The 9 Most Common Tankless Water Heater Problems (And What You Can Do About Them)

Tankless water heaters are praised for their efficiency, space-saving design, and ability to provide endless hot water. But like any advanced piece of equipment, they can run into issues. When they do, homeowners often feel stuck between a cold shower and an expensive service call.

The good news? Many tankless water heater problems are simple, common, and completely fixable, especially when you know what to look for.

Here are the 9 most frequent issues we see with tankless units, why they happen, and what can be done to prevent or resolve them.

1. Error Codes on the Display

Tankless water heaters are smart. They use onboard computers to monitor performance and will alert you with error codes when something isn’t right.

Common error codes include:

• Error 11 or 12 – Ignition or flame failure (often related to gas supply or airflow)

• Error 90 – Poor combustion due to scale buildup, venting issues, or gas flow problems

• Error 29 – Condensate or neutralizer-related issue (common with condensing units)

What to do:

Start with the basics. Make sure the unit has gas, electricity, and water. Clear any blockages in the air intake and exhaust vents. If the issue persists, a licensed plumber can run diagnostics and safely resolve the problem.

2. Blocked Air Intake or Exhaust Vent

Tankless units need proper airflow to burn gas safely and efficiently. If the intake or exhaust is blocked, you might experience shutdowns, inconsistent hot water, or even safety risks.

Causes:

• Bird nests

• Snow or ice buildup

• Leaves and debris

• Poor vent installation

Birds like to build nests inside vent pipes, posing risks to both their safety and hot showers.

Fix:

Visually inspect the outside intake and exhaust pipes. If anything is blocking the flow, remove it. Be sure the vents have the proper clearance and slope. This issue is common in Utah homes, especially during fall and winter.

3. Scale Buildup from Hard Water

Hard water is rough on everything it touches. It can dry out your skin, dull your clothes, and leave spots on dishes. Over time, it can also wear down your fixtures and appliances, causing them to fail sooner than expected. If you’re seeing white, chalky buildup on your faucets, imagine what that same scale is doing inside your water heater.

Severe hard water buildup on a faucet, illustrating the impact of mineral deposits over time.

While hard water is a known issue for traditional tank-style heaters, it’s even more destructive to tankless systems. The mineral buildup collects inside the narrow passages of the heat exchanger, reducing efficiency, triggering error codes, and shortening the life of the unit. Without proper maintenance, hard water can destroy a tankless heater faster and more aggressively. Even if the unit hasn’t stopped working yet, buildup can still cause frequent error codes and should be addressed immediately.

Signs of scale buildup:

• Drop in water temperature

• Error code 90 or similar

• Rumbling or strange sounds from the unit

Fix:

Flush your tankless system with a descaling solution (usually white vinegar or a specific descaler) every 6 to 12 months. Some units can track usage hours or alert you when service is due.

4. The Cold Water Sandwich Effect

This isn’t exactly a “problem,” but it’s one of the most common complaints we hear from homeowners with a tankless water heater.

What Is It?

You turn on the hot water. It starts out warm, then suddenly goes cold for a few seconds before turning hot again. It’s like a hot water “sandwich,” with cold water in the middle.

Why It Happens:

Tankless water heaters are designed to heat water only when needed. When someone uses hot water and then turns it off, the small amount of hot water remaining in the pipes stays there for a short time. If someone else turns on the hot water soon after, that leftover warm water is the first to come through, followed by a short burst of cold water from the pipes while the heater starts back up. Once the burner kicks in and catches up, the hot water returns.

This quick temperature fluctuation doesn’t mean there’s a malfunction. It’s just a side effect of how tankless systems work. It’s especially noticeable during short uses or when multiple people are using hot water within a short period.

Solution:

While the cold water sandwich is harmless, it can definitely be annoying, especially in multi-person households or when taking quick showers. The good news is there are ways to minimize it.

Installing a recirculating pump or a small buffer tank can help maintain a more consistent water temperature. These systems either keep hot water circulating through the pipes or hold a small amount of preheated water ready to go, reducing or even eliminating the cold burst. Some newer tankless models even have built-in recirculation features for this reason.

If this issue is a regular frustration in your home, ask your plumber about adding one of these upgrades to your system.

5. Electrical or Power Supply Issues

If your tankless unit won’t turn on or keeps cutting out, it may not be receiving consistent power.

Causes:

• Tripped circuit breaker or GFCI outlet

• Loose plug or wire

• Power surge (can damage internal components)

Fix:

Check your electrical panel and reset breakers if needed. Make sure the unit is plugged in securely. If it continues to trip, call a pro. It may be a deeper electrical issue.

