Navien Tankless Water Heater Goes Cold [Reasons + Solutions]

The main reasons behind Navien tankless water heater goes cold are setting the thermostat too low, blocked air supply or exhaust, ignition failure, and a faulty flow sensor.

navien tankless water heater goes cold

Then, there’s also system overload, flame failure, cold water sandwich, the build-up of minerals, and a malfunctioning control board. More often than not, you can solve this problem by adding a small tank water heater or descaling the unit.

However, you shouldn’t ignore the other potential reasons behind this issue as they’re quite common as well. Keep reading this guide to learn all about them and how you can fix these issues.

We’ll also discuss how to fix the Navien water heater that won’t go above 120 degrees Fahrenheit and how to prevent a tankless water heater from freezing up. Let’s dive in!

Here are the most common reasons behind your Navien tankless heater going cold:

1. Setting Thermostat Too Low

It’s annoying when you call your internet company for support and they ask you to check whether the modem has power or not. It’s a similar case here. It may seem like something you’ve already checked but it’s worth mentioning.

navien tankless water heater thermostat setting too low

So, start by checking the temperature on the thermostat of your water heater. You can usually go up to 120 degrees on Navien heaters before the locking system gets triggered. If you see a low temperature, you’ve found the problem.


Dial up the thermostat to your desired temperature. Thermostats can also malfunction with time. So, if changing the temperature settings doesn’t work, then there’s the possibility that the thermostat has gone bad. In that case, you’ll have to get it replaced.

Note: Also read why Rheem tankless water heater won’t turn on and how to fix it.

2. Blocked Air Supply Or Exhaust

If you’re wondering why your Navien tankless water heater goes cold after a few minutes, it could be due to a blocked air supply or exhaust. When they’re blocked, it triggers a safety feature in the heater to protect the unit from any fire hazards and prevent accidents.

One of the vent pipes having a hole or being disconnected is a common reason behind it. If the water heater has been installed close to another appliance, it can also cause this issue. Dirty burners can also cause this blockage.

blocked vent pipe

Check your Navien water heater for error codes and see if it has any notification about a blocked vent somewhere. Make sure the exhaust pipe is open. Inspect vents that are leading in and out as it’s not uncommon for insects like wasps to invade that space and block the vents.


To fix this issue, you must do whatever it takes to clear the blockage from the vents. Make sure that address the reasons that could be behind it as mentioned above and diagnose the problem. Once you find that, take the appropriate step.

If you find any holes upon your inspection of the vent pipes, you should replace them. The other common thing that you could come across is birds, rodents, or wasps that are blocking your vents. Examine these vents carefully and remove them to get your heater working again.

3. Ignition Failure

When there’s a problem with the ignition, you can face Navien tankless water heater with no hot water problem. Ignition failure is usually caused by supply issues like having no gas or not changing the gas line size for the heater.

If someone turns off the gas valve by mistake, that’ll also cause ignition issues. Having a tripped circuit breaker may also be the reason behind this issue as the electrical power cord on the heater won’t get power because of it.

tripped circuit breaker

The final thing to check that can go wrong with your heater is that the ignition pack has gone bad. Look for the error code in the heater. You’ll usually an E003 error code in a Navien heater when it fails to ignite.


First things first, make sure that you don’t have an empty gas tank. The heater needs sufficient fuel to do its job. Make sure you paid your gas bills and also ensure that the gas valve is fully open. Then, check for any leaks in the gas lines and replace the line if you find the leaky one.

Now, check the fuses and electrical components in your home to make sure that anything hasn’t burned out. Reset the circuit breakers if they have been tripped. If the heater works now, then your problem is solved.

But if the circuit breakers get tripped again soon, that means you have an electrical problem and the heater is fine. Call an electrician to fix it. But if the breaker is good and the heater still doesn’t work, you may have a bad ignition pack. It’ll be costly but your only option will be to replace it.

4. Faulty Flow Sensor

Tankless water heaters need a minimum flow rate of water so that they can heat it. If the water flowing through the heater is less than that, it won’t do its job properly. When the flow sensor gets corroded or damaged, it can mess up the flow rate of your tankless water heater.

navien flow sensor

You may get temperature fluctuations because of it or also get only cold water. The operation of the heater can also be shut off in extreme cases.

Even if there aren’t any issues with the flow sensor, there could be other things behind your tankless water heater not getting the required minimum flow rate.

Leaks in the pipes and stuck check valves can also cause this issue. But more often than not, the flow sensor is the culprit here.


The first fix you can try is to take the two clamps off the sensor and clean it. Hopefully, it’ll at least be a temporary fix to the problem. If that doesn’t work, you can replace it. It’s a relatively cheap solution as these sensors are typically priced between $25-$30.

