Bosch Natural Gas Tankless Water Heaters | Therm | Greentherm

Many of the questions customers ask regarding operation of this unit can be answered by following the troubleshooting steps as outlined here.

Please always refer to your owner's installation manual for complete details.


Burner does not ignite when hot
 

  • If the display is blank, verify power to outlet. (120VAC/60Hz properly grounded circuit required). Verify that the heater power button is pressed ON.
  • Verify the fuses in the control board are good. To access fuses, the control board must be removed.
  • Make sure cold water inlet connection is plumbed to the right side of heater when facing unit.
  • A minimum of 0.65 gallons per minute (GPM) (2,5 l/m) is required to activate the heater. A quart container should fill in 23 seconds or less, from hot water tap only, to activate heater.
  • Clean inlet filter screen. Inspect the water path for obstructions. Make sure all showerheads, faucet aerators and whole house filters are clear of debris.
  • The heater activates when the water flow through the unit is at or above the required minimum of 0.65 GPM (2,5 l/m). A crossover in the hot and cold plumbing pipes creates back pressure on the water flowing through the heater. Therefore, a higher flow rate than normal is needed to force the heater to activate. To check for a plumbing crossover, shut off the cold water supply to the water heater. Then open all of the hot water taps served by the heater. Wait 10 minutes and check for water flow at each tap. There should be no water flowing. Any continuous flow of water, small or large, indicates a crossover is present and must be corrected. Consult a professional plumber for help in correcting a crossover. Failing single lever faucet mixing valves are common causes of plumbing crossovers.
  • With the power button pressed to OFF and the power supply cord unplugged, remove the unit's front cover. Check wire connections between the water valve, control unit and electrode set.
  • Water heater in solar mode. If inlet water temperatures exceed the water heater's set temperature, the burners will not ignite and the solar mode indicator will show on the display.

 


Water is too hot
 

  • Selected temperature on the unit is too high. To lower output temperature, see chapter 5.4, page 32 of the manual.
  • Clean inlet filter screen.
  • Inspect the water path for obstructions. Make sure all showerheads, faucet aerators and whole house filters are clear of debris.
  • Confirm that the heater's gas type coincides with the type of gas being supplied.
  • Avoid restrictive outlets. Clean all showerheads and faucet aerators. It may be necessary to upgrade to higher flow rate shower heads if allowable by local code.
  • In areas where the water has a high mineral content, periodic descaling may necessary.
  • Ensure the outlet temperature sensor is making contact and firmly mounted on the hot water pipe.

 


Water is not hot enough
 

  • Selected temperature on the unit is too low. To raise output temperature, see chapter 5.4, page 32 of the manual.
  • Clean inlet filter screen. See chapter 6.1, page 34 of the manual to increase flow through the heater.
  • Inspect the water path for obstructions. Make sure all showerheads, faucet aerators and whole house filters are clear of debris.
  • Confirm the heater's gas type coincides with the type of gas being supplied.
  • Check inlet gas particle screen for blockage at gas inlet connection on bottom of unit.
  • Verify gas pressure is in accordance with specifications in chapter 3.13, page 29 of the manual. A gas pressure reading is needed to proceed further.
  • Contact your original installer or a local certified gas technician to obtain this reading.
  • Cold water is mixing into the hot water lines (plumbing crossover). A plumbing crossover can unintentionally mix cold water with the hot water leaving the heater. The end result is a cooler water
  • temperature than desired. To check for a plumbing crossover, shut off the cold water supply to the water heater. Then open all of the hot water taps served by
  • the heater. Wait 10 minutes and check all taps for water flow. There should be no water flowing. Any continuous flow of water, small or large, indicates a crossover and must be corrected. Consult a professional plumber for help in correcting a
  • crossover. Failing single lever faucet mixing valves are common causes of plumbing crossovers.
  • Ensure the temperature sensor is making contact and firmly mounted on the hot water pipe.

