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Bosch ProTankless™ GWH 425 EF Gas Tankless Water Heater

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 switch located behind the flip down cover plate is set to the ON position.
   
2. Verify power to the outlet where the unit is plugged in (120VAC/60Hz).
3. Confirm the cold water supply is connected to the right side of heater
4. Close installer supplied cold water shut off valve (if none installed, install before proceeding). Open all hot water taps supplied by the heater. Wait 5 minutes and check all taps. Any water running is a sign of a plumbing crossover. Consult local plumber or service person for help correcting a plumbing crossover.
5. Verify flow through the water heater is sufficient. 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 gallons per minute) is sufficient to activate the water heater.
6. If flow rate is not sufficient, 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.
7. Microswitch may be out of adjustment and may need calibration. See: TWH G1-14 Microswitch Calibration.
8. Confirm wire connections on the ECO/overheat sensor (located on the hot water outlet pipe) and the flue gas sensor (located on the right hand side of draft diverter) are secure. Inspect terminals for corrosion. To clean terminals, remove the spade connectors (note: flue sensor spades not removable) and clean with a pencil eraser.
9. The water valve assembly, which actuates the microswitch for ignition, requires periodic maintenance (every 2-5 years depending on water quality and use). Purchase water valve rebuild kit to service the water valve.
   

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. Confirm wire connection to the pilot electrovalve is secured (see components diagram in the back of manual for location of pilot electrovalve). Inspect terminals for corrosion. To clean terminals, remove the spade connectors and clean with a pencil eraser.
4. 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.
5. 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.
6. If gas is not present, verify manufacturer supplied Maxitrol regulator is in the upright position. The arrow on the back of the regulator should point in the same direction as gas flow.
7. Electric control box or pilot electrovalve maybe defective (see components diagram in back of manual for location of pilot electrovalve) Verify adequate voltage at the spade connection of the pilot electrovalve. When sparking, voltage should read 1.3VDC. If voltage is proper, pilot electrovalve is defective. If voltage is not proper, electronic control box is defective. Contact Technical Support 866-330-2730 for replacement part.
   

Pilot Sparks Continuously With No Water Running?

1. If no water is running, but the pilot electrode continuously sparks, then the micro- switch is out adjustment and needs to be calibrated. See: TWH G1-14 Microswitch calibration.

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: G1-15 Cleaning Spark Pilot Assembly.
3. Flame sensor 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, it may be dirty. Clean entire surface area of flame sensor with a pencil eraser. (Improper venting can be a cause of a dirty flame sensor. Ensure the venting is in accordance with requirements in the installation manual.)
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 electrovalve is secured (see components diagram in the back of manual for location of burner electrovalve). 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). Purchase water valve rebuild kit to service the water valve.
7. Electric control box, burner electrovalve or flame sensor maybe damaged/defective. Verify adequate voltage at the spade connection of the burner electrovalve (see components diagram in the back of manual for location of burner electrovalve). After the pilot lights, voltage should read 1.3VDC on the burner electrovalve. If voltage is proper, the burner electrovalve is defective. If voltage is not proper, electronic control box or the flame sensor is damaged/defective. Contact Technical Support (800) 642-3111 for replacement parts.

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: 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 supplied by the heater. Wait 5 minutes and check all taps. Any water running is a sign of a plumbing crossover. Consult local plumber or service person for help correcting a plumbing crossover.
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 Temperature.
5. The overheat 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. The 125BS model may be used for certain solar preheated applications. See: 125BS Manual (English PDF).
6. Flue gas sensor is tripped. Lack of adequate combustion air or failure to vent properly by reducing pipe diameter, improper use of elbows or exceeding maximum vent length will interrupt the flue gas sensor circuit. Confirm venting and combustion air is in accordance with requirements in the installation manual.
7. 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 overheat sensor (ECO) shutting off all gas flow. Descale the heat exchanger- See: TWH G1-21 Removing and Replacing Heat Exchanger.
8. A water valve, which has not been serviced, 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). Purchase a water valve rebuild kit to service the water valve.
   

Hot Water Temperature Fluctuates At Tap?

1. Close installer supplied cold water shut off valve (if none installed, install before proceeding). Open all hot water taps supplied by the heater. Wait 5 minutes and check all taps. Any water running is a sign of a plumbing crossover. Consult local plumber or service person for help correcting a plumbing crossover.
2. Check for a clogged inlet water filter screen. See: TWH G1-12 Cleaning Inlet Filter.
3. 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 Temperature.
4. The water heater is designed to modulate gas flow to the burners when water flow varies. 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 the hot water will fluctuate. The minimum water pressure for the home should be 30psi or greater. For installations on a private well system with the use of a pressure tank, the lowest pressure range setting recommended is 30-50psi (2.07-3.45 bar). The use of a pressure reducing/regulating valve before the water heater (directly after the pressure tank on well systems) is an effective way to maintain constant water pressure to the water heater. Watts brand 25AUB-3/4” or N35B-3/4” pressure reducing/regulating valves or equivalent are suggested.
5. If the inlet water temperature is not steady then the hot water temperature from the water heater will fluctuate. The water heater modulates its burners when flow rate is changed based on a steady inlet water temperature. It does not have a thermostat and will not adjust to changes in inlet water temperature. Water lines that run through hot attic spaces may create variable temperatures in the plumbing.
6. If the inlet water temperature to the water heater is above 70?F, the heater may produce outlet temperatures that are too hot for most users. A temperature balance shower valve can automatically mix in cold water to reduce such hot water temperature. This can slow the flow within the tankless water heater, decreasing it below activation point, which shuts off the burners. In the event of any temperature instability with the use of a temperature balance shower valve, refer to shower valve manufacturer instructions for internal adjustment setting. An adjustment should be made to minimize the amount of cold water the valve is adding.
7. The gas pressure also needs to be stable and adequate. 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.

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. Lower the setting and test the water temperature. This dial turns counterclockwise for lower temperatures and clockwise for higher 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. For solar preheated applications, use of the model 125BS is recommended. See: 125BS Manual (English PDF).
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 Temperatures.
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. To descale the heat exchanger, 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. Flow at one particular tap is too great or too many fixtures are running at one time. Lower 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 supplied by the heater. Wait 5 minutes and check all taps. Any water running is a sign of a plumbing crossover. Consult local plumber or service person for help correcting a plumbing crossover.
5. 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.
6. The water valve may be dirty, prohibiting it from fully opening the gas valve. The water valve requires periodic maintenance (every 2-5 years depending on water quality and use). Purchase a water valve rebuild kit to service the water valve.
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.

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 adjusted 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 obtain this reading.
   

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

4. Ventilation is not adequate. Ensure adequate combustion air is being supplied to the unit in accordance with requirements in the installation manual.
5. Verify the water heater is vented properly. Ensure the venting is in accordance with requirements in the installation manual.
6. The AquaStar 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. This part needs to be periodically serviced (every 2-5 yrs depending on water quality and use). Purchase a water valve rebuild kit to service the water valve.

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/pressure 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.

Noise when the heater is running?

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 G12 Cleaning Inlet Filter.
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.

Information Source: Bosch Water Heating