You can add a water inlet filter to keep the flow sensor from getting clogged in the future. However, if replacing the sensor doesn’t work, you need to look into other issues like check valves and the wrong showerhead. Find out if anything is wrong with them and replace them if needed.

5. System Overload

Another reason behind Navien tankless water heaters no hot water is using too many appliances at once. Your heater can only provide warm water to a certain amount of water based on its size.

navien tankless water heaters no hot water

If you’re pushing it beyond its limit and running many faucets simultaneously, you may not get any hot water. The heater can even shut down in a few cases when the level of overloading is too much for it to handle.


The first thing you can try to counter this problem is to reset your water heater. That will solve the problem in the short run. If this problem keeps reappearing, then you’re presented with a few options.

navien water heater reset button

You can add a second water heater or upgrade the size of the tankless one. But if that seems too expensive, you can make a schedule with your family members such that there’s minimal chance of overlap when using the water heater. Turn off the simultaneous use of taps to minimize the problem.

small tank water heater

Although a second water heater may seem like an expensive option in the short run, it can save you money in the long run. You’ll be using less gas to heat the water this way.

So, how do you know which is the right size of water heater for you? Here’s a quick guide to help you figure it out:

  • A 30-gallon water heater is okay for a single person or a family of two.
  • A 40-gallon water heater is okay for a family of three.
  • A 50-gallon water heater is okay for a family of four

6. Flame Failure

Supply issues can also cause flame failure in your heater just like ignition failure. So, check the gas tank, make sure it’s turned on, and that there are no supply issues before you move on any further with the diagnosis. Having a gas line that’s too small can also cause this issue.

 error codes E004

Inspect the power cord and electrical components connected to your heater. Next, check the vents and make sure that both the vents leading into the heater and from it is not blocked. The error codes E004 & E012 represent flame-related issues.


If you installed the heater yourself, then you may not know that the gas line needs to be extended around 1/2″ to 1″ in diameter for the heater to work properly. So, have that corrected in case you missed changing the gas lines?

changing the gas lines

After making sure that there are no gas supply issues, if you find that anything is wrong with the electrical components like blown fuses or tripped circuit breakers, you can easily replace the fuses and reset the breakers.

Call in an expert if doing these things doesn’t solve the problem and the error code still remains.

7. Cold Water Sandwich

Do you face a Navien intermittent hot water issue where you get hot water first, cold water after that, and then, hot water again? This is the tankless water heater cold water sandwich problem.

It’s a common issue with tankless heaters and it can get really annoying if it happens frequently. A common reason behind this issue is that the heat exchanger that’s supposed to heat the water takes some time to warm up. As it gets warm, you get cold water.

navien tankless water heater heat exchanger

Here’s how it plays out – The hot water you get first is the unused water from the last use. Then, you get cold water when the heat exchanger starts to warm up. And then, you finally have water again when it starts heating the water successfully.


One possible solution to a cold water sandwich problem is to do nothing at all. Just be patient and understand that your water heater needs some time to get going. However, if you notice the error code E016, it means you have a heat exchanger that’s overheating.

To tackle this problem, start by turning off the heater. Then, wait for half an hour and start it again. Clean the cold water inlet filter and flush the heat exchanger. Doing these things should solve the problem.

navien inlet water filter

But if you notice any cracks in the heat exchanger, you’ll have to replace it immediately as it can cause a gas leak and lead to a fire accident. That’s one situation where you must take action instead of living with it.

Even if there’s no major issue with your heat exchanger, you may still not have the patience to go through the cold water sandwich issues at times and want hot running water at all times. In that case, you can add a mini tank water heater to solve this issue.

When you have something like it, you can get a supply of warm water while the heat exchanger gets warm and starts heating up the cold water. This way, you should no longer run out of hot water at any given moment.

Generally, the longer the distance between the water heater and the faucet in which you want the water, the more you’ll be blasted with cold water. If you don’t want to add another small tank heater, you can also install a recirculating pump to have a steady supply of hot water at all times.

8. Build-Up Of Minerals

If you’re still wondering why is my Navien water heater not working properly, another reason behind it is the build-up of minerals. If you live in an area that has hard water, it’s a common issue that you’ll come across.

build-up of minerals

Hard water has more minerals present in them compared to soft water. The harder your water is, the more severe the mineral build-up issue will be. The obvious signs of this problem are to see discolored water and smelling a bad odor in it. 

The two main minerals that are the culprits behind this issue are Calcium and Magnesium. The scale build-up can also be in the narrow path that the water has to travel through when going through the heat exchanger.

When that gets clogged up, you can face the Navien tankless water heater hot and cold issue. That’s because less water will go through that passage and it’ll limit the supply of hot water.


As severe as this problem may sound, the truth is that it can be easily fixed. Descale your Navien tankless heater first to clean the build-up of minerals inside it. There are kits you can get to do it.

