Top 10 Most Common Tankless Water Heater Repair Questions Answered

1. Why is my tankless water heater not getting hot water?

When a tankless water heater isn’t producing hot water, the issue is often simple. First, confirm power or gas: make sure the breaker isn’t tripped, the unit’s power switch is on, and the gas valve is fully open. Next, check the water flow—most tankless units need a minimum flow (often 0.5–0.75 GPM) to activate. Try opening a hot faucet more fully or using just one fixture at a time.

Clogged inlet filters, frozen pipes, or a closed shutoff valve can also block flow. If you see an error code, consult the manual. Call a professional if: the burner won’t ignite, you smell gas, breakers repeatedly trip, or you can’t clear the error code.

2. Why does my tankless water heater keep shutting off mid-shower?

If your tankless water heater shuts off during use, it’s usually due to flow or safety sensors. Many units have a minimum flow requirement; when you throttle the hot water too low (or mix in too much cold), the unit may think there’s no demand and shut down. 🛁

Other common causes include:

• Dirty inlet filter reducing flow
• Overheating from scale buildup inside the heat exchanger
• Ventilation issues triggering safety shutoffs (for gas units)

Try cleaning the inlet screen, fully opening the hot tap, and checking that all isolation valves are open. Call a pro if the unit shows repeated overheat, vent, or combustion error codes, or if it shuts down even at high flow.

3. Why is my tankless water heater water temperature inconsistent or fluctuating?

Temperature swings—often called the “cold water sandwich” effect—are a frequent complaint. A short burst of cold water may occur when the burner cycles between uses. Mild swings are normal, but constant fluctuations point to issues.

Check these first:

• Avoid mixing too much cold at the faucet; instead, set a lower temp on the heater (e.g., 120°F) and open hot fully.
• Clean inlet filters and showerheads to ensure steady flow.
• Make sure only a reasonable number of fixtures run at once, so you don’t exceed capacity.

If water goes from scalding hot to cold, you may have a thermostat, flow sensor, or scaling issue. Descaling the unit often helps. Call a professional if descaling and filter cleaning don’t stabilize temperature or if error codes appear.

4. My tankless water heater is displaying an error code—what does it mean?

Error codes vary by brand, but they usually point to specific systems: ignition, venting, temperature, or flow. When a code appears:

1. Note the exact code (e.g., E1, 11, 29).
2. Check your owner’s manual or manufacturer website for its meaning.
3. Perform basic checks: power, gas valve, water valves, filters, and vents.

Common categories:

You can safely clear minor issues like dirty filters or low gas from an empty propane tank. Call a licensed technician if the code indicates venting, combustion, sensor failure, or if errors return after resetting the unit.

5. Why is my tankless water heater making loud noises?

Unusual noises can be an early warning sign:

• Rumbling or kettling: Often caused by limescale buildup on the heat exchanger. Water boils unevenly, creating rumbling sounds.
• Whistling: May mean restricted water flow through partially closed valves or clogged filters.
• Clicking: Normal during ignition, but constant or very loud clicking can signal ignition problems.
• Booming on startup: Delayed ignition—potentially dangerous with gas. 🚫

Try flushing/descaling the unit if it hasn’t been maintained according to the manual, and clean inlet filters. Make sure all valves are fully open. Contact a professional immediately if you hear booming, smell gas, or noises appear suddenly after an installation or repair.

6. Why am I getting error codes or problems after installing a new tankless unit?

New installations often reveal sizing or setup problems, not just defects. If your new system frequently errors or underperforms:

• Confirm it’s properly sized for your home’s flow rate and incoming water temperature.
• Ensure gas line size meets manufacturer specs; undersized lines cause ignition and performance problems.
• Check that venting is to code—using the right materials, length, and termination.

Also verify dip switches or software settings (elevation, gas type, recirculation options) match your situation. Many issues stem from incorrect configuration. Homeowners can verify visible items (valves open, vents unobstructed), but installation problems should be corrected by a qualified plumber or HVAC tech, especially for gas or electrical work.

7. What should I do if my tankless water heater is leaking?

Any leak around a tankless water heater needs attention quickly to prevent water damage and corrosion. First, identify the source:

• At pipe joints or unions: Often a loose fitting or failed seal.
• At the pressure relief valve: Could be normal discharge from high pressure or a failing valve.
• From inside the unit: May indicate a cracked heat exchanger or internal component failure.

Steps:

1. Turn off power and water to the heater if the leak is significant.
2. Place a bucket or towel to prevent damage.
3. Tighten accessible fittings gently; don’t overtighten.
4. Check your home’s water pressure (ideal is about 40–60 psi).

If water seems to come from inside the case or the heat exchanger area, call a pro immediately and do not operate the unit.

8. Why does my tankless water heater smell or produce discolored water?

Tankless heaters are less prone to smelly water than tank models, but it can still happen. Rotten egg or sulfur smells usually come from the water supply itself (often well water), not the heater. In these cases, a whole-house filtration or treatment system is needed.

Discolored water:

• Rusty or brown: Often from old galvanized pipes or, less frequently, corrosion inside components.
• Cloudy: Entrained air or sediment.

Steps to try:

1. Run water for several minutes to see if it clears.
2. Check if the problem is only hot water or both hot and cold.
3. Inspect and clean inlet screens; consider a sediment filter.

If smell or discoloration persists only on hot water, have a professional inspect the heater and your hot-water piping for corrosion or bacterial growth.

9. How often should I flush or descale my tankless water heater, and can I do it myself?

Most manufacturers recommend flushing/descaling once a year, or more often (every 6 months) in areas with hard water and no softener. This process removes mineral scale from the heat exchanger, preventing overheating, noise, and efficiency loss.

Basic DIY steps (check your manual for specifics):

1. Turn off power, gas, and water to the heater.
2. Close isolation valves; connect hoses to the service ports.
3. Use a small pump and bucket with a descaling solution (e.g., white vinegar or manufacturer-approved cleaner).
4. Circulate for ~45–60 minutes, then flush with clean water.

DIY is reasonable if you’re comfortable with valves and pumps. Call a pro if you’re unsure, lack isolation valves, or if the unit is already showing error codes or heavy scale symptoms.

10. When is it better to repair my tankless water heater vs. replace it?

Tankless water heaters can last 15–20 years with proper maintenance, but there are times when replacement makes more sense:

Consider repairing when:

• The unit is under 10–12 years old.
• The problem is a single component (sensor, valve, igniter).
• Repairs are significantly cheaper than a new unit and come with a warranty.

Consider replacing when:

• The heater is near or beyond its expected lifespan.
• The heat exchanger is leaking or badly scaled.
• You’ve had repeated breakdowns or expensive parts failures.
• You need more capacity or better efficiency.

A licensed technician can provide a cost comparison; if repairs exceed ~50% of replacement cost, replacement is often the smarter long-term choice.

Helpful Tips for Keeping Your Tankless Water Heater Running Smoothly

• Flush and descale regularly. Follow your manufacturer’s schedule based on water hardness (often annually). This prevents overheating, noise, and error codes.
• Install a water softener or filter in hard-water areas. Reducing scale and sediment dramatically extends heater life and improves performance.
• Keep air intake and venting clear. For gas units, periodically inspect exterior vents for debris, snow, or nests, and maintain proper clearances.
• Check filters and screens. Clean the cold-water inlet filter and, if present, recirculation or line strainers every few months.
• Set a reasonable temperature. 120°F is usually plenty for comfort and safety, and reduces stress on components.
• Schedule professional maintenance every 1–2 years. A pro can inspect burners, sensors, and combustion settings to catch issues before they become costly repairs.