March 18, 2026
A tankless water heater not working is usually caused by mineral scale buildup on the heat exchanger, a tripped error code, an ignition failure, or an undersized unit that cannot keep up with demand. Most of these problems can be diagnosed in a few minutes by reading the error code on the display and running through a short checklist before calling for service. Regular maintenance prevents the majority of tankless water heater problems before they start.
Why Is My Tankless Water Heater Not Working?
Tankless water heaters are built to last 20 years or more, but they are not maintenance-free. Unlike a traditional tank unit that holds a reservoir of heated water, a tankless system fires on demand every single time you open a hot water tap. That means the components responsible for ignition, flow sensing, and heat exchange are working constantly. When something goes wrong, the unit often displays an error code and shuts itself down, which is actually a feature, not a flaw. The code tells you exactly where to start looking.
Here is a breakdown of the most common tankless water heater problems and what is causing them.
Problem 1: No Hot Water at All
What you notice: You turn on a hot water tap and get cold water, or nothing registers on the unit at all.
What is causing it: There are a few possibilities here. The flow rate may be too low to trigger the unit. Tankless heaters require a minimum flow before they will activate. A partially closed shutoff valve, a clogged inlet screen, or a failing flow sensor can all prevent the unit from firing. A tripped gas supply or an error code from a recent fault can also cause a complete shutdown.
What to do: Start by checking the error code on the display. If the unit shows no error but still produces no hot water, check the cold water inlet filter screen because it is usually located where the cold water line connects to the unit and can become clogged with sediment over time. Remove it, rinse it under running water, and reinstall. Next, confirm the gas supply valve is fully open and that other gas appliances in the home are working normally. If the unit still does not fire, check whether it needs to be reset. Most brands have a reset procedure listed in the owner's manual, or your plumber can walk you through it.
Problem 2: Tankless Water Heater Error Codes
What you notice: A blinking light or alphanumeric code appears on the display, and the unit stops producing hot water.
What is causing it: Error codes are the unit's way of flagging a specific fault before it becomes a larger problem. The most common codes across major brands point to ignition failures (no flame detected), exhaust blockages, overheating, or flow sensor malfunctions.
What to do: Look up the specific code in your owner's manual or on the manufacturer's website. For Rinnai tankless water heater troubleshooting, Code 11 means no ignition; Code 12 is a flame failure. For Navien tankless water heater error codes, E003 is an ignition failure, and E010 is an abnormal airflow. Rheem tankless water heater error code 11 also indicates ignition failure.
Before calling for service, check the obvious first. Confirm the gas is on. Check that the exhaust vent and air intake pipes are unobstructed; bird nests, debris, and ice buildup are common culprits in winter. Try resetting the unit by powering it off for 30 seconds and back on. If the code returns immediately or does not clear after a reset, it is time to call a licensed plumber.
Problem 3: Lukewarm Water or Inconsistent Temperature
What you notice: The water starts warm, goes cold, then comes back warm, sometimes called the "cold water sandwich." Or the unit runs but never reaches the set temperature.
What is causing it: The cold water sandwich is a well-known quirk of tankless systems. When you briefly turn off a hot tap and turn it back on, the hot water remaining in the pipes from the previous use exits first, followed by a burst of cold water sitting in the pipes, then hot water from the newly activated unit. It is not a malfunction — it is a design characteristic. If the issue is the unit never reaching the right temperature, the most likely cause is scale buildup on the heat exchanger or an undersized unit being pushed past its capacity.
What to do: If a cold water sandwich is your only complaint, a small buffer tank installed before the unit can largely eliminate it. If the water simply never gets hot enough, check the temperature setting on the unit; it should typically be set to 120°F for most households. If the setting is correct and temperatures are still low, scale buildup on the heat exchanger is the most likely cause, and a descaling flush is needed. Hard water is a significant issue, and without regular water treatment or an annual maintenance flush, scale accumulates quickly and reduces efficiency.
Problem 4: Tankless Water Heater Making Noise
What you notice: Popping, rumbling, hissing, or clicking sounds coming from the unit during operation.
