Tankless Water Heater Ignition Repair in Frisco, TX

An ignition lockout is a sequence that did not complete. Knowing where in the sequence it stopped narrows the cause enormously, and you can often locate it from the kitchen by listening, because each stage makes a different noise. The order below is broadly common across brands; the timings vary by model and the service manual is the authority.

1. 1

   1. Flow detection

   A turbine or flow sensor has to see minimum activation flow, commonly in the region of 0.4 to 0.6 gallons per minute and lower on some models, before anything else happens. Below that the unit stays asleep. A blocked inlet screen or a failing pressure-reducing valve stops the sequence here, and it makes no sound at all.

2. 2

   2. Pre-purge

   The blower spins up and clears the combustion chamber of any residual gas before ignition is attempted. You hear the fan. If the fan never runs, the problem is upstream of combustion entirely.

3. 3

   3. Air proving

   A pressure switch or the blower's own speed feedback confirms the fan is moving air against the expected resistance. A blocked intake, a blocked termination, a sagging vent holding condensate, or a failed switch stops the sequence here — and the resulting code often reads as an ignition failure even though gas was never called for.

4. 4

   4. Gas valve opens and the trial for ignition begins

   The valve opens and the igniter sparks or glows. The control now has a fixed window, typically a few seconds, in which flame must be proven. You hear rapid clicking on a direct-spark unit; a hot surface unit is silent until light-off.

5. 5

   5. Flame proving

   The flame rod must return a rectified current above the control's threshold. If flame lights and dies within a second or two, the burner is fine and the sensing is not. If nothing lights at all, gas delivery or the igniter is the suspect.

6. 6

   6. Ramp to modulation

   With flame proven, the control ramps the blower and gas together to meet demand. Faults that appear here rather than at light-off are load-related, which is nearly always a supply pressure story.

7. 7

   7. Retry, then lockout

   Controls typically allow a small number of attempts before locking out and requiring a manual reset. That count is a diagnostic in itself: a unit that lights on the third attempt every time is failing, it is just failing slowly enough to be tolerated.

Flame rectification

A flame contains ions and therefore conducts electricity. When alternating current is applied between a small flame rod and a much larger grounded burner surface, the flame conducts far better in one direction than the other and behaves as a diode. The control board applies AC and looks for the resulting small direct current. Because only a real flame produces that asymmetry, it is a proof of flame that cannot be faked by a hot surface or a short circuit.

The current involved is tiny, generally single-digit microamps, and the control's minimum threshold is commonly around a microamp with the exact figure stated in the service manual for the model. That is the whole reason this circuit is so easily upset. A resistance that would be irrelevant in a lighting circuit is decisive in one carrying a millionth of an amp.

There is a maintenance use for this reading that most homeowners never get offered. Flame current degrades gradually as the rod oxidises, so a number recorded at each annual visit turns an unpredictable failure into a predictable one. A rod reading comfortably high three years running and then reading marginal is telling you it will lock out this winter. Cleaning it in October is a different experience from replacing it at six on a January morning.

One caution about doing this yourself. Reading flame current requires breaking the sensor lead and putting a true DC microammeter in series while the burner is lit, inside an opened sealed-combustion cabinet on a live gas appliance. Cleaning the rod is straightforward; measuring it properly is not, and the measurement is the part that carries the information.

Residential tankless units use direct spark ignition or a hot surface igniter, and occasionally both in a combined electrode assembly. They fail in different ways and on different timescales, so identifying which one you have narrows the possibilities before anyone opens the cabinet.

The delayed-ignition boom deserves its own note because homeowners find it alarming and they are right to. If the arc is weak or the gap is wide, gas accumulates in the chamber for a fraction of a second longer than it should before igniting, and the resulting light-off is a small deflagration rather than a smooth start. It stresses the exchanger and the chamber, and it is a fault to book rather than to live with. On a hot surface unit the equivalent is an element that has degraded to the point where it takes longer to reach ignition temperature than the control expects.

Handling matters more on hot surface igniters than people expect. Silicon carbide elements in particular are brittle enough to fail from being knocked during unrelated service, and skin oils baked onto the element create hot spots that shorten its life. If a unit stopped igniting shortly after any other work was done in the cabinet, that is worth mentioning on the phone.

Condensing units make water on every start. As the burner lights into a cold exchanger, flue gas condenses before the unit warms through, and for the first minute or so there is liquid in the combustion path by design. That is normal. What is not normal is that liquid finding its way onto the ceramic insulator of an igniter or flame rod, because a wet insulator provides a leakage path to ground and the spark then goes somewhere other than across the gap.

The classic presentation is a unit that fails to light on the first attempt of the morning and then works perfectly all day, or one that fails only after a long idle period. Cracks in the insulator, carbon tracking across its surface, and a sagging vent run that lets condensate drain backwards toward the chamber all produce the same pattern. So does an internal water leak dripping onto the burner, which is why an ignition complaint on an older unit always gets the cabinet checked for moisture that should not be there.

Grounding deserves more attention than it usually gets, because flame rectification depends on the return path through the burner. A ground screw into a painted bracket, a corroded terminal in an outdoor cabinet, a ground wire disturbed during an unrelated repair, or a receptacle with a ground that was never actually connected will all break flame sensing while every other function of the appliance appears normal. Reversed hot and neutral does the same thing more completely, and both faults are free to test with an instrument that costs less than a takeaway.

Finally, there is a category of ignition failure where nothing in the heater is wrong at all. A partially closed appliance shutoff after unrelated work. A meter sized for the house before a 199,000 BTU appliance joined it. Utility pressure dropping during peak demand on the coldest morning of the year. On a propane property, a tank low enough that the liquid surface cannot vaporise fast enough in cold weather to meet demand — propane's vapour pressure falls steeply as temperature drops, and a quarter-full tank at 15F behaves very differently from the same tank in October. We check supply before parts on every one of these calls, because supply is the answer often enough to justify checking it first every time.