Navien Water Heater Won'T Go Above 120 - Your Step-By-Step

There’s nothing quite like the jolt of a lukewarm shower when you’re expecting a steamy, relaxing wash. You head to your state-of-the-art Navien tankless water heater, check the display, and see it’s stuck. It just won’t budge past that 120°F mark. It’s a frustrating moment, and you’re right to wonder what’s going on.

I get calls about this all the time, and I want to promise you something right now: in most cases, this isn’t a sign of a broken unit. In fact, it’s often a feature, not a bug. But when it is a problem, you need to know what to look for.

In this complete guide, we’ll walk through exactly why your Navien water heater won’t go above 120 degrees. We’ll explore the built-in safety settings, cover the simple DIY checks you can perform safely, and identify the clear signs that it’s time to call in a certified pro. Let’s get that hot water flowing reliably again.

Why 120°F? Understanding Navien’s Built-In Safety Default

Before we dive into troubleshooting, let’s address the most common reason for this temperature limit. Navien, like most manufacturers, ships its tankless water heaters with a factory-default maximum temperature of 120°F. This isn’t an arbitrary number—it’s a critical safety measure.

The U.S. Consumer Product Safety Commission (CPSC) recommends setting water heaters to a maximum of 120°F to prevent scalding injuries. At 140°F, it takes only five seconds of exposure to cause a third-degree burn. At 120°F, it takes over five minutes.

This setting is especially important in households with children, the elderly, or individuals with sensitive skin. So, one of the key benefits of a Navien water heater that won’t go above 120 is that it’s actively protecting your family from accidental burns right out of the box.

How to Check and Adjust Your Navien’s Temperature Settings

Okay, so you understand the safety aspect, but you have a specific need for hotter water, like for a sanitizing dishwasher or a personal preference. Let’s look at how you can adjust the settings on the unit itself. This is the first and most important step in our navien water heater won’t go above 120 guide.

Using the Front Control Panel

For most day-to-day adjustments, the front panel is all you need. The process is straightforward:

Wake up the display by touching it or pressing any button.
Use the Up (▲) and Down (▼) arrow buttons to select your desired temperature.
Press the Set (or Enter) button to confirm your choice. The new temperature should flash a few times and then become solid.

If you try to go above 120°F and the unit won’t respond, it means the internal high-limit setting is engaged. This requires a more involved adjustment.

Adjusting the Internal Dip Switches (For Advanced Users & Pros)

Safety First: This next step involves opening the front panel of your water heater. If you are not 100% comfortable and confident in what you are doing, please stop here and call a certified Navien technician. Incorrectly adjusting internal components can create a safety hazard or void your warranty.

On many Navien models (like the popular NPE-A and NPE-S series), the absolute maximum temperature is controlled by a set of small switches on the main circuit board called dip switches.

To access them, you must first turn off the power to the water heater at your circuit breaker. Then, remove the front cover (usually held by a few screws at the bottom). On the circuit board, you will find a small bank of switches. Switch #1 and #2 typically control the maximum temperature output. The configuration guide is usually printed on a sticker inside the unit’s cover.

By changing the position of these switches, you can unlock a higher maximum temperature (often up to 140°F). After making the adjustment, replace the cover securely and restore power.

Common Problems When Your Navien Water Heater Won’t Go Above 120 (And It’s Not the Setting)

So you’ve confirmed the settings should allow for hotter water, but you’re still stuck with a lukewarm supply. Now it’s time to investigate other potential culprits. These are some of the most common problems with a navien water heater that won’t go above 120, even when it’s trying.

Hard Water and Mineral Buildup

This is, without a doubt, the number one enemy of any tankless water heater. Hard water contains dissolved minerals like calcium and magnesium. As water is heated, these minerals precipitate out and form a hard, crusty layer of scale inside the heat exchanger.

This scale acts as an insulator, preventing the heat from the burners from effectively transferring to your water. Your unit will work harder, use more fuel, and still fail to reach the target temperature. If you live in an area with hard water and haven’t flushed your unit in over a year, scale is a likely suspect.

Clogged Inlet Water Filter

Your Navien has a small filter screen on the cold water inlet to catch sediment and debris before it enters the unit. Over time, this filter can become clogged, restricting the flow of water into the heater.

A low flow rate can prevent the unit’s flow sensor from activating the burner at full power, resulting in lower output temperatures. Cleaning this filter is a simple but crucial piece of maintenance. You simply turn off the water supply to the unit, unscrew the inlet connection, carefully remove the screen, rinse it thoroughly, and reinstall it.

