Directory: Catalog > Water Heaters > Bosch Tankless Water Heaters

Bosch ProTankless™ GWH 425 HN Gas Tankless Water Heater Troubleshooting Guide

Please note: If you are unable to perform the tasks listed below or need additional assistance, please contact your original installer/licensed gas technician. For indoor installation only.

No Spark At Pilot?

1. Verify the ON/OFF button, located behind the clear cover plate, is completely depressed in the ON (I) position. Be sure that the plumbing is piped with the cold water supply on the right and hot out on the left.
2. Close installer supplied cold water shut off valve (if none installed, install before proceeding). Open all hot water taps fed by the heater. Wait 5 minutes and check all taps. Any water running is a sign of a plumbing crossover. Consult a local plumber or service person for help in correcting a plumbing crossover. Visit our Service Agent/Installer Locator.
3. Confirm that there is an adequate flow rate. With the temperature adjustment knob on the unit positioned all the way clockwise, a ½ (.5) gallon per minute (gpm) flow rate is required. When the knob is turned all the way counter clockwise, a flow of 1 gpm is required. To confirm heater activation rate see: TWH G1-06 Overcoming Fluctuating Temperatures.
4. Inspect the water path outside of the heater for obstructions. Make sure all outlets (i.e. showerheads, faucet aerators) and whole house filters are clear of debris. Also, the heater’s inlet filter screen should be inspected and cleaned. See: TWH G1-12 Cleaning Inlet Filter.
5. Verify all wire connections are secure, paying close attention to the wire connection between the piezo electrode and the ignition unit.
6. Check for voltage at the hydro-generator: Disconnect plug connector on red and black wires leading from hydro-generator (approximately 3” from hydro-generator). Verify voltage on hydro-generator side of plug connector with hot water running. The voltage reading should be 1.3VDC or greater. If voltage is lower, then remove and flush the hydro-generator. Reinstall hydro-generator and take another voltage reading. If voltage is 1.3V DC, proceed to next step. If voltage is still low, hydro-generator may be damaged/defective. Order online or contact Technical Support (866) 330-2730 if still under warranty. Please have serial number ready as it will pertain to what style hydro-generator you have.
7. Verify the temperature limit sensors (ECO) and connecting wires are plugged in securely and their terminals are free of corrosion. The terminals can be cleaned with a pencil eraser. The ECO’s are located on the left side of heater below the copper heat exchanger on the hot water outlet pipe and on the upper right side of the heat exchanger.
8. Electronic control box may be damaged or defective. Contact Technical Support (866) 330-2730 for further instructions.

Spark Appears When Hot Water Is Turned On, But Pilot And Burners Will Not Ignite?

1. Verify gas type indicated in the rating sticker located on the cover’s right hand side, coincides with the gas type you are using. NG is a natural gas unit and LP is for liquid propane.
2. If the unit was just installed or the gas lines have been worked on, there may be air in the gas line. Bleed out the air trapped in the gas line by turning the hot water faucet on and off until the pilot flame/burners ignite (Please note: depending on how much air is in the line this could take numerous cycles of turning the faucet on and off).
3. Verify gas supply is on at Natural Gas meter or Propane tank. Make sure all manual gas shut off valves are in the open position. Have a licensed gas technician confirm adequate gas pressure at the inlet tap. See: TWH G1-04 Checking Gas Pressure
4. Confirm wire connection to the pilot valve is secure. Inspect terminals for corrosion. To clean terminals, remove the spade connectors and clean with a pencil eraser.
5. Open a hot water faucet. While sparking, hold a flame from a lit match or lighter in the pilot area. If the pilot successfully lights, the pilot orifice may be dirty or clogged. See: TWH G1-15 Cleaning Spark Pilot Assembly.
6. Electric control box or pilot valve maybe defective (see components diagram in back of manual for location of pilot valve). Verify adequate voltage at the spade connection of the pilot valve. When sparking, voltage should read 1.3VDC. If voltage is proper, pilot valve is defective. If voltage is not proper, electronic control box is defective. Order online or contact Technical Support (866) 330-2730 if still under warranty.

