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How to Troubleshoot a Tankless Water Heater

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When you need to troubleshoot your tankless water heater you probably feel overwhelmed. Let's face it, even the most reliable appliances have problems from time-to-time, and your tankless water heater is no exception. But if you're scratching your head wondering where to begin, this extensive guide will help you take the first step to solving your problem.

While there's no question that a tankless water heater makes a great investment, they do need a little attention to keep them running in tip-top shape. They should be flushed regularly and a few other preventative tasks need to be completed.

But if you take care of your tankless, it'll serve you well for years to come. Still things happen, and this guide will help you troubleshoot your tankless water heater so you can get it up and running again.
Are you considering purchasing a tankless? Find out which system we have picked as the best tankless water heater in our guide.

Hands holding a burner with blue flames

Troubleshooting Burner / Ignition Issues

No Ignition

When your gas tankless water heater shows a "no ignition" message, it's an indication that the unit is attempting to light, but for some reason can not. Frequently, the issue is related to venting or the gas supply.

Common Problems Include:

  • If your water heater uses propane, check the propane tank. The problem could be as easy as running out of propane. 
  • If your fuel source is natural gas, check another appliance that uses natural gas, such as your furnance to see if it's working properly. If it isn't, the gas may have been shut off and you'll need to contact your gas company.
  • Next, check that the water and gas valves are completely open. This could also cause the tankless to not ignite. 

If none of the above steps resolve the issue, you should call a professional to assess the situation. There's a good chance it's a larger problem that needs to be addressed, and replacement parts may be needed. 

Flame Failure

When you receive a "flame failure" error message on a gas tankless water heater, the issue is typically either a gas pressure issue or an electrial problem. Always eliminate the easy fixes first, such as the natural gas being turned off or an empty propane tank. Then move to other, more complicated causes.

If the problem isn't something simple, it's generally best to call a trained technician to troubleshoot. They'll have the proper tools to measure the gas pressure and be able to quickly find and repair the problem. If it's an electrical issue, a professional plumber who specializes in tankless water heaters should be called.

Low Gas Pressure Probable Causes:

  • Gas regulator failure
  • Combustion issues
  • Gas line isn't large enough
  • Venting Issues

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Burner Will Not Ignite

Gas tankless water heaters use a burner to create the necessary heat that warms the water. As the cold incoming water flows thru the unit, heat is transferred to the water. If you're hot water is cold, there's a good chance that the burner did not ignite.

The problem could be as simple as the gas control valve not being fully open, or an issue with venting. You many be able to troubleshoot the problem, but in most cases, it's typically best to contact a professional.

There's a number of reasons that can prevent the burner from igniting. Below are a few of the common issues, but there are many others that can cause this problem.

Possible Causes:

  • Electrical Issue - Has your tankless been tripping the circuit breaker? If so, you should contact a plumber to dignose and fix the problem. 
  • Gas Flow Issue - Check for a visible pilot light. If the pilot light won't stay lit, or there isn't a pilot light at all, or you have any reason to think that there's a gas problem, you need to contact a plumber who's experienced in working with gas tankless water heaters as soon as possible.
  • Flow Sensor Issue - The flow sensor signals the tankless to activate. If there's wear or damage to this sensor, the burners will  often fail to turn on. 
  • Pilot Assembly or Burners Require Cleaning - The problem could be a build-up of dust or dirt that's preventing the pilot assembly to activate or the burners to ignite.

Noisy Burner

Tankless water heaters make noise when they're operating. However, if you notice loud or new sounds that you haven't heard before, it could be a sign of trouble.

Common Causes:

  • Check the fan for loose debris.
  • Check that the burner is still working and that the flame is clear or light blue.
  • If the burner flame is unstable, it could be because of improper venting, lack of gas pressure, or inadequate combustion.
Lady in Shower with Cold Water

Troubleshooting Water Temperature Issues

No Hot Water

You might be suprised to know that not having hot water is actually a common issue with tankless water heaters. When this happens, the first thing you should check is the electrical power and the gas and water supply to your unit. Another thing to check is the shut-off valves, as they occasionally are closed inadvertently. 

Gas Burner

Next, check the gas burner. If the flame rod is not generating sparks, the issue could be with the ignition. This is a classic sign of failure. See above: Burner Did Not Ignite.

Error Code

Check if there's an error code displaying on the tankless control panel. Your owners manual should list all the codes, as each manufacture is a little different.

If you do find an error code, the tankless will not operate until the issue has been fixed and the system is reset. When this is your problem, it's typically best to call a professional

Minimum Flow Rate

Another probable cause is if the minimum flow rate hasn't been activated. Each tankless water heater requires a minimum amount of water to flow before the unit will begin to heat.

