Navien Water Heater Constantly Running [4 Easy Fixes]

The main reasons behind your Navien hot water heater running when not using any hot water based on my experience are:

  • Preheating the water in the buffer tank.
  • Expansion tank issues.
  • Mistakes were made during installation.
  • Water leaking from condensate pipe.

I’ll explain all these issues in detail along with their solutions in this guide. You’ll also learn about replacing the buffer tank in your Navien water heater if it goes bad. So, let’s dive in!

navien water heater constantly running

I’ll talk about the most common reasons why your Navien water heater may be running continuously in this section.

01. Heating The Buffer Tank

The problem of the Navien water heater running constantly in many types. In this scenario, the water heater may turn on intermittently on its own even if there’s no demand for hot water.

buffer tank of navien tankless water heater

You may notice that the water heater will turn for a few minutes every hour or so. The main reason it happens is that the water heater heats the buffer tank. It comes in handy when a tap is turned on and off in a short period.

How To Fix:

Heating the buffer tank isn’t always needed. Especially, if the heater turning on again and again is bothering you. You can put the heater out of the intelligent preheating mode. By doing that, you’ll eliminate the need for the buffer tank as the hot water goes through the faucet.

You’ll find the switches to adjust these settings under a black rubber piece that’s right above the LCD screen. You’ll have to remove the black piece to access the dip switches.

Flipping the dip switch 1 on the dip switches above the LCD will turn the smart preheating function off. Make sure you don’t screw around with the other settings. If you mess something up, you’ll likely have to wait a long time to get the hot water.

Another word of warning for you is to only enable the intelligent preheating mode if you’re consistent with the shower times. If there are too many irregularities in the hot water uses, this mode won’t keep up. So, you’d have a longer waiting time for hot water.

One of the smart ways to counter this issue is to set up a remote control if your Navien water heater is of the NPE series. You can then set up the reheating option based on a timer. That way, you can run the heater only on the predetermined periods when you need it.

Turning off the internal circulation system as I’ve shown you will increase the wait time for hot water by a little bit. But it’ll prevent the gas from being wasted. That’s because the water heater will no longer be turned on intermittently and that won’t activate the gas.

So, the choice is yours. It’s up to you if you prioritize gas or water optimization. If it were me, I’d definitely be more concerned about saving gas. But you pick the option that suits you better.

If you want to minimize gas usage and still get hot water with no or little wait, you can install an external recirculation setup. That way, you can get the best of both worlds and get hot water while using as little gas as possible.

02. Water Coming Out Of PRV

Another common type of issue you could be facing here is the water will constantly come out of the Pressure Relief Valve. It can even happen when no hot water is being used.

pressure relief valve of navien tankless water heater

The main culprit behind this issue is if your expansion tank has failed. If you don’t currently have an expansion tank, you’d need to install the one. The tank may even make a noise before the water comes out of the Pressure Relief valve.

The main job of an expansion tank is to reduce the pressure of the hot water. But if you don’t install it or it goes bad, the pressure builds up in the water heater. That’s why the water comes out of the PRV. It’s a serious issue as the water heater can get completely damaged or blow up if not attended to soon.

Another possible culprit is that the PRV has failed. It’s not likely if you have a fairly new unit. But if you have used your water heater for more than 5 years or so, the pressure valve itself could be faulty.

How To Fix:

If you don’t have an expansion tank, you’ll have to install one immediately. If your expansion tank has failed, you’ll have to replace it with a new one. I’ll talk about the details of how you can install it in a moment.

expansion tank of navien tankless water heater

However, there’s also the possibility that you have a bad PRV. If that’s the case, the replacement is fairly easy. All you’ll have to do is locate it, turn off the water supply, unscrew it, and replace it with a new one.

Keep in mind that tankless water heaters only need a PRV. It only deals with pressure. The tank style water heater requires a T&P valve which deals with both temperature and pressure. So, don’t get your hands on the wrong part and only get a pressure relief valve.

