Ignition problems in tankless units don’t always announce themselves with a dramatic bang. More often they creep in: the shower runs lukewarm, the unit clicks repeatedly without catching, or an error code flashes and disappears after a reset. I’ve answered more “no hot water” calls caused by ignition hiccups than any other tankless issue. The good news is that most root causes fall into a short list of culprits. The challenge is reading the symptoms the right way, especially when the problem only shows up at certain flow rates or only on frigid mornings.
I’ll walk through how ignition actually happens inside a modern tankless water heater, what commonly prevents it, and a practical sequence to diagnose and fix the issue without throwing parts at the problem. I’ll reference the realities I see in the field: hard water, under‑sized gas lines, corroded grounds, and venting that was “good enough” until a remodel changed the airflow. If you’re in or near Lee’s Summit and searching for tankless water heater repair, some examples below use local conditions — long winter inlets, mixed fuel sources, and aging homes — but the principles apply anywhere.
What “ignition failure” really means
Tankless ignition is a timed choreography. The control board watches for demand, checks safeties, confirms airflow, opens the gas valve, sparks or heats an igniter, then looks for flame. Only when the flame signal is stable does it ramp to target temperature. A failure can occur at any step, and different brands label it with different codes: E001 or 11 on some units, 12 on others, “flame failure” elsewhere. The internal logic will usually try to light three to five times before locking out.
From the user’s perspective, you might notice short bursts of hot water followed by cold, or clicking with no heat. Sometimes the unit lights at high flow but not at low flow — a classic hint that the minimum gas pressure or air/gas mix is marginal. Error codes are invaluable, but symptoms matter just as much because a borderline condition might not throw a code until winter or until a coincident load runs (furnace firing, dryer running, or a stove top on high).
The three pillars: fuel, air, and spark
Every reliable ignition depends on adequate gas volume and pressure, proper combustion air and exhaust, and a healthy ignition/feedback circuit. When I see a “no light” situation, I test in that order. It mirrors the control board’s logic and avoids getting lost in the weeds.
Fuel: Even if the gas valve opens, the unit needs the right pressure at the inlet while flowing. Static pressure tests are only half the story. An undersized line that reads 8 inches water column at rest can drop below spec as soon as the valve opens, especially on cold-start when the unit calls for maximum BTUs.
Air: Most tankless units are sealed-combustion appliances. They breathe through intake piping and exhaust through venting that must be tightly assembled, properly sloped, and of the correct material. Any blockage, condensate pooling, or poorly sealed joints can slow airflow enough to prevent the pressure switch or fan-speed feedback from confirming safe combustion.
Spark and sense: Ignition systems vary — hot surface igniters and spark electrodes dominate. Either way, the flame proving rod must sense microamps of current through the flame. Dirt, oxidation, or a weak ground can prevent that tiny current from making it back to the board, so the unit thinks there’s no flame and shuts the gas.
Reading the clues in the home
Two homes can have identical units and completely different ignition issues. Field conditions matter.
A tract home built in the late 90s might have a long 1/2-inch gas run that served a 40-gallon tank with no trouble. After a tankless water heater installation in the same spot, the line is now too restrictive for 180,000 BTUs at full fire. The unit lights on low-flow hand washing but fails on a shower when the furnace kicks on. This is common during winter in Lee’s Summit when appliances stack on the same manifold.
In a lakeside home, I once traced intermittent ignition to wind effects on the concentric vent. Gusts created negative pressure at the intake, and the unit went out of range at the moment of light-off. The vent met minimum length, but its termination faced the prevailing wind. Rotating the termination and adding an elbow resolved it.
I’ve also found that homes with heavy DIY work often hide a simple answer: paint overspray on a ground lug, a reversed hot/neutral at the receptacle, or an extension cord feeding the unit. Tankless boards are sensitive to grounding; a reversed neutral can create just enough noise to fool the flame sensor.
Common mechanical causes of ignition failure
Gas supply mismatch is the headline problem. Tankless units have published minimum and maximum inlet pressures. For natural gas that’s often around 3.5 to 10.5 inches water column, for propane 8 to 14. Static pressure might pass, but dynamic pressure under load tells the truth. If the unit starves during ignition, it misfires. Undersized lines, long runs with too many fittings, or old regulators can create these conditions. When we perform a water heater service in the field, we test dynamic pressure while the unit ramps so we can see the sag.
Dirty burners and electrodes come in a close second. Dust, lint, and mild corrosion change how gas distributes across the burner and how the flame contacts the sensor. On hot surface igniters, a glaze of combustion byproduct can slow heat-up and miss the lighting window. Regular water heater maintenance that includes burner inspection and cleaning prevents this.
Vent and condensate problems might not be obvious. A slightly sagging horizontal run can pocket a half cup of condensate. On start-up, the fan is fighting a water seal. Or a shared chase picks up dryer lint that settles on intake screens. On condensing models, a clogged condensate trap can back water into the combustion chamber. I’ve cleared traps with five-year-old coffee grounds from a utility sink, a surprising but real culprit.
