Bosch Tankless Water Heater Won’t Ignite: How To Fix

The main culprits behind your Bosch tankless water heater won’t ignite are:

  • Power malfunction
  • Fuse failure
  • Water mobility is restricted 

The other reasons are wire connection problems, plumbing malfunction, loose connection in the cold water inlet, etc. The instant solutions include checking for power in the control board, replacing of fuse in the control board, and clearance of the water flow paths.

So, if you are concerned about why your Bosch tankless water heater is not igniting then surely you can have a look at this article. 

Quick Solutions To Bosch Tankless Water Heater Won’t Ignite

The possible reasons for the ignition problem in your water heater and their quick solution are listed below:

ProblemsQuick Solutions
Lack of power in the unitCheck if there is power in the electric outlet otherwise the heater will not receive power.
Non-functional fuseReplace the fuse that is mounted on the bottom of the heater.
Loose connection of wiresInspect all the wire connections in the water valve, water flow sensors, and ignition electrode sets and also replace the damaged ones.
Dust accumulation in the water-flowing pathwaysClean all the clogged pathways that obstruct the flow of water in the heater.
Clogging in the water filter screenClean the build-up in the water filter screen through pressure water
Faulty valve of cold water inletFix the leakages in the cold water inlet which can obstruct the cold water supply into the water heater
Gas supply problemsChoose the right gas type (i.e. LPG or PNG) for your heater. Replace the valve of the gas supply line if it doesn’t work.
Connection problem between pressure switch and water regulatorTighten the connection between the pressure switch and water regulator as it can cause low water flow.
Problem with ThermocoupleInstall a new thermocouple and replace the faulty one as it can obstruct the gas flow to the heater.
Plumbing connections crossoverConnect the cold water pipe to the cold water inlet and the hot water pipe to the hot water inlet.
The water heater shifted to solar modeMake sure that the temperature of the inlet doesn’t cross the limit.
bosch tankless water heater won’t ignite

Why Your Bosch Tankless Water Heater Won’t Ignite? (Explained With Solutions)

This troubleshooting chapter will help you to find out the specific culprit that is causing the ignition problem in your Bosch water heater. Here are the reasons that can cause ignition failure and their simple solutions.

01. No Power In The Unit

Power failure in the unit is one of the most common reasons why your Bosch tankless water heater won’t ignite. The ignition process mainly occurs in the burner. The ignition occurs when the burner receives power from the unit.

The burner will not work if the unit doesn’t receive any power. There are a few possible reasons why there is no power in the unit:

  • Wire malfunctions
  • Electric outlet malfunctions
  • Didn’t plug the power cable into the electric outlet.

What To Do:

First of all, make sure that there is power in the electric outlet. Otherwise, there is no need to check the burner of the heater. Because if there is no power in the electric outlet then the unit will not receive power and the burner will not work. 

If there is power in the electric outlet then try to make sure that all the wire connections of the heater are plugged in properly. Because sometimes the loose connection of the wires can halt the power transportation in the unit. If the problem still persists then you should consult with an electrician.

02. Fuse Malfunction

The malfunctioning fuse in the control board is another common reason for Bosch water heater ignition failure. If the fuse is not working properly, the unit won’t be powered and the burner will not work. 

In order to ensure ignition in the burner you must make sure that there is power in the unit.

What To Do:

If your fuse is not working then you have to follow the steps mentioned below:

  • Ensure that the power cable of the water heater is not plugged in.
  • Then you should remove the cover at the bottom of the unit by removing all the screws.
  • Eventually, remove all the power cables in the control unit.
  • Lastly, press the snaps and remove the top cover of the control unit.
  • Then you should check the fuse that is installed in the printed circuit. 
  • If the fuse is not working then you have to replace it with a new one.
  • Finally, reinstall all the parts in reverse order.

03. Wrong Cold Water Inlet Connection

If the cold water connection is not plumbed on the right side of the heater when facing the unit, it can lead to several problems:

  • Improper Water Flow: Connecting the cold water inlet to the wrong side can disrupt the intended water flow pattern within the heater. This can result in reduced efficiency and performance of the unit.
  • Insufficient Heating: The tankless water heater relies on a specific water flow path to ensure effective heating. If the cold water connection is on the wrong side, it can disrupt the heat exchange process, leading to insufficient heating of the water.
  • Inadequate Hot Water Supply: Misalignment of the cold water connection can cause a decrease in the volume and temperature of the hot water being delivered. This can result in an insufficient hot water supply for your needs.

