Bosch tankless water heater pilot light problems can be frustrating at times. But don’t worry; I’m here to help. Here are some common pilot light problems:
- The pilot does not light.
- The pilot light goes out when the button is released.
- The pilot goes out during or after hot water use.
To troubleshoot these problems, you will need to find out the reasons first. The reasons can be due to inadequate gas supply, wrong gas type, improper air combustion, sparks not being thrown at the pilot area properly, etc. So to troubleshoot these problems easily, I recommend you go through our guide.
Bosch Water Heater Quick Troubleshooting Table For Pilot Light Problems
Issues | Reasons | Solutions |
The Pilot Goes Out Upon Button Release | # Faulty thermocouple # Weak or unstable pilot flame # Issues with other components in the pilot safety circuit # Incorrect gas type # Insufficient time holding down the gas control slide button | # Check the thermocouple and replace it if broken. Ensure it generates an electrical signal and senses the pilot flame correctly. # Clean or unclog the pilot assembly. Ensure proper gas flow and adjust for stability. # Inspect and clean sensor connections. Verify functionality and replace faulty components. # Verify and use the correct gas type indicated on the rating sticker. # Fully depress and hold the gas control button for at least 20 seconds after lighting the pilot. |
Pilot Fails To Light | # Problems with the gas supply # Air in the gas line # Issues with the ignition system # Blocked pilot assembly | # Check gas supply and open manual shut-off valves. Ensure sufficient gas pressure. # Hold down the gas control slide button and repeatedly hit the pilot igniter to release the trapped air. # Check for a spark at the pilot area and ensure proper electrode connection. # Shut off the gas supply. Remove and clean the pilot orifice. Reinstall and test the gas flow. Replace the orifice if necessary. |
The Pilot Goes Out During Or After Using Hot Water | # Overheating from preheated water # Improper venting # Insufficient combustion air # Delayed burner shutdown # Faulty connections in the pilot safety circuit # Inadequate pilot flame # Internal scaling in heat exchanger # Inadequate gas pressure | # Use only cold water for the inlet to prevent overheating. # Ensure proper venting according to the manufacturer’s specifications. # Provide adequate airflow for proper combustion. # Ensure burners shut off immediately after use. # Inspect and clean connections in the circuit. # Clean pilot orifice for a better flame. # Descale heat exchanger as directed. # Verify and adjust gas pressure if needed. |
Table of Contents

Bosch Tankless Water Heater Pilot Light Problems
A malfunctioning pilot light can be a frustrating issue to deal with. However, with proper troubleshooting, you can often identify and resolve the problem. Here are some common Bosch tankless water heater pilot light problems that you could encounter along with their solutions:
01. Pilot Fails To Light
There are a number of reasons why a Bosch tankless water heater’s pilot will not light. They include problems with the gas supply, air in the gas line, or issues with the ignition system.
i) Problems with the gas supply:
The water heater may not receive gas if the gas supply has been cut off at the natural gas meter or propane tank. If the manual gas shut-off valves are closed, it restricts the flow of gas to the water heater. Insufficient gas pressure at the inlet tap can prevent the pilot from lighting.
ii) Air in the gas line:
After a new installation or work is done on the gas lines, there may be air trapped in the gas line, hindering the flow of gas to the pilot.
iii) Problems with the ignition system:
When the gas control slide button is pressed, there should be a spark in the pilot area. If there is not one, the ignition system is malfunctioning. Also, if the spark electrode is not properly connected, it can prevent the ignition system from generating a spark to light the pilot.
iv) Blocked pilot assembly:
The issue of the pilot not lighting on the Bosch tankless water heater could be due to a blocked pilot assembly.
How To Fix:
Check if the gas supply is turned on at the Natural Gas meter or Propane Tank. Ensure that all manual gas shut-off valves are in the open position. You can follow the guide on how to check gas pressure at inlet taps in the later part of this article.
If there is no gas present, verify the position of the Maxitrol regulator. The arrow on the back of the regulator should point in the same direction as the gas flow. Insufficient gas pressure can prevent the pilot from lighting.