Electrical and board issues are rarer, but they happen. A power surge fries a relay, or a corroded molex connector reduces sensor signal. I see more ground-related problems than board failures. It takes minutes to verify a tight, clean chassis ground and a correctly wired receptacle. That step saves hours.
Water-side oddities can masquerade as ignition failures. A calcified flow sensor might under-report flow at low demand, so the board believes there isn’t enough demand to start a burn. Or scale shifts water temperature readings, causing rapid fire-and-drop cycles that feel like ignition issues. Where water hardness runs 8 to 15 grains, as it often does in our region, descaling intervals matter. If your tankless has never been flushed, a thorough water heater maintenance service can clear a surprising number of “ignition” complaints without touching the gas train.
The service call: a disciplined sequence
I carry a short mental checklist. It keeps me from overthinking and keeps the visit efficient for the customer. Here’s the trimmed version I use on site:
- Verify complaint, observe behavior, and retrieve error history. Check dynamic gas pressure at ignition and mid-fire; confirm fuel type and regulator health. Inspect venting, intake screens, and condensate path; clear blockages. Inspect and clean burner, igniter, and flame rod; confirm gap and alignment. Confirm electrical ground, polarity, and connector condition; test flame-sense microamps.
That’s the first and only list of this article. Each step either clears the path or reveals where to dig deeper.
If the unit lights at high flow but not low, I narrow the suspects to gas delivery at minimum modulation, air/gas ratio, or flow-sensor reporting. If it lights on warm recirculation but not from a cold start, I look at ignition timing and gas pressure sag at high BTU call. If wind is in play, I watch fan speed feedback and put a hand near the termination to feel gusts.
A closer look at gas supply issues
On natural gas, I target 7 to 10 inches water column during steady burn for most units, but ignition events are a moment of maximum demand and transient response matters. I measure at the unit’s inlet tap with a manometer, then I cause the unit to fire and a second large appliance to run, like a furnace or range top. If pressure dips below the manufacturer’s minimum during the start sequence, it’s almost always a sizing or regulator problem.
Undersized piping turns up often in water heater replacement jobs where a tank was swapped for tankless without addressing the feed. Even if a line is technically adequate on paper, long runs with many elbows add up. I’ve had good results upsizing a final segment or adding a dedicated branch with fewer fittings. For propane systems, tank regulators sometimes lag during snap demand. Replacing an aging regulator or adding a second-stage regulator near the appliance stabilizes light-off.
Combustion tuning matters too. Some models allow manual gas valve calibration using a manometer and test points while the unit holds a specified firing rate. If your unit has this procedure, follow the manual closely. A small tweak can convert borderline starts into reliable ignition.
Venting and air management subtleties
I pay careful attention to vent slope and length. On condensing units using PVC or polypropylene, horizontal runs must slope back to the heater to drain condensate. If they slope forward, water pools in the vent and the fan works harder at light-off. If the vent rises vertically for a long run, expansion and contraction can loosen joints. I always inspect for primer and solvent-weld coverage, and I look for white chalky streaks that signal minor leaks.
Intake screens collect dust and pollen. On sidewall terminations near landscaping, spiders and cottonwood fluff love to build homes. A blocked intake won’t always trip a code, but the fan speed will spike as the board tries to achieve target airflow. Clearing the screens takes seconds and can solve the trouble on the spot.
Wind effects are real. Concentric terminations should be oriented with guidance from the manufacturer. In open exposures across the Lee’s Summit area, I’ve installed wind guards or redirected terminations to prevent gusts from pushing exhaust back into the intake at the worst time — ignition.
Igniters, flame rods, and the magic of microamps
The flame sensor circuit measures a tiny DC current — often in the 1 to 10 microamp range — flowing through the flame to ground. If the current is too low, the board abandons the ignition sequence. Dirt on the rod, misalignment, or a poor ground will all lower the signal. I clean flame rods with a fine abrasive pad, not sandpaper, to avoid grooves. On spark systems, I check electrode gap and shield spacing. On hot surface systems, I look for hairline cracks and discoloration that indicate a tired igniter.
Measuring flame-sense microamps with a series meter leads can be revealing. An otherwise healthy system showing 0.3 microamps is likely to short-cycle or drop out under marginal conditions. After cleaning and re-seating grounds, I often see that jump to 2 or 3 microamps. If it still reads low, I check for resistance on the ground path and verify that the house receptacle is properly wired. I have fixed a surprising number of “bad boards” with a ten-minute ground cleaning.
Water quality and flow sensors: the quiet saboteurs
Scale builds inside heat exchangers and on flow sensors. A stuck or sluggish flow sensor may under-report, causing the control to refuse ignition below a threshold. When I perform water heater maintenance, I disassemble the flow sensor to check for debris. In homes without softening, a descaling flush using manufacturer-approved cleaner can restore normal operation. Many units benefit from annual descaling in hard water areas, possibly twice a year if usage is high.
Thermistors can drift. If the unit misreads inlet temperature, it might attempt ignition at an inappropriate modulation, then drop out. This is less common but worth considering when everything else checks out. Comparing displayed inlet temperature to an actual reading at the cold feed will tell you if the sensor is in the ballpark.