What To Do:

It is important to ensure that the cold water inlet connection is plumbed to the right side of the heater when facing the unit. Connecting the cold water inlet to the correct side allows for efficient heating and proper operation of the unit.

Notes: To ensure proper installation of the water heater, one should place it centrally and ensure that the ¾” cold connection is on the bottom right and the hot connection is on the bottom left when facing the unit. This placement is recommended to ensure even distribution of hot water throughout the structure, minimizing wait times and providing more efficient use of the heater.

04. Debris Build-up In The Water-Flowing Paths

The dust accumulation in the water-flowing paths might be one of the common problems that cause ignition failure in the water heater. The blockage in the water paths in the drain valve, hot water inlet, or cold water inlet might cause a stoppage in the water supply throughout the heater. 

You have to ensure that there is no presence of blockage in the water-flowing paths. Or else, it can cause specks of dust to build up in the paths. 

What To Do:

You should check whether all the water-flowing paths are free of any debris build-ups or clogging of bags of dust. Make sure that you have checked all the water paths in the faucets, filters, and shower heads. 

In addition, follow our troubleshooting steps that simply explain how you can easily flush the water heater.

  • Firstly you have to turn off the main power of the water heater.
  • Locate the service valves on the cold water inlet and hot water outlet pipes. These valves should be located near the water heater.
  • If you suspect the blockage is in the drain valve, then drain the valve after switching off the water supply to the tank. Attach a hose or pipe to drain the water. Then you have to clear the blockage by using a plumbing snake.
  • Give time to refill the tank and turn on the main power supply.
  • Test the water heater. 

05. Mixing In The Plumbing Connections

If there are not any problems in the control module then you can check all the plumbing connections which means the hot and cold water connections. The burner assembly won’t start if there is a crossover in the plumbing connections. A crossover in the plumbing means you have installed the hot water pipes in the cold water outlet or vice versa.

The plumbing crossover can create back pressure in the water flow. Usually, the heater will ignite if there is a higher pressure of water flow than normal. The back pressure of the water flow will not let the heater ignite.

What To Do:

  • Firstly, start by checking if there is any plumbing malfunction in the heater. 
  • Then shut off the cold water flow and then you will need to open the hot water inlet. 
  • Wait for at least 15 minutes to observe that there is any water coming from any of the faucets in the hot water inlet.
  • If there is any flow of water (even if it’s low pressure) then there is surely a plumbing crossover present in the heater. Then consult with a plumbing expert in order to fix this problem.

06. Blockage In The Water Filter Screen

The water filter screen in a water heater is typically a mesh or screen filter located at the cold water inlet of the heater. Its purpose is to trap sediment and debris from the incoming water supply, preventing them from entering the water heater tank.

Your burner assembly won’t be ignited if there is dust, debris and minerals clogged in the inlet water filter screen. Because the water will not pass through the build-up and it will make the unit unable to operate. Eventually, the burner won’t be ignited if the unit doesn’t activate.

What To Do:

Here are the needed procedures to clean the water filter screen effectively: 

  • Close the connection of the water shut-off valve that is installed. 
  • The next step will be to make way for the water using the discharge drain. 
  • Now remove the front access panel of the heater & then locate the water filter screen from the upper part of the water heater.
  • Uninstall the brass hex cap that keeps the water filter stable in that position. 
  • Finally, take out the water filter screen and then clean the screen with pressure water. 
  • Then reinstall all the parts again in reverse order. 

The most important thing is to ensure that the water is flowing freely in your water heater so that the burner assembly can be activated.

07. Problem With Cold Water Inlet Valve

If the valve of the cold water inlet is not functioning properly due to water leakage then your Bosch water heater won’t ignite. Either the valve is clogged or it is not plumb correctly. 

For that reason, the cold water is failing to enter the water heater from the main water supply. That is why there is an interruption in the ignition process. Therefore, make sure you inspect the valve of the cold water inlet.