How To Install An Expansion Tank?

If you want to replace the expansion tank on your own, it’s possible. Trust me, I’ve done it before. You can follow these steps to install an expansion tank:

Step 01 – Match The Expansion Tank Pressure With The Faucets

You can set up the thermal expansion anywhere on the cold water side. Mounting it to the wall next to the cold water shutoff valve is a good choice. All thermal expansion tanks come with a precharged pressure.

You’d have to match the pressure of the expansion tank with that of the water faucets in your home. For example, let’s say the water pressure at your house is 40 PSI and the expansion is 20 Watts. So, you’ll need to bump up the pressure of the expansion tank by 20 PSI so that it can be at 40 PSI.

You’d need a pressure gauge to measure the pressure of your water faucets. You’ll notice a spot on the expansion tank just like you would on a football to let in more air. Then, use a bicycle pump or a compressor to increase the pressure in the tank.

Step 02 – Connect The Expansion Tank To The Studs

Install the straps for the thermal expansion tank into the studs. If you’re having difficulty finding the studs, you can use a studs locator. Then, set up the expansion tank in the right place and secure its position by tightening it with the straps.

Step 03 – Connect The Expansion Tank And Water Heater

The next step is the most crucial as you’d have to connect the water heater and the expansion tank. Depending on the piping system of your water heater, you can go about it in many ways. 

One of the easiest ways is to use a copper Tee fitting. You’ll need a couple of flex connectors. Connect the first flex connector to the expansion tank from the tee-fitting. The second flex connector will be connected from the other end of the Tee-fitting to the water heater.

Once you do that, you’ve completed the installation. Turn on the water supply and see if there are no leaks. If there are any leaks, tighten the areas where the leak is coming from. If there are no leaks, congratulations on a job well done.

It’s understandable if the process seems too complicated for you. All the tools needed to execute this job may not even be available to you. In that case, it’s perfectly fine to go with a qualified technician for this job.

03. Installation Mistake

If Navien gas tankless water heaters or combi boilers aren’t installed properly, you can run into many issues when facing them. One of these issues is the unit running on its own and wasting gas.

When installed in a closed-loop system, it can cost you hundreds of dollars in additional gas bills. One such mistake could be not insulating the pipes well. If that happens, the need for hot water will always remain. So, the water heater may even turn on every 30 minutes or so.

Similarly, there could be a plethora of installation mistakes that could lead to this issue. Using internal parts of the heater for recirculating water is also another mistake. There are countless numbers of mistakes that can be made by the installer here.

How To Fix:

If you want a permanent fix for the issue you’re facing, you’d have to find the installation mistake first. Then, you’d have to take the right steps to fix it. That’s why it’s so important to hand the project to an expert when it comes to these sensitive things.

However, that can be too time-consuming, or may not have the budget for it right away. In the meantime, you can utilize an annoying but simple solution. Simply turn the water heater off when you don’t need it and turn it on when you need it.

To take it a step further, you can also turn the gas control valve on and off depending on when you need to use the water. But eventually, you’ll have to address the root issue to fix it in the long run.

04. Water Leaking From Condensate Drain

Another annoying issue that’s faced by many Navien owners is that the water will drain constantly from the condensate drain. It can even happen when the heat exchanger isn’t active at all. First of all, you have to realize that the water continuously leaking like that isn’t normal.

condensate drain of navien tankless water heater

There could be many reasons behind it. The most common one is that the O-ring between the flow sensor and the inlet can be damaged. But that’s not all. The internal drain valve could be damaged. Especially, if you have freezing weather. The cold weather can crack and damage the valves.

On top of that, the heat exchanger could also be leaking. If the primary or secondary seals of the heat exchanger get cracked, it could be leaking water. 

How To Fix:

If you want to fix this issue, you’d have to find what’s causing the problem first. Remove the front panel of the water heater and check if you can find where the leak is coming from. As I said, the leak will come from a defective O-ring in most cases. Inspect if that’s happening with you.