Check that the hot water tap is completely opened, and that there aren't any obstructions within the plumbing.

Winter Weather

During the cold months of winter special care needs to be taken to prevent your tankless water heater from freezing. Even indoor units can have issues.

The pipes leading to the unit can freeze, or even the tankless itself. See below: Cold Weather Concerns.


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Water Too Cold or Too Hot

If your tankless water heater is delivering water that's too hot or too cold, there's a number of issues that could cause this problem.  

Water is Too Cold

  • It could simply be that the water temperature is set too low. If this is the case, adjust the maximum temperature between 125° and 130°F. 
  • The gas valve isn't providing enough pressure. This can be the result of reduced gas pressure, or if the gas valve is not completely opened. You should also check if the gas line has been sized properly, it may be too small.
  • If it's been awhile since you've flushed your tankless, you may have a build-up of limescale inside your heat exchanger. When limescale accumulates, it'll serve as an insulator and prevent the heat exchanger from transferring the heat to the water. See below:  Hard Water / Mineral Build-up.
  • It could be a venting issue. Check the venting system. It should be clean of all debris and able to provde plenty of fresh air for proper combustion. 
  • There may be a plumbing crossover issue where the cold water is mixing with your hot water. It's not uncommon for a single-lever mixing valve to fail and cause a crossover issue. 

How to Check for a Crossover Issue

  1. Turn off the cold water supply to your tankless water heater.
  2. Open all of the hot water taps.
  3. Allow approximately 10 minutes to pass.
  4. If the water flow is still present, then there's an issue with crossover.

Water is Too Hot

  • The temperature on the unit has been set too high. This is an easy fix and something you should be able to do yourself. There's often a knob, but if it isn't obvious you can check your owners manual for directions on how to reduce the temperature.
  • The flow of water has been reduced because of an obstruction in the plumbing or the water filter. If this happens, then only a very small amount of water is heated in the tankless. This problem can be fixed by cleaning the water passageways. 
  • There's always the possibility that the temperature sensor is broken or improperly positioned. You can try to reposition it so that it's properly attached to the pipe or replace it with a new one.

Cold Water Sandwich

If you have a busy household full of people, its likely that back-to-back showers are happening. If this is the case, you may have first-hand experience of what's called the "cold water sandwich."

The name, cold water sandwich, comes from hot water being trapped in the pipes between uses. When the first person's shower has ended, there's still some hot water in the plumbing leading to the shower. When the second person turns the water on, the water is warm, but then turns cold before the tankless kicks in and delivers hot water again.

Many people think that since tankless units are often referred to as an instantaneous water heater, that there shouldn't be a delay for hot water. However, although the tankless activates nearly immediately, it still takes time for the water to heat and travel thru the plumbing.


The further the hot water needs to travel, the more noticeable the cold-water gap will be. However, there are a couple of work-arounds to solve this problem.

  1. You can simply be aware of the delay and wait until the cold water is replaced by hot. This is by far the most cost-effective solution, but many homeowners prefer to have instant hot water throughout their home . . . without waiting.
  2. You can install a point-of-use water heater. They come in either tank or tankless water heater styles, and are particularly helpful when hot water is needed in a remote area. Many point-of-use water heaters can also be used as a booster to your main tankless. Filling in the gap until your main water heater can take over.
Copper Pipes and Plumbing Tools

Troubleshooting External Tankless Issues

Venting Obstruction

For the majority of tankless water heaters, if you have air supply or exhaust blockage issues you're going to receive an error code. This code is designed to let you know that the exhaust or air supply has become blocked and the unit is not able to vent or combust air properly.

The first step in fixing this problem is to inspect all of the vent pipes and make sure that they are properly connected without any holes or gaps. It's never a bad idea to contact a professional plumber who specializes in tankless water heaters. If there's a small undetected gas leak, a professional will have specially designed equipment to locate such a leak. 

Common Venting Issues Include:

  • Installing the unit too close to another object that may block the air supply, or even create a fire hazard.
  • A wasp, rodent or bird nest may be blocking the venting.
  • Clearance requirements not met when unit was installed or not maintained after installation.

If you're still unable to find the problem, it's possible that the blockage is inside the tankless unit itself. It could be within the fan motor or another part. We recommend contacting a professional as some of these areas can be very difficult to inspect. 


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Low Water Pressure

If your tankless water heater isn't producing the level of water pressure that you expect, there's a number of different issues that could be responsible. In fact, the problem may not even be the water heater. 

It's a good idea to check with the water company first to make sure that they aren't experiencing any water pressure issues. If the problem isn't with the water company, then the next step is to check for obstructions in your home's plumbing, the water heater itself, or your fixtures.