 


Low water flow/pressure
 

  • Too many hot water applications are being used simultaneously or too much flow is demanded. The water heater will effectively support two 2.0-2.5 GPM shower heads simultaneously or multiple sink applications. Greater draws will result in a water pressure drop and reduced flow at taps.
  • Ensure that gas pressure is in accordance with specifications in chapter 3.13, page 29 of the manual. A gas pressure reading is needed to proceed further.
  • Contact your original installer or a local certified gas technician to obtain this reading. If gas pressure is inadequate, the water heater will close its motorized water valve, reducing the hot water flow rate in an attempt to reach the selected output temperature.
  • If selected temperature on the unit is set too high for the demanded flow rate, the water heater will close its motorized water valve, reducing the hot water flow rate in an attempt to reach the selected output temperature. Lowering the selected temperature will allow the motorized water valve to open up for increased water flow rate.
  • Clean inlet filter screen.
  • Inspect the water path for obstructions. Make sure all showerheads, faucet aerators and whole house filters are clear of debris.

 


Hot water temperature fluctuates
 

  • Hot water is very hot out of the water heater, requiring mixing in cold water in order to attain a useable hot water temperature. The addition of too much cold will overpower hot water flow from the tankless water heater. If this slows the flow through the tankless water heater below its activation point, it will shut off the burners. The end result is nothing but cold water coming out of the outlet. Consult Service Bulletins for a detailed service bulletin on overcoming temperature fluctuations.
  • Unbalanced pressure in water lines. Any restriction in the water heater, such as a clogged inlet filter screen, can result in unequal pressures between the cold and hot water lines. In such cases, when mixing in the higher pressure cold water at the tap, the lower pressure hot water can be overpowered. This will shut down the burners because the hot water flow rate fell below the minimum flow rate required for activation. Verify inlet filter screen is clean and clear of debris. See chapter 6.1, page 34 of the manual for inlet filter cleaning instructions.
  • Heater deactivated by temperature balancing valves.
  • If the outlet water temperature is set too high, the heater can produce temperatures that are too hot. A temperature balance shower valve will automatically mix in cold water to reduce the hot water temperature. In the event of any temperature instability at a fixture using a temperature balancing valve, refer to the valve manufacturer for instructions on internal adjustment setting. An adjustment should be made to minimize the amount of cold water the valve is adding. Additionally, the temperature setting on the heater can be lowered to prevent the temperature balance valve from mixing in too much cold.
  • Inlet water pressure is erratic due to fluctuating supply water pressure. For installation on a private well system with the use of a pressure tank, the lowest pressure range setting recommended is 40 - 60 psi (2.75 - 4.15 bar). Consult your installer or local plumber for effective ways to maintain constant water pressure to the appliance when on a well system.

 


Noisy burner/heater during
 

  • Sealed combustion leak. Make sure combustion cover is securely fastened. Ensure the exhaust vent adaptor is properly sealed with supplied gasket.
  • Leaky seals create improper combustion resulting in noise.
  • Improper venting. Venting that is unsealed, the wrong material, too big in diameter or too long in run will
  • result in unstable burner flames and noise. Ensure venting is in accordance with specifications in chapter 3.3, page 9 of the manual.
  • Lack of adequate combustion air. Drawing
  • combustion air from a room area of inadequate size will result in unstable burner flames and noise. Improper piping of combustion air to the outside or insufficient space will result in unstable burner flames and noise. Ensure adequate combustion air is
  • provided from the unit in accordance with specifications in Section 3.4, page 20 Combustion Air Requirements of the manual.
  • Cross contamination. Ensure that intake and exhaust terminations maintain required clearances as stated in chapter 3.3, page 9 of the manual. Cross contamination between intake and exhaust may cause unstable burner flames and noise.
  • Lack of gas pressure. Inadequate gas pressure will cause the fuel-to-air mixture (CO2) to be out of adjustment. This will result in unstable burner flames and noise. Ensure gas pressure is in accordance with specifications in Section 3.13, page 29 Measuring
  • Gas Pressure of the manual. A gas pressure reading is needed to proceed further. Contact your original installer or a local certified gas technician to obtain this reading.
  • 6. Verify proper CO2 readings per chapter 6.5, page 36 of the manual. CO2 adjustments must be done by a certified
  • gas technician with a calibrated combustion gas analyzer.