Bosch Tankless Water Heater Won't Ignite

Bosch Tankless Water Heater Won’t Ignite [7 Easy Solutions]

Bosch Tankless Water Heater won’t ignite or come on due to one of the following reasons: 

  • There is no power at the unit
  • Fuse in the control board goes bad
  • Plumb the cold inlet connection wrongly
  • Inlet water filter screen gets clogged
  • Water paths get restricted
  • Plumbing crossover
  • Wire connections come loose

If you want your Bosch Tankless Water to ignite again, you must address and resolve those issues. 

And this troubleshooting guide will help you determine the culprit that causes this issue. So, keep reading to discover the reason and its solution. 

Table of Contents

Bosch Tankless Water Heater Won’t Ignite [7 Easy Solutions]

This chapter will break down 7 common reasons that prevent your Bosch Tankless Water Heater from igniting. 

1. Unit Is Not Receiving Power

The burner of your Bosch Tankless Water Won’t won’t come on if the unit receives no power. 

Generally, the unit won’t get any power if: 

  • You don’t plug the power cord into the electric outlet
  • Faulty electric outlet 
  • Problem with wire connections

How To Fix: 

First off, check the electric outlet and verify there is power at the outlet. 

This unit requires a 120VAC/60Hz properly grounded circuit to power up the water heater. If your water heater doesn’t get sufficient power, you will never turn on the unit. 

If you are electrically inclined, check all the wire connections. Otherwise, it’s preferable to call a certified electrician to check the wire connections professionally. 

2. Fuse In the Control Board Is Bad

Fuse Position

If the fuse in the control board goes blown out, there will be no electric continuity at the unit. Consequently, the burner won’t come on. 

To ignite the burner, you should verify the fuse in the control board is good. Otherwise, you should replace the fuse. 

How To Fix: 

Begin with checking the fuse in the control board. To check the fuse, follow the following procedure: 

  • Disconnect the water heater  from the power supply before removing the front cover.
  • Remove the front cover by removing the two screws at the bottom of the unit.
  • Detach each and every single cable from the control unit.
  • Press the four snaps of the control unit and take the cover out. 
  • Check the fuse in the printed circuit board and ensure it’s in good condition. Otherwise, you must replace it. 
  • Once you check and replace the fuse, reinstall all parts in reverse order. 

3. Plumb the Cold Inlet Connection Incorrectly

Ensure you plumb the cold water inlet connection correctly. Otherwise, you can’t ignite your Bosch Tankless Water Heater. 

When facing the water heater, the ¾” cold water inlet connection should be on the top right side of the unit. If it’s on the left side, detach that connection and plumb it into the right side. 

Lastly, tighten the water connection (cold side) with the support of a wrench. 

4. Inlet Water Filter Screen Gets Clogged

It’s not possible to ignite the burner if the debris or mineral contents clog the inlet water filter screen. 

A blockage in the inlet water filter screen will reduce the water flow rate to prevent the unit from activating. 

How To Fix: 

Clearing the restriction from the inlet water filter screen will fix this issue. To clean this filter, locate its position. 

You can find the inlet water filter screen on the top of the water heater, at the cold water inlet connection. 

Then, close the installer supplied water shut-off valve. Use the discharge drains to purge the water. 

Next, remove the brass hex cap that keeps the filter in place. After that, take the inlet water filter screen out of its place and clean it under running water.  

Once you clean the filter evenly, reinstall it by following the procedure in reverse. 

5. Blockage In The Water Paths

Restrictions in the water paths like the shower head, faucet aerators, and whole house filters can also throw a wrench into the normal operation of the unit. 

If they get restricted, it will affect the water flow rate through the unit. In the worst case, it can even prevent the water heater from igniting. 

How To Fix: 

Insect the water paths for restrictions. Ensure all the shower heads, water faucets, and whole house filters are clear of debris or lime buildup. If needed, clean them to let the water flow through them freely. 

6. Plumbing Crossover

Plumbing crossover means when you plumb the cold connection into the hot outlet or the reverse. 

A crossover in the hot and cold plumbing pipes creates back pressure on the water flowing through the water heater. 

As a consequence, a higher flow rate than normal is needed to force the heater to activate. 

How To Fix: 

Begin with checking for a plumbing crossover. In this case, shut off the cold water supply to the water heater. 

Next, open all the hot water faucets served by the water heater. Wait for 10 minutes and check for water flow at each faucet. There should be no water flow. 

If you notice any continuous flow of water (it may be small or large), I bet a crossover is present there. And you need to correct the crossover. 

I recommend you to consult with a professional plumber for help in correcting the crossover. 

7. Wire Connections Come Loose

If the wire connections between the water valve or water flow sensor, control unit, and electrode sets come loose, you can’t ignite the unit. 

I highly recommend you to check the wire connection between the water valve, control unit, and electrode set. Of course, you must unplug the power cord of the unit before checking those wirings. 

However, you can call a certified electrician to have the wiring checked professionally. 

How Do I Reset My Bosch Tankless Water Heater?

It’s very simple to reset the Bosch Tankless Water Heater compared to other tankless units in the market. Just long-press the power (On/Off) button for 3 seconds and the unit will initiate the startup sequence. In other words, it will reset your tankless unit. 

However, if you own an electric tankless water heater from Bosch, go along with the following procedure to reset the unit: 

  • Turn the power off to the water heater at circuit breakers.
  • Remove the front cover from the unit.
  • Locate the thermal cutouts on the top of each heating module. 
  • The red button on each cutout is the reset button. Press the reset buttons which will reset the unit. 
  • Close the front cover and restore the power to the unit. 

Read more about Bosch Tankless Water Heater Reset Button.


One of the 7 culprits I mentioned throughout this guide is responsible for your Bosch Tankless Water Heater to not ignite.  

Just perform each troubleshooting step and you can easily discover the reason behind this issue. 

However, if you are unable to fix the igniting problem by following this guideline, don’t hesitate to call a qualified plumber. 

You can also choose the below comment box to let me know the problem you are experiencing- in detail. 

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