Tankless Water Heater Troubleshooting: What to Check Before Calling a Plumber

No hot water. No error code. No idea what went wrong. If your tankless water heater suddenly stops working or starts acting up, you might assume it’s time to call a plumber. And in some cases, you should. But there are a few things you can check first that might save you the service call.

Tankless systems are efficient, compact, and reliable but they’re also more sensitive to small issues that can throw off performance. Before you reach for the phone, here’s a simple checklist to run through. These are the most common problems homeowners can safely troubleshoot on their own.

1. Is the Unit Getting Power?

It may sound obvious, but the first step is to make sure the unit has power or fuel.

If electric:

• Check the breaker panel. A tripped breaker is more common than you’d think.
• Make sure the unit’s power switch is turned on (often near the bottom of the unit).

If gas:

• Confirm that the gas valve is open.
• Make sure your home has gas service. If other gas appliances aren’t working, the issue may not be the water heater.

2. Are You Overloading the System?

Unlike tank systems, a tankless water heater heats water on demand. If multiple fixtures are running hot water at once like a shower, dishwasher, and laundry, you may exceed the unit’s capacity.

What to do:

• Turn off all hot water taps.
• Wait 30 seconds, then run a single tap to see if hot water returns.
  If it works with one tap, the system is overloaded. You may need to stagger usage or consider a higher-capacity unit.

3. Have You Checked the Water Filter and Inlet Screen?

Tankless systems often include a small inlet screen or filter to catch sediment and debris. If your water supply has a lot of minerals or particles, that screen can clog and restrict water flow.

Steps:

• Turn off power and water to the unit.
• Locate the cold water inlet filter (check the manual).
• Remove and rinse the screen under clean water.
• Reinstall, turn everything back on, and test the system.

If the unit works after this, sediment was the issue—and you may need a water softener or more regular maintenance.

4. Are There Error Codes on the Display?

Most modern tankless heaters have a digital display that will show an error code if something’s wrong. Check your manual or look up the code online. Common ones include:

• Flame failure (ignition problem)
• No water flow (inlet valve shut or clogged filter)
• Overheating (scaling or sensor issue)

Do not ignore codes. They’re there for a reason and can help pinpoint what’s wrong faster when you do need professional help.

5. When Was the Last Descaling?

If your water has a high mineral content, scale can build up inside the heat exchanger. This reduces efficiency and can cause the system to shut off or overheat.

If it’s been more than a year since your unit was flushed or descaled, that buildup may be causing the issue.

Signs of scale:

• Hot water cuts out quickly
• Temperature fluctuates during use
• Error code related to overheating or flow rate

Flushing the system requires a pump, vinegar solution, and basic knowledge of your unit’s valves. If you’re not comfortable doing it, this is where calling a licensed plumber makes sense.

When to Stop and Call a Pro

Troubleshooting only goes so far. If you’ve run through the basics and your system still isn’t working properly, it’s time to bring in an expert. Continuing to test or reset the unit without solving the root issue can cause more damage.

At Rockwater Plumbing, we diagnose tankless issues quickly, handle cleaning and descaling, and replace failing components when needed. We’ll also let you know if it’s a one-time fix—or a sign your system is wearing out.

A Little Knowledge Goes a Long Way

Tankless systems are smart but they’re not self-healing. When something stops working, it usually starts with a warning sign you can spot. Knowing what to check first helps you avoid unnecessary repairs, save time, and explain the issue more clearly if you do need plumbing service.

And if you’re stuck, or just want peace of mind, give Rockwater Plumbing a call. We’ll get your hot water running again the right way, without the guesswork.