A tankless water heater watches its own operation constantly: water flow, inlet and outlet temperatures, flame presence, fan speed, exhaust. When something falls outside its limits, it shuts down and logs a code. That makes tankless repair a mix of reading what the unit has recorded and checking the physical causes behind it.
No hot water from a tankless unit? Start here
- Note any code on the display, plus the make and model on the label.
- Try a hot tap at full flow; tankless units need a minimum flow to fire.
- Check whether other gas appliances are working normally.
- If it's been freezing, mention that; frozen or cracked heat exchangers are a known failure.
Common tankless faults
- Scale in the heat exchanger: overheating shutdowns, reduced flow, temperature swings, maintenance codes
- Ignition and flame sensing: clicking without lighting, lighting then shutting off
- Flow sensor and inlet filter: unit won't fire at normal flow, or cuts out
- Gas supply: undersized lines or low pressure, showing up as lukewarm water or faults at high demand
- Venting and air: blocked terminations, disconnected vent sections, dirty fans
- Electronics: circuit boards, sensors and wiring
- Freeze damage: cracked heat exchangers or fittings
Why scale causes so many tankless calls
A tankless heat exchanger has narrow passages with water moving through them at high heat. In hard water, minerals deposit on those hot surfaces just as they do in a tank, but here the build-up restricts flow and insulates the exchanger. The unit's sensors detect overheating or poor heat transfer and shut it down, or it simply delivers less hot water than it used to. Across the inland south and much of the Central Valley, descaling resolves a large share of tankless complaints. Hardness varies by district; your water provider can tell you about your supply.
Other California factors
Winter inlet temperatures in the north, the valley and the foothills push units closer to their limits, exposing marginal sizing and scale that went unnoticed in summer.
Outdoor units, common in the southern metros and on the coast, face sun, wind, insects in vent terminations and occasional cold snaps.
Freezing is a real risk in foothill and mountain communities, particularly in second homes where power may be off and freeze protection can't run.
Gas supply in older homes may never have been upgraded when a tank was swapped for tankless, which shows up as performance problems at peak demand.
Mounting and connections. Tankless units hang on the wall; secure fastening and connections that tolerate movement matter here as much as strapping does for a tank. The technician can tell you what they see.
We'll connect you with someone who works on tankless systems regularly.
What to leave alone
For the technician only
- The unit's front cover, burner, gas valve and circuit board
- Gas connections and venting
- Power to the unit's controls beyond a normal outlet
- The relief valve
It's fine to note codes, check that the unit's power cord is plugged in where it uses one, and keep vent terminations clear of debris you can see from outside.
How the technician diagnoses and repairs
They start with the fault history, then observe the unit firing if it will. They check water flow and the inlet filter, gas supply pressure under load, venting and air intake, and the sensors and igniter. If scale is suspected, they look at inlet and outlet temperatures and the unit's behavior, and usually descale through the isolation valves. Failed components are replaced, and the unit is tested at low and high flow to confirm stable temperature.
Repair or replace a tankless unit
Because there's no tank to rust through, many tankless faults are worth repairing even on older units: sensors, igniters, fans and boards are replaceable. The big exception is a heat exchanger that has cracked from freezing or failed from long-term scale and overheating. That part is expensive, and on an older unit replacement is often the better value. Parts availability for older or less common brands can tip the balance too. You'll get prices for both before deciding.
Protecting a tankless unit from freezing
Freeze damage is one of the most expensive tankless failures, and it's largely preventable. Many units include built-in freeze protection, small heaters that keep the heat exchanger above freezing, but those only work while the unit has power. In foothill and mountain homes, second homes left with the power off, and outdoor units in the colder north, that's where trouble starts. A cracked heat exchanger or fitting usually shows up as a leak or a unit that won't run once the weather warms. If your property sits empty in winter, ask the technician to set up a plan: keeping power on, draining the unit properly before a long absence, or insulating exposed piping. Doing that once is far cheaper than a heat exchanger.
What affects the cost
| Factor | Effect |
|---|---|
| Descaling only | Often the least costly fix for hard-water faults. |
| Sensors and igniters | Moderate; model-specific. |
| Circuit board or fan | More costly; availability varies. |
| Heat exchanger | The most expensive repair; weighed against replacement. |
| Gas supply fix | Line upgrades are a separate piece of work. |
How long it takes
Descaling and common part replacements are often completed in one visit. Boards, heat exchangers and less common parts may need ordering. The technician will tell you the likely wait before you agree to anything.
The code and model help us send the right technician.
Tankless repair questions
Codes vary by manufacturer and model, so we won't guess. Write down the code, the make and the model and pass them on when you call. The technician will check the unit's history as well as the current code, because the pattern of faults often tells more than the latest one.
That's often called a cold-water sandwich: a brief slug of cold water between hot runs, usually when a tap is turned off and on again quickly. Some of it is normal for tankless units. If it's severe, or temperatures swing during a steady shower, scale, a flow sensor or gas supply issues may be involved.
Common causes include scale overheating the heat exchanger, low flow below the unit's activation threshold, a clogged inlet filter, a venting or combustion air problem, a flame-sensing fault, or a gas supply that can't keep up. The technician works through them in turn rather than guessing.
The unit may be at its limit for the flow you're asking of it at the current inlet temperature, especially on cold mornings or with two showers running. Scale reduces capacity further. A gas supply that can't deliver full fire, or a fault in the gas valve or sensors, can also cause it.
It depends on water hardness, use and the manufacturer's guidance. In hard-water parts of California, many units benefit from descaling on a regular schedule, and some display a maintenance code when scale builds up. The technician can recommend an interval after seeing your unit.
Often, yes. Tankless units don't have a tank that rusts out, so many repairs are worth doing on older units. If the heat exchanger itself has failed, or parts are no longer available, replacement may make more sense. You'll get the price of both options.
The igniter is trying but the unit isn't confirming a flame. It could be the igniter, the flame rod, the gas supply, the gas valve, or a venting or air problem that stops combustion. The technician checks each in turn. If you smell gas, leave the building and call your gas utility and 911 from outside.
Yes. Water left in the heat exchanger can freeze and crack it, which is a common costly failure in foothill and mountain homes and in unheated spaces. Many units have freeze protection that relies on power being on. A technician can advise on protecting yours.