In some cases, it could be that the water pressure is reduced because several appliances are operating at the same time. This will cause the overall water draw to be higher, which will reduce the water pressure for each.

Another cause could be from a reduction in gas pressure. When gas pressure is lower, your water heater will automatically reduce the water flow. This allows the unit to deliver the proper output temperature.

Hard Water / Mineral Build-up

Mineral build-up, also known as limescale, is a problem that can occur with any type of water heater. But its expecially a concern with a tankless unit.

Simply put, hard water is water that's full of minerals. Primarly magnesium and calcium. When hard water is heated, the minerals are deposited on metal surfaces, which over time, will build-up within your heat exchanger and reduce the effectiveness of your tankless to heat water.

The harder water you have, the more potential there is for issues and damage due to the build-up of limescale. Read more about hard water HERE.

Reducing Limescale Build-up

There are two critical things you can do to reduce the build-up of limescale. The first is to flush your tankless unit on a regular basis. Our post will guide you step-by-step how to flush both an electric and gas tankless water heater.

The second thing you can do is install a scale inhibitor prior to the cold water entering your tankless. This can make a huge difference since it will help prevent limescale from  forming in the first place. 

Our favorite scale inhibitor is a 3M product called the Aqua-Pure Hot Water System Protector. Once installed, you can easily replace the cartidges to help give your tankless an added layer of protection.

Aqua-Pure AP430SS Hot Water System Protector

Aqua-Pure Hot Water System Protector

The Aqua-Pure Hot  Water System Protector is a scale inhibitor by 3M. Once installed, you'll need to change cartridges periodically so it can help protect your tankless from scale build-up.

Cold Weather Concerns

One common problem in the winter is to see white smoke coming from your unit's exhaust vent. This isn't anything to worry about, and simply means that the exhaust temperature is much higher than the current temperature of the air.

Freezing Precautions

There are a lot of potential problems that can occur if your tankless water heater freezes. Even indoor units are prone to freezing, especially if they are located in a basement or a garage.

In order to protect a tankless from freezing, it's important to make sure the unit is supplied with gas and electricity to help activate the built-in freeze protection. Read our full post on caring for a tankless water heater in cold weather. 

We also highly recommend installing auto drain down solenoid valves as a safety precaution. If there's ever a power outage during cold weather, the valves will automatically drain your tankless. This will not only give you peace of mind, but can save you hundreds of dollars in repairs.

Rinnai 104000059 Freeze Protection Solenoid Valves Kit

Freeze Protection Solenoid Valves

Freeze protection solenoid valves will automatically release water within the tankless system should there be a power outage.

Reaching to grab a key hanging from the ceiling

Troubleshooting Other Tankless Water Heater Issues

System Overload

Tankless water heaters come in different sizes. They're sized by the amount of gallons of hot water they're capable of delivering per minute (GPM). Basically, this translates to how many sinks, showers, or appliances a tankless is capable of servicing at a single time.  

Based on the capacity of your tankless, there may be too many hot water applications working at once. If this happens, it can overload your unit.

Generally, you'll know this is the problem if your water heater struggles to provide the necessary hot water, or if it shuts down completely. 

If your tankless is overloaded, you'll need to reduce your demand for hot water. Turn off a shower, a sink faucet, or another hot water application that's being used.

It may also be necessary to reset the unit completely. However, if you notice that your tankless is overloaded on a regular basis, then you may need to upgrade to a larger, higher capacity unit. Another option is to install a second tankless.

Tankless Shuts Off During Showers

If you have a tankless water heater that's over 10 years old, the problem may be that the minimum flow rate is too high. Each tankless has a specific minimum flow rate that must be met for the unit to operate.

The flow rate is simply the minimum level of water (measured in gallons) that must be flowing thru the tankless before the unit will activate and begin heating.  

If the total amount of hot water you require is under the minimum flow rate for the unit, then the tankless will likely shut down as a safety measure. 

Since there isn't enough cold water constantly moving over the heat exchanger, the tankless is at risk of overheating. It shuts down to prevent damage to the heat exchanger and to ensure that it won't deliver extremely hot water that could cause severe burns.

You many want to consider installing a new tankless water heater. The majority of older units have a minimum flow rate of 1/2 to 3/4 Gallons Per Minute (GPM). However, newer units are designed with extremely low minimum flow rates. Some are even as low as 1/4 GPM.


When a leak develops within the heat exchanger there's a huge risk of corrosion. In most cases, a corroded tankless water heater will need to be replaced.

Another issue that can cause a tankless to corrode is improper venting. When the exhaust gases don't vent out of the unit as they should, the heat exchanger can corrode, and even worse, create an unsafe environment within your home.

If you suspect corrosion, you should contact a professional to assess the situation and help you determine the best solution.


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