Last updated: July 2026 · Reviewed against Rinnai, Navien and Noritz service documentation
Most tankless water heater problems trace back to three things: scale buildup, a clogged inlet filter, or a venting/ignition fault — and the unit will usually tell you which with an error code on its display. Unlike a tank heater, a tankless unit is a diagnostic-friendly appliance: read the code, and you are halfway to the fix. Here is what the common faults mean and which you can handle yourself.
Start With the Error Code
Tankless heaters display a numeric code when something is wrong. Codes vary by brand, but the families are consistent across Rinnai, Navien, Noritz and others: ignition failure, flame loss, exhaust/venting blockage, overheating, flow sensor issues, and scale-related faults. Look up your specific code in the manual (or on the inside of the front panel) before assuming anything — it saves guessing and often a service call.
Common Problems and What They Mean
| Symptom | Likely cause | DIY? |
|---|---|---|
| No hot water, ignition error | Gas supply, igniter, or flame sensor | Check gas first; then pro |
| Water not hot enough | Scale buildup, undersized unit, low flow | Descale first |
| Hot then cold (“cold water sandwich”) | Normal cycling, or flow sensor | Often normal |
| Error on high demand | Unit undersized for simultaneous use | Sizing issue |
| Reduced flow | Clogged inlet filter or scale | Clean filter (DIY) |
| Exhaust/venting code | Blocked intake or exhaust vent | Clear debris (DIY) |
Fix 1: Descale It (Solves More Than You Think)
If the water is lukewarm, output has dropped, or scale-related codes appear, descaling is the first move — and it resolves a surprising share of “broken” tankless units. If you have never flushed it, that is almost certainly your problem. Full instructions: tankless water heater maintenance.
Fix 2: Clean the Inlet Filter
A clogged cold-water inlet screen restricts flow, which can prevent the unit from firing at all (it needs minimum flow to ignite) or cause flow-related error codes. Shut off the water, unscrew the filter, rinse, reinstall. Five minutes, zero cost.
Fix 3: Check Venting and Gas
Blocked intake or exhaust vents (debris, nests, snow) trigger safety shutdowns and are a common seasonal cause. Also confirm the gas supply valve is open and other gas appliances work — a tankless unit demands high gas flow, and an undersized gas line is a classic installation-related fault that shows up as ignition errors under heavy demand.
When to Call a Pro
Call a technician for ignition failures that persist after checking gas, any code pointing to the gas valve, heat exchanger, or control board, combustion problems (yellow flame, soot), or if descaling and filter cleaning do not restore performance. Tankless service visits run $150–$300; major component repairs can be significant, so weigh them against the unit’s age. Cost comparison: how much is a tankless water heater.
FAQ
Why is my tankless water heater not producing hot water?
The most common causes are scale buildup in the heat exchanger, a clogged inlet filter restricting flow below the ignition threshold, a gas supply problem, or an ignition fault. Read the error code first, then descale and clean the filter before calling for service.
What is the most common tankless water heater problem?
Scale buildup from hard water, by a wide margin. It reduces efficiency and hot water output, triggers error codes, and eventually damages the heat exchanger. Annual descaling prevents the majority of tankless service calls.
Why does my tankless water heater go hot then cold?
Brief temperature swings (the “cold water sandwich”) are often normal as the unit cycles between draws. Persistent cold spells point to scale, a flow sensor issue, or a unit undersized for your simultaneous hot water demand.
Sources: U.S. Department of Energy — Tankless Water Heaters · manufacturer service documentation (Rinnai, Navien, Noritz).