What To Do:

Checking the valve of the cold water inlets can be a complex task for you. Therefore, I recommend you read the guide that I noted below:

  • Turn off the water supply of the heater.
  • Then locate the cold water inlet valve. Basically, it is located on the top side of the water heater tank. Typically it looks like a small valve or pipe with threads to connect other pipes.
  • The next step will be to inspect the valve of the cold water inlet. You have to look for any signs of leakage whether the water is dripping or leaking from the valve.
  • If there is any leakage fix it as soon as possible. Because the leakages or cracks in the valve can obstruct the cold water supply to the heater. The leakage in the seal of the valve can be fixed by installing a new seal on top of the old one.
  • If there is no leakage, ensure the cold water inlet connection is plumbed correctly. It should be on the top right side of the unit.
  •  Finally, turn on the water supply. Afterwards, again look for any leakages in the valve of the cold water inlet.
  • After fixing the connections, you can finally test the heater whether the ignition problem occurs or not.

08. Gas Supply Issues

The lack of proper gas supply to the heater can cause ignition failure in your Bosch tankless water heater.  Problems in the valve of the gas line as well as air in the gas line can hamper the flow of gas into the heater. In addition, choosing the wrong gas type can also cause ignition problems.

What To Do:

Ensure that the gas valve is turned on because the ignition is triggered when it receives the gas from the gas supply. If the valve of the gas supply line is not working then replace it with a new one.

The constant flow of gas in the burner assembly will not cause any ignition failure. However, be absolutely sure that you choose the right type of gas for water heaters which is Propane or Natural gas.

In addition, it will be best to consult with an expert regarding air issues in the gas lines. Because you cannot inspect the air issue by yourself unless you are an expert.

09. Minimum Flow Requirement Is Not Met

The water flow rate must be at least 0.5 gallons per minute to trigger the heating mechanism in the tankless water heater. This requirement ensures that the heater operates efficiently and effectively, providing hot water as needed. 

What To Do:

To verify that the minimum flow rate of 0.5 gallons per minute (1.9 liters per minute) is met to activate the heater, you can perform a simple test using a container and a timer. Follow these steps:

  1. Get a quart-sized container (32 fluid ounces or approximately 0.95 liters) that is suitable for measuring liquid volume.
  2. Place the container under the hot water tap that is connected to the heater.
  3. Turn on the hot water tap to its maximum flow.
  4. Start the timer as soon as the water begins to flow into the container.
  5. Stop the timer when the container is completely filled.
  6. Check the recorded time on the timer.

If the container fills in 23 seconds or less, solely from the hot water tap, it confirms that the minimum flow rate requirement of 0.5 gallons per minute is being met to activate the heater. If the time exceeds 23 seconds, it indicates that the flow rate may be insufficient, and the heater may not activate properly.

In certain uncommon situations where the inlet water temperatures are high and the set temperatures are low, the heater may increase the activation rate beyond the standard 0.5 gallons per minute (gpm) to prevent overheating at low flow rates. To test this, gradually increase the water flow (hot water demand) through the heater and observe if the heater activates with a higher flow rate.

10. Loose Wire Connections

Problems with wire connections between the water valve, control unit, and electrode set can lead to various issues with the operation of the water heater. Here are some problems that can arise from faulty wire connections:

  1. Intermittent or No Heating: Loose or disconnected wires can interrupt the flow of electricity and prevent the heating elements from functioning properly. This can result in intermittent heating or no hot water at all.
  2. Inconsistent Temperature Control: Faulty wire connections can disrupt the communication between the control unit and the water valve, causing inaccurate temperature readings or inconsistent temperature control. This can lead to water that is either too hot or not hot enough.
  1. System Failure: If the wire connections are severely damaged or completely disconnected, the water heater may fail to function entirely. This can result in a complete loss of hot water and require professional repair or replacement.

What To Do:

It is important to regularly check the wire connections between the water valve, control unit, and electrode set of the water heater. Proper and secure wire connections are crucial for the efficient and safe operation of the heater. Here are the key advice to follow while checking the wire connections:

  1. Ensure safety: Before performing any checks or maintenance, make sure to turn off the power supply to the water heater and follow proper safety procedures.
  2. Inspect the connections: Carefully examine the wire connections between the water valve, control unit, and electrode set. Look for any signs of loose or damaged wires, corrosion, or poor connections.
  3. Tighten loose connections: If you notice any loose wire connections, use appropriate tools to tighten them securely. Be careful not to overtighten, as it can damage the wires or components.
  4. Repair or replace damaged wires: If you find any damaged or frayed wires, they should be repaired or replaced immediately. Damaged wires can lead to electrical issues or malfunctions.

