ignition apparatus. It’s far safer, and you’ll reduce the risk of replacing the wrong part(s) and costing yourself extra money.

Free estimates help: Mother’s licensed techs diagnose your tankless water heater ignition issues during a free estimate- identify the problem before you spend a dime on repairs.

Electrical problems

If you can’t locate any issues with the gas, air or water intakes in your tankless water heater, and there are no faulty components within the igniter- you may just have an electrical issue.

Dillon urges homeowners who can’t identify their ignition problem to call their plumber before diving in for repairs- for reasons just like this.

Imagine tearing apart your instant-on or combination boiler looking for faulty parts or blocked vents, when it was just your circuit breaker.

Advice: “The circuit box should be your first and last check,” Dillon says. Even if there isn’t a tripped breaker, you still need to turn off the power supply to perform most tankless water heater diagnostics safely.

Call Mother for Dallas Tankless Water Heater Repair

Did you walk through all the DIY steps in this guide, and your tankless water heater still won’t ignite? If you live in the DFW Metroplex, we’re here to help.

Trust Mother for modern, tech-driven solutions that improve tankless heater system efficiency and performance. Our team of licensed plumbers is constantly trained and upskilled on advances in tankless technology- you always get the single best solution to your problem.

Get your flame burning, and get back to on-demand hot water. Call Mother 7 days a week- we’ll be there to diagnose your ignition problem within 24 hours.

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