Pilot Lights But Burners Will Not Come On?

1. Verify gas type indicated in the rating sticker located on the cover’s right hand side, coincides with the gas type you are using. NG is a natural gas unit and LP is for liquid propane.
2. Pilot flame should be blue in color and completely engulfing the flame sensor. If not, the pilot orifice may be dirty or clogged. See: TWH G1-15 Cleaning Spark Pilot Assembly.
3. Flame sensor is not recognizing pilot flame. Verify wire connection to the bottom of the flame sensor is secure. If flame sensor is fully engulfed by the pilot flame, the flame sensor may be dirty. Clean entire surface area of flame sensor with a pencil eraser.
4. Inspect the water path outside the heater for obstructions. Make sure all outlets (i.e. showerheads, faucet aerators) and whole house filters are clear of debris. Also, the water heater’s inlet filter should be inspected and cleaned. See: TWH G1-12 Cleaning Inlet Filter.
5. Confirm wire connection to the burner servo valve is secured (see components diagram in the back of manual for location of burner servo valve). Inspect terminals for corrosion. To clean terminals, remove the spade connectors and clean with a pencil eraser.
6. The water valve assembly, which allows gas flow to the burners, requires periodic maintenance (every 2-5 years depending on water quality and use). See: TWH G1-32 Water Valve Rebuild. Purchase water valve rebuild kit to service the water valve. Order online or contact your local wholesaler to order parts.
7. Ignition unit, burner servo valve or flame sensor maybe damaged/defective. Verify adequate voltage at the spade connection of the burner servo valve (see components diagram in the back of manual for location of burner servo valve). After the pilot lights, voltage should read 1.3VDC on the servo valve. If voltage is proper, the servo valve is defective. If voltage is not proper, Ignition unit or the flame sensor is damaged/defective. Order online or contact Technical Support (866) 330-2730 if still under warranty.
   

Main Burners Go Out During Hot Water Use?

1. Hot water flow rate dropping below required activation flow rate. Turn temperature adjustment knob all the way clockwise. Fully open a hot water faucet and fill a quart container. If the container fills in 30 seconds or less, the flow rate (1/2 gallon per minute) is sufficient to activate the water heater. If flow rate is not sufficient, the water heater’s inlet filter should be inspected and cleaned. See: TWH G1-12 Cleaning Inlet Filter.
2. Close installer supplied cold water shut off valve (if none installed, install before proceeding). Open all hot water taps fed by the heater. Wait 5 minutes and check all taps. Any water running is a sign of a plumbing crossover. Consult a local plumber or service person for help in correcting a plumbing crossover. Visit our Service Installer Locator.
3. The minimum water pressure for the home should be 30psi or greater. For installation on a private well system with use of a pressure tank, the lowest pressure range setting recommended is 30-50 psi (2.07-3.45 bar).
4. Hot water is very hot out of the tap, requiring a lot of cold water to be added with it in order to attain a useable hot water temperature. The addition of too much cold will overpower and slow the flow within the tankless heater, decreasing it below activation point, which shuts off the burners. The end result is nothing but cold water coming out of the outlet. See: TWH G1-06 Overcoming Fluctuating Temperatures and TWH G1-07 Lowering Temperatures.
5. The temperature limit sensor (ECO) is tripped. The water heater does not have a thermostat. If the inlet water is preheated, the unit will overheat, stopping the flow of gas. Plumb inlet with cold water only.
6. If the water has a high mineral content, the heat exchanger may be scaled internally. This restricts the water path, causing the water to overheat and trip the temperature limit sensor (ECO) shutting all gas flow. To descale the heat exchanger, see: TWH G1-21 Removing and Replacing Heat Exchanger.
7. The water valve may be struggling to keep the gas valve open. The water valve requires periodic maintenance (every 2-5 years depending on water quality and use). See: TWH G1-32 Water Valve Rebuild. Purchase water valve rebuild kit to service the water valve. Order online or contact your local wholesaler to order parts.