Hire a plumber to clean the lines and pipes as they’re most likely affected by the mineral build-up as well.

tools for descaling water heater

Check your water filter when flushing the tankless water heater as it can get clogged up as well. Cleaning it will solve the issue. If you can’t get it cleaned up and the dirt is stuck in a stubborn manner, you’ll have to replace it.

After flushing your system, it’s a good idea to use a water softener to prevent the build-up of minerals again. It’ll keep your water heater and pipes from getting corroded.

Taking good care of your water heater will prevent this problem from happening in the first place. So, how often to flush Navien tankless water heater? We’d recommend doing it once at least every six months.

9. Malfunctioning Control Board

Although it’s not the most common reason behind getting cold water, keep in mind that heaters don’t last forever.

With time, they can go bad, especially, the control board. If you’ve been using your heater for many years now, then this could be the reason in your case.

malfunctioning control board

If you see Error Code 03 which means ignition failure, it could be due to a bad control board. The unit turns on and off on its own multiple times is also a sign of a bad control board.


The temporary fix to this problem is to turn your heater off and on or reset it. If that works, you have more reasons to believe that your control board is bad. Eventually, you’ll have to replace it.

How To Fix Navien Water Heater Won’t Go Above 120

If you normally press the up and down arrow buttons or the + and – buttons depending on the model of your Navien water heater, you’ll see that you won’t normally be able to get past the 120 degrees Fahrenheit limit.

navien water heater won't go above 120

It’s a safety mechanism meant to protect people from accidental scalding. However, there is a simple way to bypass it. Before we show you how to do it, keep in mind that you may be putting yourself in a dangerous situation when you get past 120 degrees.

It can cause severe burns or even deaths if you’re not careful about changing the temperature setting. Here’s a quick list for you to show you the time in which a child can suffer a third-degree burn.

Temperature Of WaterTime
120 °F (49 °C)Around 10 Minutes
130 °F (55 °C)Around 10 Seconds
140 °F (60 °C)1 Second
160 °F (70 °C)Less Than 1 Second

So, now you know the risks of going past the safety temperature limit. There’s a reason why Navien put it in place. We don’t recommend pushing that limit, especially if you have a child in the house. Someone may forget to readjust the temperature and bring a whole world of trouble.

But if you have made up your mind that you want to turn up the temperature, the process of doing it is really easy. Instead of pressing the + or up arrow button like you normally would, press and hold it for two to five seconds. You’ll see the temperature rising at 5 degrees increments.

press and hold this button

Okay, so that should take care of this issue. Remember to lower the temperature settings when you’re done using the heater.

Another common problem that Navien users face is that sometimes it can get locked at 110 degrees Fahrenheit even though it’s meant to lock at 120 degrees. In case you’re facing that problem, here are the potential fixes you can try to solve:

  • Try resetting the water heater. As Navien heaters have a reset button, you can do it easily by pressing the button and the unit will reset.
  • If you have any circulation pumps, make sure you turn them off. Flip the pump off by pushing both dip switch 1 and 2 down. Readjust the temperature and restore the original position of the dip switches.
  • Press and hold the button to increase the temperature for 2 to 5 seconds.
  • If doing these things doesn’t work, you can call the Technical Support team of Navien to take a look at the problem and fix it.

How To Fix Navien Tankless Water Heater Freezing Up

Your Navien tankless water heater is prone to get frozen when the temperature starts to plunge and the weather gets cold. To prevent this nightmare, it’s important to winterize your heater as well as the pipes.

There are plenty of things you can do to ensure that your heater is protected during cold weather. Here are the most effective tips to keep your tankless heater from getting frozen:

i). Use Navien Tankless Freeze Protection Options

Just like any good quality tankless water heater, most Navien units also come with in-built freeze protection settings. Make sure you take full advantage of it. These heaters use an electrical element to provide internal heat to the system so that it doesn’t get frozen.

 freeze protection settings

For the freeze protection settings to work, you’ll have to make sure that the heater is always connected to a power source. You can only turn it off for routine maintenance. Otherwise, keep it on at all times. It’ll work no matter if it’s turned on or not but the electrical supply must be connected.

Go to the “Navien Freeze Protection Setting” and use this menu to set the temperature of circulation pump freeze protection. It’ll most likely be at 50 degrees Fahrenheit by default. You can set it anywhere between 43-50°F.

So, how does it work? The water heater will run the circulation pump to prevent freezing if the heating supply temperature goes below the value you set for longer than 10 seconds.

ii). Maintain A Power Source

For the freeze protection settings to work, the tankless water heater must always be connected to a power source. If the power supply is interrupted, there’s nothing keeping the tankless water heater from being frozen.

maintain a power source

It’s common for the electrical grid to get messed up during extreme temperatures and cause power outages. When that happens, the freeze protection system will be deactivated.