Hold down the gas control slide button while repeatedly hitting the pilot igniter button to release air from the line. It may take several minutes to completely bleed the air out.
Ensure that a spark is being thrown at the pilot area when the gas control slide button is centered in the single flame position and firmly depressed. Repeatedly push the pilot igniter button to ignite the pilot. If there is no spark, check the wire connection to the electrode to ensure it is properly connected.
The pilot orifice may be clogged with dirt, debris, or mineral deposits, preventing the gas from flowing properly. Here is a step-by-step guide to address the issue of a blocked pilot assembly on your Bosch tankless water heater:
Step 1: Shut off the gas supply
Locate the manual gas shutoff valve. Turn it to the “Off” position to cut off the gas supply to the water heater. Ensure that the gas is completely shut off before proceeding.
Step 2: Take off the pilot gas tubing
Find the pilot gas tubing mounted in the center of the burner assembly. It is a small tube that supplies gas to the pilot assembly. At the base of the pilot gas tubing, you will see a retaining clip. Release the pilot gas tubing from this clip to detach it from the burner mount.
Step 3: Remove the pilot orifice
Near the base of the pilot gas tubing, you will find a brass hex-headed pilot orifice connected to the gas valve. Use an appropriate-sized wrench or tool to carefully unscrew and remove the pilot orifice from the gas valve.
Step 4: Soak the orifice and pilot gas tubing
Place the removed pilot orifice and pilot gas tubing in a container filled with carburetor cleaner. Allow them to soak in the cleaner for approximately 30 minutes. This will help dissolve any dirt, debris, or mineral deposits clogging the orifice and tubing.
Step 5: Dry off and reinsert the orifice
After soaking, remove the pilot orifice and pilot gas tubing from the carburetor cleaner and thoroughly dry them off. Ensure that they are completely dry before reinserting the orifice back into the gas valve. If your heater uses liquid propane (LP) as the gas type, replace the orifice with the appropriate one for LP. Follow the removal instructions in reverse to reinstall the pilot gas tubing.
Step 6: Open the gas supply and test
Once the pilot gas tubing is reinstalled, open the gas supply by turning the manual gas shutoff valve to the “On” position. Verify that the gas is flowing correctly. Once you have confirmed that the gas supply is restored, you can return the heater to service. If cleaning the pilot orifice does not resolve the issue, it may be necessary to replace the orifice.
02. The Pilot Goes Out Upon Button Release
If the pilot light keeps going out on your water heater upon button release, it indicates a potential problem. Here are some possible causes and solutions:
i) Faulty thermocouple:
The thermocouple may be broken or positioned incorrectly. That prevents it from sensing the pilot flame and generating the necessary electrical signal to keep the gas valve open. A faulty thermocouple can result in the pilot flame going out when the button is released.
ii) Weak or unstable pilot flame:
If the pilot flame is weak or unstable, it may not produce enough heat to keep the thermocouple active. This can be caused by a dirty or clogged pilot assembly.
iii) Issues with other components in the pilot safety circuit:
The pilot safety circuit consists of various components, such as the overheat sensor (ECO), flue gas sensor, safety spillage switch (for models with power vents), and electromagnet. If any of these components are faulty or not functioning correctly, they can interrupt the pilot flame when the button is released.
iv) Incorrect gas type:
Using the wrong gas type for your Bosch water heater can result in it not producing hot water. It is crucial to verify that the gas type matches the one supplied before installation.
v) Insufficient time holding down the gas control slide button:
If the gas control slide button is not fully depressed and held down for a sufficient amount of time after lighting the pilot, the thermocouple may not receive enough heat to generate the necessary electrical signal. This can cause the pilot flame to extinguish when the button is released.
How To Fix:
When lighting the pilot, ensure the gas control slide button is fully depressed and held down for at least 20 seconds after the pilot is lit. This allows the thermocouple to heat up sufficiently and generate the necessary electrical signal to keep the pilot flame ignited.