Hot Water Temperature Fluctuates At Tap?

1. Hot water flow rate diminished below activation rate and has caused the unit to deactivate---see next step. Additionally, review symptom: Main burners go out during hot water use.
2. Unbalanced pressure in the water lines. The added restriction caused by the AquaStar in the hot water system can result in uneven pressures between the cold and the hot water supplies. In such cases, when mixing cold water at tap, the lower hot water pressure will be overpowered by the cold water pressure, which can cause the AquaStar burners to shut down. Make sure faucet aerators and showerheads are free of minerals and dirt. Do not add any flow restrictor to a shower head. Also clean the AquaStar's inlet filter screen. See: G1-12 Cleaning inlet filter screen. Also, see step 3.
3. If the AquaStar's burners remain on with hot water flow but the temperatures fluctuate then the water pressure or gas pressure may be unstable. The AquaStar 125HX is designed to modulate gas flow to the burners when water flow varies, however if the water pressure in the home is erratic and the water flow is not consistent while a tap is opened, then the temperature of hot water will swing. The gas pressure also must be stable and adequate. Check gas pressure at inlet tap on unit. See: G1-04 Measuring inlet gas pressure.
4. If the inlet water temperature is not steady then the hot water temperature from the AquaStar will fluctuate. The 125HX modulates its burners when the flow rate is changed. It does not have a thermostat and so will not adjust to noticeable changes in inlet water temperature.

Water Is Too Hot?

1. Verify gas type indicated in the rating sticker located on the cover’s right hand side, coincides with the gas type you are using. NG is a natural gas unit and LP is for liquid propane.
2. The temperature adjustment knob maybe set too high. Adjust the temperature adjustment selector and test the water temperature. This dial turns counterclockwise for lower temperatures and clockwise for higher temperatures. If water is still too hot, adjust the gas control slide to the left for lower temperatures.
3. Inlet water is preheated. This model does not have a thermostat and will not modulate flame height in order to prevent overheating. It is designed for a cold water feed only. Replumb the heater with cold water to the inlet.
4. Increase flow rate. Restrictions in the water path can slow the flow of water through the heater, resulting in very hot outlet temperatures. Restrictive showerheads and faucet aerators should be cleaned out or upgraded with less restrictive ones. In addition, the inlet filter on the water heater should be cleaned and inspected. See: TWH G1-12 Cleaning Inlet Filter.
5. If water is still too hot, follow further steps to lower outlet temperatures. See: TWH G1-07 Lowering Temperature.
6. If the water has a high mineral content, the heat exchanger may be scaled internally. This restricts the water path, causing the water to boil and produce extremely hot temperatures. See: TWH G1-21 Removing and Replacing Heat Exchanger.

Water Is Not Hot Enough?

1. Verify gas type indicated in the rating sticker located on the cover’s right hand side, coincides with the gas type you are using. NG is a natural gas unit and LP is for liquid propane.
2. Water flow at one particular tap is too great or too many fixtures are running at one time. Lower the flow to stay within heater’s specifications.
3. The temperature adjustment knob maybe set too low. Raise the setting and test the water temperature. This dial turns counterclockwise for lower temperatures and clockwise for higher temperatures.
4. Close installer supplied cold water shut off valve (if none installed, install before proceeding). Open all hot water taps fed by the heater. Wait 5 minutes and check all taps. Any water running is a sign of a plumbing crossover. Consult a local plumber or service person for help in correcting a plumbing crossover. Visit our Service Installer Locator.
5. Inspect the water path outside of the heater for obstructions. Make sure all outlets (i.e. showerheads, faucet aerators) and whole house filters are clear of debris. Also, the heater’s inlet filter screen should be inspected and cleaned. See: TWH G1-12 Cleaning Inlet Filter.
6. The water valve may be struggling to fully open the gas valve. The water valve requires periodic maintenance (every 2-5 years depending on water quality and use). See: TWH G1-32 Water Valve Rebuild. Purchase water valve rebuild kit to service the water valve. Order online or contact a wholesaler to order parts.
7. Gas pressure is too low. Ensure gas pressure is in accordance with specifications in the installation manual. A gas pressure reading is needed to proceed further. Contact your original installer or a local certified gas technician to obtain this reading. See: TWH G1-04 Checking Gas Pressure.
   