You’ll need a backup power source to prevent that from happening. Here are the most common options you can go with:

  • Portable Home Generator.
  • Whole Home Generator.
  • Computer UPS.

If there aren’t too many people living in your home, a small home generator will be the ideal choice for you. It’ll keep supplying power to the heater even if there’s load shedding. You can also choose a whole home generator if you think that the smaller generator won’t be enough to fulfill your demands.


Finally, you can choose a battery power bank or UPS to give backup in case of power cuts. The ones that are used for desktop computers can also be used for tankless water heaters. Get one with a bigger battery so that you can run the heater longer without electricity.

iii). Drain Water From The Heater

Even in a tankless water heater, water residue inside it can create a lot of problems regardless of how small it is. When the water gets frozen inside the unit, the ice can damage many of the plastic components in the heater.

So, it’s important to drain the heater at regular intervals during the cold months. You should especially take this step if you’re planning to go on a vacation and won’t be home for a while.

iv). Keep The Heater In A Warm Place

Choose the warmest place in your house to store your heater. If you can keep it in a heated area, it’ll naturally fight against being frozen during the cold months. You can also use vent terminations with wind-resistant features to cut down the wind that’s getting to the heater.

v). Insulate Your Pipes

Remember that the freeze protection settings of the heater don’t extend to the pipes. There’s still a strong chance that the pipe can get frozen during winter. Use heat tape, polyethylene insulation, or fiberglass to protect the pipes in your home from getting frozen.

 insulate your pipes

You can also install a recirculation system so that the heater will continuously warm the pipes as the water will keep going through them. That’ll keep the pipes and other parts of your plumbing system from being frozen over.

vi). Keep Running Water

The last resort option to keep your tankless heater from freezing over is to run a trickle of water overnight. It can be as little as 0.1 to 0.2 gallons per minute from the hot side to the furthest faucet. It’ll minimize the chances of freezing.

How To Choose The Right Tankless Heater For Winter

Have a plumber inspect your house. The three most important things when it comes to choosing the right heater are the flow rate, the required temperature rise, and the number of people that will use the heater.

i). Flow Rate

The flow rate is essential because it’s the amount of water that flows in a minute. It’s measured in GPM or Gallons Per Minute. If the flow rate in your house doesn’t match the minimum flow rate required by the heater, it won’t work properly.

ii). Temperature Rise

The second crucial factor is how much the heater will have to work to keep a steady supply of warm water. If the temperature of the groundwater is already on the higher side, the heater will have to do less work.

iii). Number Of People

The right size of the heater for your house will depend on how many people will use it. If you foresee multiple taps being operated at once, you’ll have to opt for a bigger heater.

number of people

Otherwise, a small heater will be fine. When it comes to tankless water heaters, size matters.


How well does a tankless water heater work in cold weather?

An important factor that determines how well the heater will work is whether it’s gas or electric. Gas tankless heaters outperform electrical ones as is the case with tank-style heaters.

Can mineral build-up cause changes in water pressure?

As the passage of the water becomes narrow due to the blockage, there’s less room for the water to travel. That changes the pressure. It’s unsafe and should be treated right away. Thankfully, it’s an easy fix.

Why does my Navein tankless water make noises?

The common reasons behind it are the vibration of loose parts, a defective pressure valve, and a dirty flow sensor. Contact the Navien support team to solve this problem, especially, if your unit is under warranty.

Can frozen pipes stop the supply of hot water?

If you’re living in an area where the climate is extremely cold, the pipes can get frozen and prevent the hot water from traveling. Thaw your pipes safely and see if you can get hot water by trying again.

Why does the water in my house smell like rotten eggs?

The build of minerals can make the water smell bad and look discolored. Fungus, mildew, and bacteria can grow in your unit. You can solve this problem by descaling the heater or using a water softener.

How to turn the heat up on Navien tankless water heater?

Depending on the model, you’ll either see an “upward arrow” or a “+” button on the control board of your Navien tankless water heater. You can press these buttons to incrementally raise its temperature.


As frustrating as it is to see your Navien tankless water heater goes cold, the problem won’t be fixed on its own. You need to find out what’s causing it and fix that. And hopefully, this guide will help you do just that.

If one of the issues mentioned in this guide turns out to be the problem in your case, you can solve it now. Otherwise, you should contact the Navien support team to see what’s up. Comment below if you have any questions about these tankless heaters.

About William

William is the founder of He has real life practical skills in fixing smoker & heating appliance issues. He loves to share his knowledge & helps others in fixing their appliances & saving their money. William firmly believes that anyone can repair his or her unit with the correct guidance & knowledge. See more at about us.

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