Tankless water heaters are reliable, but like any appliance, they can sometimes stop working or show error codes. Problems such as no hot water, low pressure, ignition failure, or strange noises are common and usually have simple causes. Some performance problems are caused by scale buildup or poor maintenance, which can reduce efficiency over time. See the tankless water heater efficiency guide to understand how performance changes as the unit ages.
This tankless water heater troubleshooting guide explains the most common problems, error codes, and fixes. Use the chart and sections below to quickly diagnose the issue and find the correct solution.
Quick Tankless Water Heater Troubleshooting Chart
Quick Diagnosis Chart
| Problem | Likely Cause | Quick Fix |
|---|---|---|
| No hot water | Gas supply, power issue, ignition failure | Check breaker, gas valve, reset unit |
| Water goes hot then cold | Flow rate issue, scale buildup | Increase demand, descale unit |
| Error code 11 | Ignition failure | Check gas supply |
| Loud rumbling | Mineral buildup | Flush and descale |
| Low water pressure | Clogged filter | Clean inlet filter |
Tankless Water Heater Troubleshooting Flowchart
Tankless Heater Problem
↓
No Hot Water?
↓
Check Power
↓
Check Gas Supply
↓
Check Water Flow
↓
Check Error Code
↓
Flush Heater if Needed
↓
Call Technician
Tankless Water Heater Not Heating Water
Why It Happens
If your tankless water heater is not producing hot water, the most common causes include:
- Tripped breaker or power interruption
- Closed gas valve (gas models)
- Ignition failure
- Blocked venting
- Unit not detecting sufficient water flow
Quick Fixes to Try
- Check your circuit breaker.
- Confirm the gas valve is open.
- Turn the unit off and reset it.
- Run multiple hot water fixtures at once to increase flow rate.
- Clean the inlet water filter.
If your unit displays an error code, consult the manufacturer’s manual.
When to Call a Professional
If resetting the system does not solve the issue, you may have:
- A failed ignition component
- Gas pressure issues
- Internal sensor failure
In those cases, contact a licensed plumber or HVAC technician.
Tankless Water Heater Goes Hot Then Cold (The “Sandwich Effect”)
This is one of the most searched tankless heater problems.
Common Causes
- Low water flow rate
- Mineral buildup inside heat exchanger
- Mixing valve issues
- Undersized unit for household demand
What You Can Do
- Increase water flow by turning on another hot fixture.
- Check for clogged aerators.
- Descale the unit if it hasn’t been flushed in 12 months.
Many homeowners use a tankless flush kit and descaling solution to remove mineral buildup during annual maintenance.
If temperature fluctuation continues, the unit may be undersized for your home’s peak demand. Use the tankless water heater size chart to make sure the heater is correctly sized for your household.
One of the most common issues homeowners experience is when a tankless water heater goes hot then cold, which can be caused by low flow rates or mineral buildup.
Tankless Water Heater Error Codes
Modern tankless water heaters display error codes when something is wrong. Click the code below to see the cause and how to fix it.
Rinnai Error Code 11
Ignition failure.
Check gas supply and ensure there is no air in the line.
Rheem Error Code 29
Condensate blockage (common in condensing units).
Inspect and clean condensate drain.
Navien Error Code E003
Ignition failure.
Verify gas pressure and electrical supply.
EcoSmart Error Code E5
Water flow issue or sensor problem.
Check inlet filter and flow rate.
If error codes persist after basic troubleshooting, professional servicing is recommended.
Tankless Water Heater Making Noise
Tankless units are typically quiet. If you hear rumbling, knocking, or high-pitched whining, scale buildup is usually the cause.
Why It Happens
Hard water minerals accumulate inside the heat exchanger. As water flows through, trapped sediment causes vibration and noise. Regular maintenance helps prevent these issues. See the tankless water heater maintenance guide for yearly service recommendations.
Flush and descale the system using a pump and cleaning solution designed for tankless heaters.