Verify that the gas type is indicated in the rating sticker located on the right-hand side of the cover. Make sure it matches the gas type you are using. NG stands for natural gas, and LP is for liquid propane.
Check all connections of the pilot safety circuit that I previously mentioned. If corrosion is evident, clean the sensor connections using light sandpaper. The electromagnet connection can be tightened by turning the 5mm nut from the thermocouple, which screws into a larger 17mm nut. Ensure the nuts are snug but not over-tightened.
Contact a licensed gas technician to measure and adjust the gas pressure according to the manufacturer’s specifications. This will ensure a stable and properly sized pilot flame. Clean or replace the pilot orifice if it’s dirty or bad to get a clear pilot flame.
Finally, if the millivoltage reading from the thermocouple is below 24mVDC, it indicates a potential defect. Contact Bosch support for further instructions on how to proceed. They will provide guidance on troubleshooting or arrange for a replacement thermocouple if necessary.
03. The Pilot Goes Out During Or After Using Hot Water
Here are some reasons and problems why the pilot light goes out during or after hot water use:
i) Overheating due to preheated inlet water:
If the water entering the heater is already hot, it can cause the unit to overheat and trigger the safety overheat/high-limit sensors. This can result in the pilot going out to prevent further overheating.
ii) Improper venting:
Incorrect venting, such as using incorrect pipe diameter, improper placement of elbows, or not meeting the required vent length, can lead to poor airflow and improper combustion. This can cause the pilot to go out during the operation.
iii) Insufficient combustion air:
If the combustion air requirements are not met, it can result in an inadequate oxygen supply for combustion. This can lead to a weak or unstable pilot flame, which may go out during or after hot water use.
iv) Delayed burner shutdown:
If the burners in the water heater do not shut down immediately after hot water use then it can cause problems. It will overheat the sensor (ECO) to activate and interrupt the pilot circuit. This can result in the pilot flame going out.
v) Faulty connections in the pilot safety circuit:
Corrosion or loose connections in the thermocouple, overheat sensor (ECO), flue gas sensor, or electromagnet can disrupt the pilot safety circuit. This can lead to the pilot flame going out during or after hot water use.
vi) Inadequate pilot flame:
If the pilot flame is too small or weak, it may not provide enough heat to keep the thermocouple heated and generate the necessary electrical signal. This can cause the pilot to go out when the water heater is in use.
vii) Internal scaling in the heat exchanger:
High mineral content in the water can lead to scaling inside the heat exchanger. This can restrict the water flow, causing the water to overheat and triggering the safety mechanisms, resulting in the pilot going out.
viii) Inadequate gas pressure:
Insufficient gas pressure can lead to an unstable or weak pilot flame. If the gas pressure is too low, the pilot may not receive enough fuel to stay lit during or after hot water use.
How To Fix:
Plumb the inlet with a cold water line only to prevent preheated water from causing the unit to overheat. Also, Confirm that the venting system meets the manufacturer’s specifications regarding pipe diameter, elbow use, and vent length. Proper venting ensures the safe and efficient operation of the water heater.
Follow the guide for providing adequate combustion air to the water heater in the later part of this article. Also, remember that proper airflow will help maintain a proper draft and prevent issues with the pilot light. Confirm that the burners shut off immediately when hot water is turned off.
Clean the water valve assembly periodically to ensure its proper functioning. Here’s a step-by-step guide on how to clean the filter in the water valve assembly of your Bosch tankless water heater:
Step 1: Shut off the installer-supplied cold water isolation valve to the heater. If there isn’t one already installed, make sure to install it before proceeding. Open the nearest hot water tap to drain the plumbing lines. This will help relieve pressure and allow any remaining water to drain out. Position a bucket or container under the water valve assembly of the heater to catch any water that may drain from it during the process.
Step 2: Locate the inlet water pipe connected to the back of the water valve. Disconnect this pipe carefully to separate it from the water valve. Next, locate the water pipe leading out of the water valve to the heat exchanger. Remove the retaining pin on the water valve assembly and pull the pipe free from the water valve.