11. The Water Heater Is In Solar Mode

Sometimes the temperature in the inlet might cross the set temperature fixed by the water heater. As a result, the burner will not ignite and you will face ignition failure. The LED screen will definitely show the solar mode of the water heater.

What To Do:

You have to make sure that the temperature of the inlet doesn’t cross the fixed temperature set by the heater. If it does consult with an expert who will give you the necessary suggestions in order to fix this problem.

How Do You Reignite The Bosch Tankless Water Heater?

After solving the ignition failure, reignite the Bosch tankless water heater. However, reigniting the water heater can be difficult for some people. That is why, I’ve listed the steps that you can follow to reignite the water heater.

  • Step 1: Before you begin, turn off the main power source of your water heater.
  • Step 2: Turn off the gas supply of your water heater. You can do it by turning the valve of the gas line that is located at the bottom of the water heater. Then wait for 5 minutes because the accumulated gas needs to dissipate. After waiting for 5 minutes, turn on the gas supply.
  • Step 3: Head on to the button panel. Afterwards, press the far left button which will let the gas pass through the pilot light. 
  • Step 4: Then press the far right button which will cause a spark and your burner of the heater will reignite.

How Can You Reset The Bosch Tankless Water Heater?

Well, resetting the Bosch tankless water heater is very easy. Just you can follow the following steps:

  • Step 1: Turn off the main power source of the water heater. 
  • Step 2: Remove the cover on the water heater. 
  • Step 3: After finding the reset button on the center of the thermal cutout, you have to press it for at least 30-40 seconds.
  • Step 4: Then release the reset button and wait for a few minutes. Finally, you can turn on the main power source of the heater and your water heater will be in reset mode.

What Can You Do To Prevent The Ignition Problem?

Maintaining your tankless water heater is really important because it will ensure that the water heater operates efficiently and doesn’t face ignition problems. Here are some tips that you can follow to maintain your Bosch tankless water heater:

air filter of bosch tankless water heater
  • Check the air filter: Most tankless water heaters have an air filter that protects the heater from dust and debris. You have to ensure that you have cleaned the air filter regularly. Also, the air filter is doing its job properly.
  • Inspect the pressure relief valve: This safety mechanism keeps the tankless water heater from building up too much pressure. To make sure it is operating properly, you should check this valve once a year.
  • Thoroughly clean the heat exchanger: Mineral accumulation from hard water may lead the heat exchanger to lose efficiency and shorten its lifespan. In order to avoid this, you should descale the heat exchanger either once or twice a year. This entails cleaning the system with vinegar or a descaling solution.


Why does my Bosch tankless water heater keep shutting off?

The main reason your water heater keeps shutting off is that the heater can’t supply enough hot water per minute. If you are using the shower, doing laundry, or dishwashing at the same time with hot water then your water will cut off.

Why does my Bosch tankless water heater pilot light not stay lit?

The main reason that your Bosch tankless water heater pilot light is not keeping on is because of the damaged or bend thermocouple. The faulty thermocouple will make the pilot light go out every time you try to light it up.

How do I check if the thermocouple is faulty?

The thermocouple is one kind of safety device that ensures that the pilot flame is on and sends a signal to the gas valve to keep the gas flowing. If the thermocouple is faulty, then the gas valve will not let the gas flow and the burner will not ignite.

How can I check the ignition system?

To check the ignition system, first, turn off the gas supply and pilot flame. Then check the electrode sets to see if they’re damaged or not. If the electrode sets are damaged, the ignition system won’t work.


In this troubleshooting guide, I’ve explained everything in detail on how to fix the ignition issues with your Bosch tankless water heater. Start by checking the gas supply and clearing any debris in water-flowing paths. 

Gradually progress to more complex solutions like checking the minimum water flow or inspecting the loose wire connections on the water valve, control unit, and electrode set. If you are looking for more troubleshooting steps, you can check out our Bosch tankless water heater troubleshooting guide.

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