Burners Do Not Burn Cleanly; Yellow Flames When Operating?

1. Verify gas type indicated in the rating sticker located on the cover’s right hand side, coincides with the gas type you are using. NG is a natural gas unit and LP is for liquid propane.
2. Gas pressure may be inadequate resulting in improper combustion. Ensure gas pressure is in accordance with specifications in the installation manual. A gas pressure reading is needed to proceed further. Contact your original installer or a local certified gas technician to obtain this reading. See: TWH G1-04 Checking Gas Pressure.
3. For proper combustion at high elevations the inlet gas pressure should be set at installation to the value shown below (while operating at maximum BTU). A gas pressure reading is needed to proceed further. Contact your original installer or a local certified gas technician to make this adjustment.
   
   

   Altitude	Natural Gas Inches W.C.	Liquid Propane Inches W.C.
   0 ft - 2,000 ft	5.7"	10.5"
   2,000 ft - 4,500 ft	4.6"	8.4"

Above 4,500 ft consult your local gas provider to see if adjustment is needed.

4. Confirm the clearance requirements around the heater are being met in accordance with specifications in the installation manual. Obstructing the venting and/or the combustion air supply to the heater may result in improper combustion and yellow flames.
5. The GWH-425HNO has burners much like a furnace. It is essential that they remain clean and are supplied with adequate air for combustion. If the flames burn yellow or orange it is strongly recommended that the burners be removed from the unit and cleaned. Once the burners are removed, the fins in the top of the heat exchanger can be accessed and brushed or vacuumed clean. See: TWH G1-13 Removing and Cleaning Burners.

Burners ignite without water flow?

1. The water valve may be dirty and sticking, prohibiting it from fully closing the gas valve. The water valve requires periodic maintenance (every 2-5 years depending on water quality and use). See: TWH G1-32 Water Valve Rebuild. Purchase water valve rebuild kit to service the water valve. Order online or contact your local wholesaler to order parts.

Low hot water pressure?

1. Confirm adequate flow rate through the unit. With cold supply shut off to the unit, disconnect hot outlet side to the unit. Place a gallon jug under hot water outlet to catch water. Open cold water supply and measure flow rate out of the unit. With temperature adjustment knob set all the way clockwise, the flow rate should be close to 2GPM (filling the jug in 15 seconds). If the flow rate is not adequate, inspect and clean inlet filter screen. See: TWH G1-12 Cleaning Inlet Filter.
2. Confirm the incoming water pressure to the unit is above the required 30psi.
3. Inspect the water path outside the heater for obstructions. Make sure all outlets (i.e. showerheads, faucet aerators) and whole house filters are clear of debris.
4. Be sure to run only one major fixture at a time with this water heater. Opening too many taps at one time can disperse water flow resulting in diminished flow at all outlets.
5. If the water has a high mineral content, the heat exchanger may be scaled internally. This restricts the water path, causing diminished water flow. See: TWH G1-21 Removing and Replacing Heat Exchanger.
   

Noise when the heater is running?

1. Confirm that there is an adequate flow rate. With the temperature adjustment knob on the unit positioned all the way clockwise, a ½ (.5) gallon per minute (gpm) flow rate is required. When the knob is turned all the way counter clockwise, a flow of 1 gpm is required. See: TWH G1-06 Overcoming Fluctuating Temperatures.
2. Inspect the water path outside the heater for obstructions. Make sure all outlets (i.e. showerheads, faucet aerators) and whole house filters are clear of debris. Restrictions in the water path can slow water flow through the heat exchanger causing it to boil and make noise.
3. If the water has a high mineral content, the heat exchanger may be scaled internally. This restricts the water path, causing the water to boil resulting in noise and overheating. See: TWH G1-12 Removing and Replacing Heat Exchanger.

   

   Information Source: Bosch Water Heating