When replacement makes more sense than repair
Not every ignition failure gets a cost-effective fix. On older units where parts availability is spotty, you can spend more time waiting for a discontinued gas valve than the unit is worth. If the heat exchanger shows corrosion or leaks and the board is original, I have frank conversations about water heater replacement. The economics pivot on age, part availability, and gas/venting noncompliance discovered during diagnosis.
A newer unit under warranty leads me toward documented repairs. An older out-of-warranty unit with multiple marginal systems pushes me toward recommending a modern replacement with known-good venting and water heater service a correctly sized gas line. In Lee’s Summit, many early tankless installations from the mid-2000s are now reaching this point. Upgrading can also solve efficiency and noise issues and reduce service calls.
Installation choices that prevent ignition trouble later
Good water heater installation prevents half the ignition calls I see. Proper line sizing, thoughtful vent routing, and clean electrical supply are givens, yet I still encounter corner-cut solutions that worked “most of the time.” When we handle water heater installation in Lee’s Summit, we size gas lines for full fire plus concurrent appliance loads, not just nameplate BTUs in isolation. We keep vent lengths within conservative limits, use manufacturer-recommended materials, and protect terminations from wind.
Combustion analysis at commissioning is worth the extra time. Some units self-calibrate, but verification with instruments catches oddities like a slightly lazy flame at low fire or an intake restriction that might pass unnoticed until the first big cold snap. We also document dynamic gas pressure, intake/exhaust deltas, and flame-sense readings at start-up. These “birth stats” make future service faster and more precise.
Maintenance rhythm for long-term reliability
Tankless equipment thrives on light but regular attention. An annual water heater service is usually enough for city-water homes with moderate hardness. For well water or very hard water, I nudge the schedule to every six months for the first year, then adjust based on scale evidence. A maintenance visit that actually does the work — descaling the exchanger, cleaning the burner and sensor, checking condensate traps, inspecting venting, verifying gas pressure, and confirming flame-sense microamps — is not a checkbox exercise. It directly supports trouble-free ignition.
If you prefer a homeowner routine between professional visits, keep it simple:
- Keep the intake and exhaust terminations clear of debris and snow; glance monthly in winter. Listen for repeated clicking without ignition; if it happens more than once, call before it becomes a lockout.
That’s the second and final list. Small habits prevent emergency calls at the worst times.
Special notes for the Lee’s Summit climate and housing stock
Cold incoming water in January forces maximum burner output during ignition. If gas supply is tight or venting is marginal, this is when you notice it. Houses built with 1/2-inch branch lines feeding multiple appliances are the usual suspects. We often reroute or upsize the last leg to the heater. Also, garages and mechanical rooms that seem draft-free can still create pressure imbalances when large appliances or bath fans run. Sealed-combustion models help, but proper termination and intake shielding are crucial.
Local water hardness runs in the mid-range to high, depending on neighborhood. Without conditioning, expect scale to show up inside a year, especially on high-use systems. Building a water heater maintenance schedule around real measurements — flow sensor cleanliness, descaling residue volume — pays off more than a calendar reminder alone.
When to call for professional help
DIY steps can rule out the obvious: checking for power at the receptacle, clearing visible lint from the intake, verifying gas supply is on, and confirming error codes from the screen. Once you step into gas pressure testing, flame-sense measurement, and combustion inspection, you need the right tools and training. Safety comes first. A gas leak or incomplete combustion is not worth the risk.
A qualified technician will arrive with a manometer, combustion air tools, electrical testers, and brand-specific knowledge. They’ll also know when to stop and recommend replacement instead of stacking bills on a failing unit. If you need tankless water heater repair in Lee’s Summit or surrounding areas, choose a provider who talks as easily about line https://maps.app.goo.gl/iDyXY2SdtXV9NEGi8 sizing and vent routing as they do about error codes. That’s a sign they’ll solve the root cause, not just reset the board.
Tying it back to installation and lifecycle choices
Tankless systems repay careful planning. If you’re considering water heater installation or evaluating water heater replacement, use ignition reliability as a litmus test for the installer’s approach. Ask how they size gas lines, how they route and support venting, whether they record start-up measurements, and what their water heater maintenance program includes. Good answers reduce emergency calls and extend service life.
On the service side, an ignition failure is rarely a mysterious gremlin. It’s usually a small imbalance in fuel, air, or sensing. The fix might be as simple as cleaning a flame rod or as involved as adding a new gas branch. Either way, a structured diagnostic process beats guesswork. And if you keep up with water heater service, you’ll prevent most of these problems from surfacing at all.
For homeowners in and around Lee’s Summit, local specifics are familiar: winter cold starts, mixed appliance loads, and aging lines. Addressing those conditions during installation and reaffirming them during maintenance is the surest path to dependable hot water. Whether you need tankless water heater repair, water heater service, or a fresh start with a new unit, the same principles hold: size it right, vent it clean, ground it well, and check the numbers. The burner will light, every time.