The solution is super simple if you have a bad O ring. You’ll have to replace it. Thankfully, the replacement job is so easy that anyone can do it. You can follow these steps to do it:

  1. Turn off the gas, electric, and water supply to the water heater.
  2. Drain the water remaining in the heater.
  3. Remove the front panel by undoing the four screws and locate the flow sensor.
  4. Loosen the screws holding the flow sensor in place and take it out.
  5. Replace the O-ring and put the flow sensor back in its place.
  6. Tighten the screws and put the front panel of the heater back on.
  7. Restore the water, electric, and gas supply of the water heater.

That’s how easy it is to replace the O-ring in the water heater. It shouldn’t be too hard to find it. You’d find it in plumbing stores or you can also order it online. Similarly, you’d have to replace the faulty valves if they’re damaged due to being frozen.

The most unfortunate possible issue in this scenario is if you have a bad heat exchanger. You’re better off replacing the water heater if your unit is past the warranty period because replacing the heat exchanger can be really expensive. 

But if you have a warranty, you’d at least get the replacement part for free. If you hire a technician, you’d have to pay the labor charges. You can even save that if you’re daring enough to do this task on your own.

Replacing the heat exchanger isn’t the easiest thing to do on your own. But I’ve shared the instructions on how to replace the heat exchanger in another guide. Check it out if you’re interested in doing the job yourself.

How To Replace The Buffer Tank In Navien Water Heater?

The buffer tank is an important component that makes sure you get hot water fast when you turn on a hot water faucet. Heated water is stored in the buffer tank and goes to the faucet when there’s a demand for hot water.

When there’s anything wrong with the buffer tank, you won’t be able to enjoy hot water on demand. So, you’d have to replace it in that case. You can follow these steps to replace the buffer tank on your own:

Step 01 – Remove The Fan

First, keep in mind that it’s important to follow safety precautions. So, start by turning off the gas and power supply to the water heater. Then, turn off the water supply as well and drain the water from the heater.

Remove the screws holding the front cover of the water in place and remove it. Then, remove the two screws holding the fan in place. Pull out the fan and you should see the screws holding the buffer tank in place.

Step 02 – Remove The Old Buffer Tank

Remove the fasteners “A” and “B”. Then, you can loosen the 4 buffer tank screws to make the tank loose. Once that happens, you can pull out the buffer tank from the water heater.

Step 03 – Install The New Buffer Tank

Before putting the new buffer tank in, replace the O rings included in the copper pipe and the buffer tank. Then, place the buffer tank correctly from the copper pipe and plastic adapter.

Replace the fasteners of the buffer tank. Then, tighten the four screws of the buffer tank and make sure it’s secure in its place. After that, install the fan and put the front cover back in place.

Tighten the four screws to make sure that the front panel is locked in its position. Restore the power and gas supply to the water heater. Then, turn on the water supply and check for leaks.

If there’s a leak, you made a mistake somewhere during the installation. Check where the leak is coming from and fix it. If there’s no leak, then you have done a perfect job on the installation.

What To When Navien Heat Exchanger Is Leaking?

You may notice water leaking from your tankless water heater. Upon further inspection, you could see that a cracked heat exchanger is causing this issue. A Navien water heater leaking is a dangerous issue.

The water leaking is the least of your worries. The bigger concern is that a damaged heat exchanger will allow gas to escape. That can be extremely harmful for you.

There’s no simple fix when a heat exchanger goes bad. If your unit is under warranty, you can contact Navien to replace it. Otherwise, you may have to replace the heat exchanger or even the entire water heater to fix this issue.

The water heater can also leak from areas other than the heat exchanger. I’ve compiled a detailed guide on how to keep the Navien tankless water heater from leaking. Check it out if you’re interested.

What Does Call For Heat Mean On Water Heater?