Regular maintenance reduces noise and improves efficiency. In many cases, mineral buildup inside the heat exchanger can cause performance issues. Learning how to flush a tankless water heater can often restore normal operation. Heavy scale buildup can also shorten tankless water heater lifespan, so regular descaling is important for long-term durability. See the tankless water heater lifespan guide to learn what affects durability.
Low Water Pressure from Tankless Heater
If hot water pressure is lower than cold:
Possible Causes
- Dirty inlet filter
- Mineral buildup
- Partially closed valve
- Plumbing restriction
What to Check
- Turn off water supply.
- Remove and clean inlet filter.
- Inspect isolation valves.
- Descale if needed.
If both hot and cold water pressure are low, the issue may be in your home’s plumbing system rather than the heater.
Common Tankless Water Heater Problems and Error Codes
Tankless water heaters are reliable, but homeowners occasionally experience performance issues. Below are some of the most common problems and their causes.
• Tankless water heater goes hot then cold – Often caused by low flow rates or mineral buildup.
• Tankless water heater temperature fluctuation – hot and cold water changes during use.
• Tankless water heater making noise – Often caused by scale buildup or water pressure problems.
• Tankless water heater low water pressure – Can result from clogged filters or restricted plumbing.
• Tankless water heater not turning on – causes, error codes, and startup problems.
• Tankless water heater not igniting – Often caused by gas supply problems, blocked vents, or ignition component failures.
• Tankless water heater keeps shutting off – causes, overheating, low flow, and safety shutdown problems.
• Tankless water heater reset – how to restart the unit after shutdown or error code.
• Tankless water heater no hot water – Can be caused by power loss, gas supply problems, clogged filters, or system shutdown.
• Tankless Water Heater Error Code 03 – flow, sensor, or abnormal operation problem.
• Tankless Water Heater Error Code 10 – air supply or venting problems.
• Tankless water heater error code 11 – Usually indicates an ignition failure or gas supply issue.
• Tankless water heater error code 12 – Flame failure during operation, often caused by gas pressure, venting, or sensor problems.
• Tankless water heater error code 13 – Usually caused by flame detection, combustion, or gas supply problems.
• Tankless water heater error code 29 – Usually caused by venting, airflow, or combustion problems.
• Tankless Water Heater Error Code 61 – fan or airflow problem that can cause shutdown.
• Tankless Water Heater Error Code 76 – communication or wiring problem.
• Tankless Water Heater Error Code 90 – exhaust or combustion temperature problem.
• Tankless Water Heater Error Code 99 – venting or exhaust problem.
Each of these problems has specific causes and solutions. Use the links above to find detailed troubleshooting steps for your specific tankless water heater problem.
Frequently Asked Questions
How do I reset a tankless water heater?
Most units can be reset by turning the power off at the unit or breaker for 30–60 seconds, then turning it back on. If an error code returns, additional troubleshooting may be required.
Why does my tankless heater run out of hot water?
Tankless heaters don’t “run out” of hot water, but they can be overwhelmed if multiple fixtures exceed the unit’s maximum flow rate (GPM capacity).
How often should I descale a tankless water heater?
Most manufacturers recommend flushing the system once per year. Homes with hard water may require descaling every 6–9 months.
Can low water pressure affect a tankless water heater?
Yes. Tankless units require a minimum flow rate to activate heating. If flow is too low, the burner may not engage.
According to the U.S. Department of Energy, proper repair and maintenance can extend the lifespan of tankless systems.
Conclusion
Most tankless water heater problems are caused by scale buildup, low flow rate, gas or power supply issues, or incorrect installation. Improper installation can also cause performance problems. See the tankless water heater installation cost guide to understand what proper setup requires. Using this troubleshooting guide can help you quickly find the cause and decide whether the fix is simple maintenance or professional repair.
Regular flushing, proper sizing, and yearly maintenance help prevent most common tankless water heater problems and extend system lifespan.