6. Low Gas Pressure or Supply

Tankless water heaters require a consistent and sufficient gas supply, especially during peak demand. Undersized gas lines are a common installation mistake, and one we see all the time. Unfortunately, some homeowners and even plumbers get so excited about installing a tankless system that they overlook whether the existing gas line can actually handle the increased demand.

Without proper sizing, the unit may fail to operate correctly, struggle to produce hot water, or throw error codes. It’s crucial to evaluate the full system and ensure the gas supply matches the specifications of the tankless unit being installed.

Symptoms:

• Flame failure errors

• Unit shutting down under high demand

• Inconsistent hot water temperature

One of our trained technicians uses a manometer to ensure proper gas supply and volume in a water heater.

Fix:

Have a licensed plumber verify the gas line sizing and check pressure at the unit using a manometer. The gas supply should match the manufacturer’s specs for BTU input and distance.

7. Water Flow Too Low to Activate Heating

Tankless water heaters require a minimum flow rate, usually around 0.5 gallons per minute (GPM), to activate the burner and begin heating water. If the flow doesn’t meet this threshold, the unit may not turn on at all, or it might rapidly cycle on and off, causing frustrating hot-cold fluctuations.

This issue is especially noticeable when using fixtures that naturally have a lower flow rate, like modern faucets, low-flow showerheads, or when you’re only slightly opening a tap (like while brushing your teeth). If your tankless heater seems unresponsive during these times, low water flow could be the culprit.

Common Causes:

• Clogged faucet aerators – Mineral buildup from hard water can restrict water flow significantly.

• Dirty inlet filter – The built-in filter on your unit may be blocked, reducing water flow into the heater.

• Low home water pressure – System-wide pressure problems can impact the unit’s ability to operate.

• Partially closed shut-off valves – Valves upstream from the unit may not be fully open.

How to Fix It:

• Clean faucet aerators by unscrewing them and soaking in vinegar to remove scale.

• Check and flush the unit’s inlet filter, typically located where the cold water enters the heater.

• Inspect shut-off valves to ensure they’re fully open and not partially restricting flow.

• If you suspect your home has low water pressure overall, you may need a plumber to assess the issue. A pressure-boosting system might be required in some homes.

Even though this seems like a minor problem, it can make a big difference in your experience with a tankless system. Staying ahead of these issues with routine maintenance can keep things running smoothly.

8. Frozen Pipes or Unit in Winter

Most modern units include built-in freeze protection, but it only works if the unit has power. If a power outage hits during a freeze, pipes inside or around the unit can burst.

Fix:

Make sure the unit stays plugged in year-round. Install in climate-controlled areas when possible. Use pipe insulation and heat tape for exposed water lines.

9. Improper Setup or Settings

Surprisingly, many tankless water heater issues can be traced back to incorrect setup during installation. Whether it’s a dip switch that was never adjusted, a temperature setting left at default, or a venting mode set wrong, small oversights can cause big headaches later on.

These problems can lead to lukewarm water, inconsistent temperatures, or even repeated error codes. Homeowners might think the unit is faulty when in reality, it’s just not properly configured.

Common Setup Issues:

• Temperature setting too low – Many units default to 100°F or lower, which isn’t hot enough for most uses.

• Dip switches not adjusted – These internal switches control things like gas type, altitude, or recirculation mode.

• Incorrect venting or recirculation settings – Some installers may skip these steps, especially if they’re unfamiliar with the unit.

• Factory settings not customized for the home’s specific needs

How to Fix It:

• Check the temperature setting on the control panel and adjust it to 120–125°F for typical residential use.

• Review the installer’s manual and verify that dip switches and system configurations match your setup.

• Perform a factory reset if you suspect the settings are off and want a fresh start.

• If you’re unsure, have a qualified technician inspect the settings to confirm everything is dialed in properly.

Proper configuration is key to long-term performance. Even the best tankless system won’t work right if it’s not set up to meet your home’s specific needs.

Prevent Problems with Regular Maintenance

Many of these issues can be avoided with regular maintenance:

• Annual descaling flush

• A properly installed, consistently used, and well-maintained water softening system

• Cleaning filters

• Checking venting and gas pressure

• Keeping software or firmware updated (for smart units)

At Colossus Plumbing, we install, service, and maintain all major brands of tankless water heaters. We also help homeowners understand how to keep their systems running efficiently for years to come.

Don’t wait until it gets worse. Book your service now and get peace of mind.

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