What actually fails on a tankless unit
A tankless heater has fewer parts than a storage tank but far more sensors. The heat exchanger, flow sensor and control board talk to each other constantly. One weak reading can shut the whole appliance down.
That is why a tankless fault often looks dramatic and turns out to be small. A clogged inlet filter can mimic a failing heat exchanger, and a blocked condensate line can pass for a dead circuit board.
The diagnosis is elimination, not guesswork. It starts with the code on the display and the water flow at the fixtures, then moves inward.
What the error code is telling you
Manufacturers number their codes differently, but the families repeat across brands. A code narrows the system, not the exact part.
| Code family | Usual meaning | First check |
|---|---|---|
| Ignition or flame | The burner did not prove a flame | Gas supply, igniter, flame rod |
| Overheat | Outlet temperature climbed too high | Scale, flow rate, temperature sensor |
| Flow or volume | Too little water is moving through | Inlet filter, isolation valve, aerator |
| Air pressure or fan | The fan or vent is not proving draft | Vent blockage, fan, pressure switch |
| Condensate | The drain is blocked or slow | Trap, drain slope, neutraliser |
| Sensor or comms | A reading is missing or out of range | Harness, sensor resistance, board |
Write the code down before you reset anything. A cleared code removes the evidence the provider needs, and the same fault then takes longer to find.
Eight symptoms and what they point to
- Hot water starts strong, then fades. Scale in the exchanger or a unit running at its limit.
- Water never gets hot at all. Ignition or gas supply, not the tap.
- One fixture stays cold while others work. A clogged aerator or a mixing valve passing cold water.
- Banging or kettling noise. Mineral scale on the exchanger surface.
- Code on the display, then shutdown. The unit is protecting itself from a fault it can see.
- Water inside the cabinet. An internal seal, a condensate leak or a cracked exchanger.
- Fan running without a flame. A vent or pressure switch fault, often a blocked termination.
- Sulfur smell near the unit. Treat it as a gas concern and ventilate before anything else.
The faults behind most calls
Scale in the heat exchanger
Heating water leaves mineral deposits behind, and a tankless exchanger packs them into narrow passages. Flow drops, the outlet temperature climbs and the unit eventually trips on overheat. Descaling restores most of the performance when it is caught early.
Left alone, scale overheats the metal and can split a passage. At that point the exchanger leaks and the repair becomes a replacement, which is why the descaling interval matters more than most owners expect.
Inlet filter and flow sensor
The inlet filter catches grit before it reaches the exchanger, and it clogs quietly. A restricted filter tells the unit that demand is low, so the burner modulates down or refuses to fire. Cleaning it is quick, and it is the first thing a provider checks.
The flow sensor reports how much water is moving. When it drifts, the unit misreads demand and cycles. Sediment and scale are the usual culprits, and the sensor is usually cleaned or replaced rather than repaired.
Gas and ignition faults
A tankless burner needs a clean flame signal within a set time or it locks out. A weak igniter, a dirty flame rod or gas pressure that sags under load all produce the same symptom: ignition, then shutdown. Gas pressure has to be tested while the unit is firing, not while it sits idle.
Undersized gas lines show up here first. The unit lights, then drops out when several fixtures call at once. That is a supply problem rather than an appliance fault, and gas water heater repair covers the checks that confirm it.
Venting and condensate
Condensing units drain acidic water through a trap and a neutraliser. A blocked trap shuts the unit down, and a frozen or sagging drain does the same in cold weather. Vent terminations block with debris, nests and storm material.
Outdoor units on the coast face salt air as well. Corroded terminals and degraded seals cause intermittent faults that only appear in wind-driven rain. The fix is often a service kit and a better mounting position.
How the provider finds the fault
The visit follows a fixed order so that no part gets replaced on a hunch.
- Read the history. Stored codes and the sequence of resets tell more than a single fault.
- Measure flow and temperature. Inlet and outlet readings show whether the unit is starved or overheating.
- Test gas or power under load. Pressure and current are checked while the unit fires, not before.
- Inspect the exchanger and filter. Flow restriction and scale are confirmed by eye before descaling.
- Quote the repair in writing. Parts, labour and any descaling are listed separately, and you approve before work continues.
Ask which test confirmed the diagnosis. A clear answer separates a technician who traced the fault from one who guessed at the most common part.
When a tankless unit is worth repairing
Most faults are economic to repair: filters, sensors, igniters, fans, gas valves and condensate traps. The unit stays, and the repair restores it for a fraction of a new appliance.
A leaking heat exchanger changes the maths. The part carries most of the cost of the unit, and the water that damaged it has touched everything else. A provider should show you the leak before recommending replacement.
Age and parts availability decide the rest. A model that has been discontinued can still be repaired while spares last, but a second major fault within a year is a signal to stop spending. Our tankless installation page covers what a replacement involves beyond the appliance.
Stopping the same fault coming back
Descaling on the manufacturer's interval is the single biggest factor. Hard water shortens that interval, and a whole-house softener changes it again. A provider sets the schedule from your water rather than a generic card.
Rinse the inlet filter when the flow at the fixtures drops. Check the condensate trap for debris and confirm the drain still slopes. Look at the vent termination after a storm, because nests and debris cause lockouts that look electrical.
If the unit sits far from the bathrooms, as it can on larger lots in White City, the wait for hot water is longer and the flow at the far tap is weaker. That is plumbing layout, not a fault, and a recirculation option may suit better than repairs.
A yearly service visit covers the flush, the filter and the safety checks in one appointment. Our flush and maintenance page lists what that visit includes for tankless and tank models.
Tankless repair questions
Can I just reset the error code?
You can reset once to see whether the fault returns, but write the code down first. A reset clears the unit's clue trail, and the same code often comes back later in the week. If it returns after one reset, stop resetting and book the repair.
Why does my tankless unit go cold after a few minutes?
That usually means the unit is reaching its limit or losing its flame signal. Scale in the heat exchanger, a restricted inlet filter or a gas supply that cannot hold pressure under load all cause it. The provider tests flow and gas pressure at the same time.
Is descaling part of a repair visit?
Sometimes. If scale is the confirmed fault, descaling is the repair. If the fault is a sensor or a gas valve, descaling is a separate maintenance item. Ask which one appears on the invoice.
How long do tankless units last?
It depends on water quality, maintenance and how hard the unit works. Hard water and skipped descaling shorten the life of the heat exchanger. A unit that has been serviced usually lasts longer than one that has only been reset.
Can a tankless unit be repaired if the heat exchanger leaks?
A leaking heat exchanger is usually a replacement rather than a repair. The part often costs more than a new appliance, and the rest of the unit has seen the same water. The provider should show you the leak before you decide.
Do you repair commercial tankless systems?
Calls for multi-unit and commercial tankless systems are routed to providers who handle that work. Describe how many units are on site and whether they are linked when you call. That decides who is sent.