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Bosch Model GWH 425 User Manual

Indoor model flow modulated with standing pilot suitable for heating potable water only -
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GWH 425 PN
Flow Modulated with Standing pilot
Suitable for heating potable water only - Not approved for space heating purposes
(Intended for variable flow applications with steady cold water inlet temperatures)
Warning: If the information in this manual is not
followed exactly, a fire or explosion may result
causing property damage, personal injury or death.
Do not store or use gasoline or other flammable
vapor and liquids in the vicinity of this or any other
service or maintenance can cause injury or
property damage. Refer to this manual. For
assistance or additional information consult a
qualified installer, service agency or the gas
In the Commonwealth of Massachusetts this
product must be installed by a licensed plumber or
gas fitter.
Upon completion of the installation, these
instructions should be handed to the user of the
appliance for future reference.
What to do if you smell gas
• Close gas valve. Open windows.
• Do not try to light any appliance
• Do not touch any electrical switch; do not use any
phone in your building.
• Immediately call your gas supplier from a neighbor's
phone. Follow the gas supplier's instructions.
• If you cannot reach your gas supplier, call the fire
• Installation and service must be performed by a
qualified installer, service agency or the gas supplier.



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  Summary of Contents for Bosch Model GWH 425

  • Page 1 GWH 425 PN INDOOR MODEL Flow Modulated with Standing pilot Suitable for heating potable water only - Not approved for space heating purposes (Intended for variable flow applications with steady cold water inlet temperatures) GWH-425-PN-N GWH-425-PN-L Warning: If the information in this manual is not followed exactly, a fire or explosion may result causing property damage, personal injury or death.
  • Page 2: Table Of Contents

    Gas piping & connections Measuring gas pressure Water connections 3.10 Recirculation application Operation instructions For your safety read before operating your water heater 14 Lighting instructions To turn off appliance Setting the water temperature Draining water from heater Maintenance and service...
  • Page 3: Appliance Details

    Appliance details GWH 425 PN specifications (Technical data) Approved in US/Canada Capacity GWH 425 PN: 4.3 GPM Maximum output GWH 425 PN: 91 525 Btu/hr Maximum input GWH 425 PN: 117 000 Btu/hr Efficiency in % Recovery efficiency 78% Min. Output GWH 425 PN: 23 906 Btu/hr Gas Requirement Gas connection (inches) - ¾”...
  • Page 4: Unpacking The Gwh 425 Pn Heater

    The appliance and its gas connection must be leak tested before placing the appliance in operation. 4. Keep water heater area clear and free from combustibles and flammable liquids. Do not locate the heater over any material which might burn.
  • Page 5: Dimensions And Installation Clearances

    SIDES BOTTOM (C) Fig. 4 Minimum clearances 6 720 608 034 Table 1 Dimensions in inches (mm) Model GWH 425 12 inches (306 mm) 4 inches (100 mm) 0 inches 4 inches (100 mm) 12 inches (306 mm) Dimensions inches...
  • Page 6: Heater Placement And Clearances

    Please contact Bosch Water Heating with any questions. Proper location for installing your heater Carefully select the location of the water heater. For your safety and for proper heater operation, you must provide combustion air to the heater and a proper exhaust vent system.
  • Page 7: Mounting Heater

    Mounting Heater Warning: before starting installation: B check that there are no loose parts inside the appliance B ensure that gas pipe, gas valve, and burner have no damage and are properly fitted. B Read chapter 2.2 to verify proper gas type and to check all parts are included in box.
  • Page 8: Combustion Air Requirements

    Combustion air requirements The BOSCH PRO water heater holds cold water in its copper heat exchanger and water valve when not in use. Because of this, any cold air that comes down through the unit's vent pipe is capable of freezing these components.
  • Page 9: Venting

    Fig. 10. Under no circumstances should the vent pipe be reduced in size. B An approved gas vent connector must be attached to the top of the water heater and rise vertically at least 12" before entering into an approved gas vent connector elbow.
  • Page 10 Masonry chimney Masonry chimneys shall be built and installed in accordance with NFPS 211 or local codes. A minimum 5" diameter gas vent pipe (metal double wall Type B), or an approved clay flue liner or a listed chimney lining system must be used when venting into a naturally drafting, internal masonry chimney.
  • Page 11: Gas Piping & Connections