Insufficient Gas Pressure

A tankless water heater needs a high volume of natural gas or propane to heat water on demand. If the gas line is undersized, the pressure is set too low, or there’s an issue with the gas meter or regulator, the unit will be starved for fuel.

When the heater can’t get enough gas, it can’t achieve the high BTU output required to raise the water temperature to 130°F or 140°F, especially during cold winter months when the incoming water is much colder. Checking and adjusting gas pressure is a job exclusively for a licensed gas technician.

The “Cold Water Sandwich” Effect

Sometimes the issue isn’t a steady low temperature, but a blast of cold water in the middle of a hot shower. This is the “cold water sandwich.” It happens when you turn off the hot water for a moment (like to lather up) and then turn it back on. The heater shuts down, leaving a pocket of cold water in the pipes. When you turn the tap back on, you get that cold slug before the hot water arrives again.

While not strictly a temperature limit problem, it feels like one. Navien’s newer models with ComfortFlow™ recirculation technology help minimize this, but it can still occur in systems without it.

Your Essential Navien Water Heater Care Guide for Peak Performance

The best way to ensure your heater performs flawlessly for years is with consistent maintenance. Following these navien water heater won’t go above 120 best practices will not only keep your water hot but also improve efficiency, making it a more sustainable and eco-friendly navien water heater choice.

Annual Descaling: This is the most important task. Once a year (or every 6-9 months in very hard water areas), you need to flush the unit with a descaling solution (like plain white vinegar). This dissolves the mineral buildup and keeps the heat exchanger clean and efficient.
Clean the Inlet Filter: Check and clean the cold water inlet filter screen every six months to ensure strong water flow.
Inspect the Condensate Trap: Your high-efficiency Navien produces condensate. Ensure the drain line and trap are clear of debris to prevent water from backing up and causing a shutdown.
Check for Error Codes: Get familiar with your unit’s display. If it shows an error code, consult your manual or call a technician. Don’t ignore these warnings!

When to Call a Certified Navien Technician

As a homeowner, there are plenty of checks you can do yourself. But it’s just as important to know when to step back and call a professional. Your safety is always the top priority.

Call a pro if:

You smell gas near the unit. Leave the area immediately and call your gas utility’s emergency line from a safe distance.
The unit is displaying an error code you can’t resolve.
You have performed all the basic checks (settings, inlet filter) and the problem persists.
You suspect low gas pressure or need any work done on the gas line.
You are not comfortable opening the unit’s cover to access internal components like dip switches.

Frequently Asked Questions About Navien Temperature Issues

Is 120°F hot enough for a shower?

Absolutely. Most people find a comfortable shower temperature to be between 100°F and 110°F. A setting of 120°F provides a safe buffer and is plenty hot for virtually all household needs, including washing dishes and laundry.

Can I override the 120 degree limit on my Navien water heater?

Yes, on most models, the absolute maximum temperature can be raised to 140°F by adjusting the internal dip switches on the main circuit board. However, due to the scalding risk, this adjustment should ideally be performed by a qualified technician who can confirm the system is operating safely at that higher temperature.

Why does my hot water temperature fluctuate?

Fluctuations can be caused by several factors. The “cold water sandwich” is a common cause. It can also be due to a partially clogged filter causing flow rate variations, or another appliance (like a dishwasher) suddenly drawing hot water and affecting the pressure and flow to your shower.

Your Path to Reliable Hot Water

Dealing with a Navien water heater that won’t go above 120°F can be a puzzle, but it’s one you can solve. Start by remembering that the 120°F limit is a powerful safety feature. If you need more heat, carefully check the settings and internal switches, always prioritizing safety.

If the settings are correct, shift your focus to maintenance. A clean and well-maintained unit is a happy and effective unit. Regular descaling and filter cleaning are the keys to long-term performance and efficiency.

With this guide, you now have the knowledge to diagnose the issue like a pro. You know what to check, what to clean, and when it’s time to make the call for professional help. Stay safe, and enjoy that perfectly hot shower!

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David Clark

Founder & Lead HVAC Specialist at HVACRLY

David Clark is the founder of HVACrly and a certified HVAC specialist with 10+ years working on residential and light-commercial systems. He turns complex heating, cooling, ventilation, and refrigeration topics into clear, actionable guides focused on safety and energy efficiency. When he’s not on a service call, he’s testing tools and writing step-by-step tutorials.