Step 3: Look for two set screws at the bottom of the water valve where it connects to the gas valve. Loosen these set screws. If the set screws are seized or difficult to loosen, it is important to stop and call Bosch tech support before proceeding. Forcing or snapping seized set screws may cause irreversible damage to the water valve and gas valve assemblies. Once the set screws are loosened, you can remove the water valve by pulling it to the right.
Step 4: Separate the two halves of the water valve by removing the five screws located on the left side of the water valve. This will allow you to access the internal components for cleaning. Apply lithium or faucet & valve grease to the pushrod and o-ring. This helps ensure smooth operation and proper sealing of the water valve assembly. Take a moment to inspect the venturi, which is a component inside the water valve. Clean the venturi if necessary, using a soft brush or cloth.
Step 5: Once all required parts have been thoroughly cleaned and inspected, reassemble the water valve. Lining up the two halves and fastening them together with the five screws that you previously removed. Reconnect the water pipe leading to the heat exchanger by inserting it into the water valve and securing it with the retaining pin. Reconnect the inlet water pipe to the back of the water valve. Tighten the two set screws at the bottom of the water valve, ensuring a secure connection to the gas valve.
Step 6: Close the hot water tap that was opened earlier to drain the plumbing lines. Open the installer-supplied cold water isolation valve to allow water to flow back into the water heater. Check for any leaks around the water valve assembly and ensure everything is properly sealed.
Inspect all connections of the pilot safety circuit, including the thermocouple, overheat sensor (ECO), flue gas sensor, and electromagnet. Clean any corroded sensor connections with light sandpaper. Ensure a secure connection between the electromagnet and the thermocouple.
Make sure the pilot flame fully engulfs the thermocouple tip. If the pilot flame is too small, clean the pilot orifice to improve its size and quality. If the water has a high mineral content, scaling may occur in the heat exchanger, restricting water flow and causing overheating. Follow the instructions provided by the manufacturer for descaling the heat exchanger to restore proper operation.
Finally, if the above steps do not resolve the issue, verify that the gas pressure is within the manufacturer’s specifications. Contact your original installer or a licensed gas technician to obtain a gas pressure reading and make any necessary adjustments.
What Are The Combustion Air Requirements For The Bosch?
The combustion air requirements can vary depending on the model. However, I have made a simple guide for Bosch Pro heaters so that you can easily understand it.
i) Determine the Type of Installation:
If the water heater is installed in an unconfined space (greater than 50 cubic feet per 1000 Btu/h), follow the instructions for combustion air in unconfined spaces. If the water heater is installed in a confined space, follow the instructions for combustion air in confined spaces.
- Determine the volume of the unconfined space. If the structure has been tightly constructed (air infiltration rate of 0.40 ACH or less), consult an HVAC specialist to ensure proper combustion air supply.
- The confined space must have two permanent openings—one starting within 12 inches from the top and the other within 12 inches from the bottom of the enclosure.
ii) Calculate Free Area:
Each opening should have a minimum free area of one square inch per:
- 1000 Btu/hr if all air is taken from inside the building.
- 2000 Btu/hr if all air is taken from the outside through horizontal ducts.
- 4000 Btu/hr if all air is taken from the outside through direct openings or vertical ducts.
iii) Consider Louver Blockage:
Louvers, grills, and screens may block airflow. Increase the sizes of the openings by 400% for wood louvers or 135% for metal louvers if the effective free area is unknown. If the confined space cannot accommodate two openings, it should have one permanent opening or duct within 12 inches of the ceiling.
iv) Calculate Free Area:
The opening or duct should have a minimum free area of one square inch per:
- 3000 Btu/hr if all air is taken from the outside through a direct opening or vertical duct.
v) Consider Louver Blockage:
Apply the same adjustments as mentioned in step 3a if the effective free area is unknown. Louvers, grills, and screens reduce the effective free area. If the free area is unknown, increase the sizes of the openings by 400% for wood louvers or 135% for metal louvers. In tightly constructed buildings, it is recommended to take all combustion air from the outside.
vi) Consult an HVAC Specialist:
If you are unsure about the adequacy of the combustion air supply or need assistance with providing more combustion air, consult an HVAC specialist. They can assess the situation and provide guidance on ensuring proper combustion air for the Bosch PRO water heater.