What is causing it: Clicking during ignition is normal because it is the unit firing. Persistent clicking that does not resolve into a flame is an ignition failure (see error codes above). Rumbling or popping during operation typically means mineral scale has built up on the heat exchanger, and the water is boiling against the deposits. Hissing can indicate a small water leak at a connection point.
What to do: If you hear sustained rumbling or popping, schedule a descaling flush as soon as possible. Running a scaled-up heat exchanger causes it to work harder, raises operating costs, and shortens the unit's lifespan. If you hear hissing, check the connections at the inlet and outlet lines for moisture or visible dripping. A water heater leak that is ignored will worsen — do not wait on that one.
Problem 5: Pilot Light or Ignition Failures (Gas Units)
What you notice: The unit attempts to fire, but no hot water is produced, often paired with an error code.
What is causing it: Most modern tankless gas water heaters use electronic ignition rather than a standing pilot light, so there is no pilot flame to relight. An ignition failure usually means the gas supply is interrupted, the igniter itself has failed, or the venting is obstructed, and the unit's safety system is preventing ignition.
What to do: Confirm the gas supply to the home is normal by checking another appliance. Inspect the air intake and exhaust vents for any visible blockage. If both check out and the unit still will not ignite, the igniter or gas valve likely needs to be inspected by a professional. These are not DIY repairs; they involve the gas system and require a licensed plumber.
Tankless Water Heater Maintenance: What Keeps These Problems Away
The single most effective thing you can do to prevent tankless water heater problems is to schedule an annual maintenance flush. During a flush, a descaling solution is circulated through the heat exchanger to dissolve mineral deposits before they can restrict water flow or cause overheating. Annual flushing is what keeps the warranty valid and the unit running efficiently.
Beyond the annual flush, clean the cold water inlet filter screen every 6 to 12 months and visually inspect the venting twice a year, especially after winter. If your home has particularly hard water, a whole-house water softener or a dedicated water conditioner on the tankless line will significantly reduce scale accumulation between service visits.
For a full look at what a tankless water heater installation and service involves, our water heater page outlines what homeowners should expect. And if you are comparing a tankless unit to a traditional tank, our water heater service page covers both options.
When to Call a Licensed Plumber for Tankless Water Heater Repair
Some tankless water heater troubleshooting steps are well within reach for most homeowners, but there are situations where calling a professional is the right move and the safer one.
Call a licensed plumber if: the unit repeatedly throws the same error code after resetting; you suspect a gas supply issue; the heat exchanger needs descaling and you do not have the right equipment; you notice water dripping from the unit or connections; or the unit is more than 10 to 15 years old, and the repairs are starting to approach replacement cost.
If you are searching for tankless water heater repair near me, Tom Falk Plumbing & Heating has been servicing water heaters across the area since 1961. Schedule service here or call us at 717-872-2850.
Frequently Asked Questions About Tankless Water Heater Problems
Why is my tankless water heater producing no hot water? The most common causes are a flow rate too low to trigger the unit, a clogged inlet screen, a tripped error code, or an interrupted gas supply. Start by reading the error code on the display and checking the inlet filter screen before calling for service.
How often does a tankless water heater need to be serviced? Most manufacturers recommend an annual maintenance flush to remove mineral scale from the heat exchanger. In hard water areas, this is especially important. Skipping annual maintenance is the fastest way to shorten the unit's lifespan and run into costly repairs.
What does a tankless water heater error code mean? Error codes indicate a specific fault detected by the unit's internal sensors. Common codes across Rinnai, Navien, Rheem, and Noritz units point to ignition failures, exhaust blockages, overheating, or flow sensor malfunctions. Check the code in your owner's manual and start with the basics before escalating to a service call.
Can a tankless water heater be repaired, or does it need to be replaced? Most tankless water heater problems are repairable. Replacement typically makes more sense when the heat exchanger itself has cracked, the unit is more than 15 to 20 years old, or repair costs are climbing toward the price of a new unit.
How much does tankless water heater repair cost? Repair costs vary depending on the problem. Cleaning the inlet screen or a basic reset is minimal. Descaling a heat exchanger, replacing a flow sensor, or repairing an igniter will typically run a few hundred dollars. A new unit with installation generally ranges from $1,500 to $3,500, depending on the size and fuel type.