If your water heater was to run all the time in a fully-fledged style, your utility bill would be sky-high. So, there are mechanisms in all types of water heaters to reduce the time when it’s fully active.

In tankless water heaters, it’s quite simple. The water heater is only supposed to be heated when you turn on the hot water faucet. So, the heater becomes active only when there’s a demand for hot water. So, the heater calls for heat and does the rest.

Even tank-style water heaters work in a similar way. When the tank is filled with a water heater, the main burner goes off. Only the pilot light remains on so that it can turn on the main burner when there’s a demand for hot water. 

When there’s a call for hot water, the water heater comes on. Similarly, the heating element in an electric water heater comes on when there’s a call for hot water. Hopefully, that makes sense to you.

If your water heater keeps calling for heat or doesn’t heat the water at all, something is wrong with your heater. In that case, I’d recommend getting it checked soon and taking the appropriate steps to fix it.

How Long Should Hot Water Heater Run?

When it comes to a regular tank-style water heater, the water is first stored in a tank. Then, it’s heated. So, it takes some time for the water to heat up. But tankless water heaters work much faster.

There is no need for storage in a tankless water heater. If there is a buffer tank in your tankless unit, only then a little water will be preheated. The goal of that is to preheat the water and eliminate the waiting period for hot water.

So, there’s no doubt that a tankless water heater will run for a shorter period on average compared to its tank-style counterpart. Tankless water heaters have virtually no waiting time. Compared to that, gas water heaters take about 30-45 minutes and electric heaters take 60-80 mins to heat the tank.

As for how long the water heater should run, it depends on your needs. In an average household, the water heater runs for about 3 hours per day. However, it can be more or less depending on your family’s needs.

If you use a tankless water heater, it won’t have to be on for nearly as long. In an average home, the tankless water heater only needs to be on when it’s used. So, it would be just about an hour or two in most cases.

How Long Should Hot Water From A Heater Last?

If you’re someone who enjoys a long nice shower, you may find that you run out of hot water soon. For the users of tank-style water heaters, that’s a common problem. You’d only have 15-20 minutes of hot water and then, it’ll be finished. Cold water will come pouring in after that.

If you’re facing this issue, there are two solutions. The first one is to shower faster and get it done within that time. The second option is to install a shower head with low flow. It’ll add the shower time as less hot water will pass by it.

The good news is that you can avoid these issues by using a tankless water heater. As it’s set to heat the water heater on demand, it will continue to heat the water for as long as you need.


What causes a vibrating noise in my water heater?

Navien tankless water heaters are reputed to be pretty quiet. But if your unit is vibrating, it could be due to the movement of the exhaust fan motor. However, it’s normal and the vibration shouldn’t be too bothersome at this stage.

Is a closed-loop system essential when installing Navien tankless water heaters?

You’d get hot water quickly if you installed the heater in a closed system. But it can lead to more gas bills as the heater will constantly be on. So, it’s not worth it and you’re better off waiting for a little while when using hot water.

Should the condensate fittings be made of metal?

Keep in mind that condensate is acidic. So, using metal fittings isn’t right for them as they’ll get damaged and start leaking soon. You should opt for plastic fittings here instead.

Why does my Navien water heater make excessive noise?

If the Navien water heater makes too much noise or vibrates excessively, there could be some serious issues. The main potential problems here are bad inducer motors, loose pipe fittings, or poor combustion.

How long is the warranty of heat exchangers on Navien water heaters?

In the Navien NP and NR residential water heaters, the heat exchanger is warranted for 15 years. In the NP commercial use series, the heat exchanger has a 10-year warranty period.


Why is my Navein water heater constantly running? By now, you should have a clear idea as you’ve read this guide. It can take a bit of trial and error to find out the exact issue that’s making your heater act up. Hopefully, the fix will be easy to implement.

Some issues are more difficult than others. So, hire qualified technicians when a job is out of your comfort zone. Another common issue with these heaters is the Navien exhaust smells like gas. Check out my guide on it if you’re facing that issue.

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