How To Measure Gas Pressure At Inlet Taps?
Measuring the gas pressure at inlet taps is not that easy. Therefore, I recommend that you go through the below steps:
01. Shut off the Gas:
Ensure that the gas supply to the water heater is completely shut off before proceeding with the measurement. Remove the front cover of the water heater according to the manufacturer’s instructions. Look for the inlet gas pressure measuring point on the right side of the unit.
02. Loosen the Screw and Connect Manometer Tube:
Locate the test point on the right side of the water heater. Loosen the screw from the test point to create an opening. Connect the tube of the manometer securely to the test point.
03. Gas Pressure Measurement:
Make sure the manometer is properly connected and ready for measurement.
04. Static Pressure Test:
Turn the gas supply back on. Observe and record the static gas pressure reading from the manometer. This measurement represents the gas pressure when there is no flow or demand.
05. Operating Pressure Test:
All of the water heater’s hot water faucets should be turned on. Open the taps fully to create a high hot water flow rate. Set the flow control knob of the water heater to its maximum (fully clockwise) position. Set the slide control of the water heater to the right position. Observe and record the operating gas pressure reading from the manometer. This measurement represents the gas pressure under normal operating conditions.
06. Record Measurements:
Note down the static gas pressure and operating gas pressure readings. You can note it in the designated space provided on the back page of the manual or any other appropriate location for future reference.
How Do You Reignite The Bosch Tankless Water Heater?
To reignite a Bosch tankless water heater, follow these steps:
- Turn off the main power source and gas supply of the water heater.
- Wait for 5 minutes to allow any accumulated gas to dissipate.
- Press the button on the panel to allow gas to pass through the pilot light.
- Press the spark button to ignite the burner.
Follow this Bosch tankless water heater ignition troubleshooting guide to gain more information that will help you reignite the Bosch tankless water heater and restore the hot water supply.
How To Reset The Gas Water Heater After Troubleshooting?
To reset a gas-powered Bosch tankless water heater, look for a dedicated reset button labeled “RESET” on the front of the unit. Or you can use the power button.
Press and hold the reset button or power button for more than 3 seconds until the reset process begins. Release the button once the start-up sequence is complete, and the water heater should be reset and ready to operate.
FAQ:
How often should I clean or replace the pilot orifice?
Cleaning or replacing the pilot orifice may be necessary if the pilot flame is too small. The frequency of maintenance depends on factors such as water quality and usage. As a general guideline, periodic maintenance every 2-5 years is recommended.
Why is it important to check the pilot light on a tankless water heater?
Checking the pilot light ensures that the water heater is functioning properly and ready to provide hot water when needed. A properly lit pilot light indicates that the gas supply is working and the ignition system is operational.
How can a tankless water heater’s pilot light be checked?
To check the pilot light on a tankless water heater, turn the knob to “Pilot,” push it down, and look for a flame. If it’s lit, you’re good to go.
Why does my Bosch tankless water heater keep shutting off?
The main reason your water heater keeps shutting off is that it can’t supply enough hot water per minute. Because if you are using the shower, doing laundry, or dishwashing at the same time with hot water, then your water will cut off.
Conclusion
In conclusion, Bosch tankless water heaters may occasionally experience pilot light problems. Some common issues include the pilot light not igniting or going out frequently. These problems can often be resolved by ensuring proper gas supply, checking for clogs or obstructions in the pilot assembly, and verifying the functionality of safety devices.
Also, to solve the other issues with your water heater, you can check out our detailed solution guide on Bosch tankless water heater troubleshooting for both gas and electric. We will be happy to see you there!