    Gas piping & connections Before connecting the gas supply, check the rating plate on the right side of the heater to be sure that the heater is rated for the same gas to which it will be connected. In the United States: The installation must conform with local codes or, in the absence of local codes, the National Fuel Gas Code ANSI Z223.1/NFPA 54.
  • Page 12 FOR NATURAL GAS Maximum Capacity of pipe in Cubic Feet of Gas per Hour for Gas Pressure of 0.5 Psig or less and a Pressure drop of 0.3 in Water Column (0.75mbar).(Based on a 0.60 Specific Gravity Gas) Btu numbers given in thousands. Copper tubing is prohibited for use with Natural Gas in the Commonwealth of Massachusetts and not recommended elsewhere.
  • Page 13: Measuring Gas Pressure

    B Record static gas pressure reading on back page of manual. Operating pressure test B Turn on all hot water taps served by the water heater. B Run high hot water flow rate. B Set flow control knob fully clockwise.
  • Page 14: Recirculation Application

    A timed or thermostatically controlled operation of the pump is commonly done. The GWH 425 PN should be plumbed in line before the mini-tank water heater. Contact Bosch Water Heating if further instruction is needed. Operation instructions...
  • Page 15: Lighting Instructions

    Lighting instructions B STOP! Read the previous safety information. B The gas valve must be turned off by sliding the gas valve button to all the way to the left. Fig. 18 B Wait five (5) minutes to clear out any gas. If you smell gas, STOP! Follow “B”...
  • Page 16: Setting The Water Temperature

    Setting the water temperature The GWH 425 PN has a gas control that modulates burner input in response to flow. Its purpose is to ensure that the hot water temperature will remain steady, although the water flow demand might vary. The output temperature can be adjusted with either the temperature adjustment knob or the gas control slide.
  • Page 17: Water Valve

    Maintenance and service Maintenance intervals Failure Warning: recommended maintenance may result in complete failure of the unit over time. The warranty does not cover failures improper maintenance. The GWH 425 PN requires periodic maintenance. The below time maintenance intervals should kept the unit operating for many years.
  • Page 18: Main Burners

    5.6.1 Descaling heat exchanger 1. Disconnect electrical supply from the water heater. 2. Shut off the water supply to the water heater using (installer supplied) isolation valve. 3. Open hot water taps to drain and relieve pressure from the plumbing system 4.
  • Page 19: Troubleshooting

    3. If using a power vent, check the safety spillage switch reset button (the safety spillage switch is wired through the water heater's thermocouple circuit). The spill switch should be located at top of water heater close to draft diverter (Consult manual that came with powervent).
  • Page 20: Burners Do Not Light With Water Flow

    6. If the inlet water temperature to the water heater is above 70°F, 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.
  • Page 21: Water Is Too Hot

    3. Ensure the gas control slide button is all the way to the right. In addition, the temperature adjustment knob on water heater maybe set too low. Turn the knob clockwise for higher temperatures and test the water temperature.
  • Page 22: When Operating

    (see chapter 3.7). 6. The water heater 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.
  • Page 23: Protecting The Environment

    Protecting the environment Packing The packing box may be fully recycled as confirmed by the recycling symbol Components Many parts in the heater can be fully recycled in the end of the product life. Contact your city authorities for information about the disposal of recyclable products. Saving water resources: B Make sure you close all the taps after any use.
  • Page 24: Interior Components And Diagram Parts List

    Interior components and diagram parts list Interior components Fig. 28 Functional scheme Heat exchanger Burner Temperature adjustment selector Water valve Gas connection Flexible hot outlet Gas control slide Pilot gas tubing Piezo 6 720 608 034...
  • Page 25: Components Diagram

    Components diagram Fig. 29 Components Diagram 6 720 608 034 6720608126-02.1 JF...
  • Page 26: Parts List

    Parts list Item Description Front cover Shield Temp control knob Draft Diverter Exhaust pipe union Ø128 Angle bracket Trade mark badge Heat exchanger Cold water pipe Main burner NG Main burner LP Connector Piezo clip (x10) Sparking plug Pilot gas pipe Tube (x10) Pilot screen Gas valve LP...
  • Page 27: Fifteen Year Limited Warranty

    BBTNA shall not be liable for any loss or damage directly or indirectly arising from the use of the hot water heater, or for any consequential damages arising (including damages from water leakage).
  • Page 28 Installer Checklist, to be performed by installer upon installation Serial Number Gas Pressure Reading* Building Water Pressure Average Winter Temperature Average Summer Temperature Installing Company Installer name Address Phone * See Chapter 3.8 and gas pressure table (to be filled out by installer) Installation manual should be left with the owner after the installation is tested and completed BBT NORTH AMERICA Bosch Group...

This manual is also suitable for:

Protankless gwh-425-pn-nProtankless gwh-425-pn-l