Start by writing down the error code. Tankless units self-diagnose, and that code narrows the problem substantially before anyone arrives. Then call — our technicians work on tankless systems around the clock, and many issues are resolved in a single visit.
Read the Code Before You Reset
The instinct when a tankless water heater throws an error is to power-cycle it and hope. Sometimes that works, and it is worth one attempt. But note the code first.
Clearing a code without recording it destroys the most useful piece of diagnostic information available, and intermittent faults are exactly the ones that are hardest to catch on a service call. A homeowner who says “it throws a 12 about once a week, usually in the morning” has just saved an hour of diagnosis.
Codes vary by manufacturer, but the categories are consistent: ignition failure, flame loss, overheat, flow sensor, venting or air supply, and combustion faults.
The Faults We See Most
Ignition failure. Usually gas supply related. Either the gas line cannot deliver the volume the unit demands when it fires, or there is a valve, regulator, or supply issue. On propane, it can simply mean the tank is low. Undersized gas line capacity is the most common underlying cause on units that worked fine initially and then began failing intermittently as other appliances were added.
Scale-related overheating. Mineral buildup in the heat exchanger causes hot spots and triggers overheat protection. This presents as fluctuating output temperature or shutdowns during long draws.
Flow sensor problems. Debris in the sensor or a clogged inlet screen makes the unit misread demand, so it fails to fire or cycles oddly.
Venting and air supply. Obstructed intake or exhaust — nests, debris, snow, or a termination too close to something — trips combustion safeties.
Condensate blockage. On condensing models, a clogged condensate drain shuts the unit down entirely.
Symptoms Without a Code
Not every problem announces itself. The classic tankless complaint is the cold water sandwich — hot, then briefly cold, then hot again — which happens when a short draw ends and another begins before the unit restabilizes. Some of that is inherent to the technology; a lot of it can be improved with proper sizing or a recirculation setup.
Water that is hot but not hot enough usually means the unit is undersized for winter conditions, scaled, or being asked for more flow than it can deliver at the required temperature rise.
Declining flow at hot taps generally points to the inlet screen or scale rather than the plumbing.
What to Check Before Calling
A few things take two minutes and occasionally solve it. Confirm gas is on and other gas appliances are working. Confirm the unit has power and the breaker has not tripped. Look at the vent termination outside for obstruction. Check whether the problem happens only when multiple fixtures run, which points to capacity rather than failure.
Beyond that, tankless units involve gas, high-temperature exhaust, and sealed combustion. They are not a good candidate for homeowner repair.
Repair or Replace
Most tankless repairs are worth doing. These units are built to last twenty years, so a fault at year six is almost always a component issue rather than end of life — a flow sensor, an igniter, a flame rod, a gas valve.
The exception is a failed heat exchanger on a unit that was never descaled. That part is expensive enough that replacement sometimes makes more sense, and it is the argument for maintenance in its clearest form.
Preventing the Next One
Most tankless service calls trace back to scale or gas supply. Annual water heater maintenance addresses the first, and a water softener prevents it from recurring.
If you have no hot water and the unit will not restart, our 24/7 emergency plumbers are dispatched at any hour.
We repair tankless systems throughout Fredericksburg, Kerrville, Boerne, Comfort, and the surrounding Hill Country. Contact 72 Degrees or call (830) 468-5316.
