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Bosch AquaStar 125B Tech Info
Tankless Hot Water Heaters by Bosch

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Installation info for the AquaStar Hot Water Heater Model 125B

Follow information provided on this page.

Installation and service must be performed by a qualified installer, service agency or the gas supplier.

Other tips: when venting it is best to rise 12" off the top of the hot water heater before making any 45 or 90 degree elbows , then rise ¼" per foot when running any horizontal vent pipe. Size gas line properly. Install the provided gas regulator with its directional arrow facing the gas flow path (also be sure to properly regulate Liquid Propane (LP) or Natural Gas at source when supplying AquaStar). The small regulator that is provided is NOT to be used on a high pressure line.


·Required Clearances
AquaStar Model 125B Amount
TOP 12"
FRONT 4"
BACK 0"
SIDES 4"
FLOOR 12"


·Gas & Venting AquaStar Model 125B
Gas 1/2" 5/8" 3/4" 7/8" 1"
Liquid Propane (LP) Not Recommended 20 feet 60 feet 150 feet   
Natural Gas 10 feet    40 feet    150 feet
Venting Min. Diameter Min. Height
5 inches 6 feet


·Dimensions AquaStar Model 125B
Height Width Depth Weight
29-3/4" 18-1/4" 8-3/4" 39 lbs.


·Combustion Air Required AquaStar Model 125B

In order to ensure an adequate supply of combustion air, the National Fuel Gas Code requires that any conventional vented gas appliance be installed in a room with a minimum volume of 50 cubic feet per 1,000 Btu/hr rated input. The following is the minimum size of the open space in which the AquaStar Tankless Hot Water Heater Model 125B may be installed in compliance with this requirement:


Model Volume Area with an 8' ceiling
125 B 5,900 cubic feet 738 square feet


If the AquaStar 125B Water Heater is installed in an area smaller than indicated above, then the installer must make special provisions for combustion air in the form of two openings, each one within 12" of the top and 12" of the bottom of the space, having a total free area of 1 square inch per 1,000 Btu/hr of rated input if the air is taken from inside the house. This requirement reduces to 1 square inch per 4,000 Btu/hr if the air is taken from the outside.

Note: This calculation assumes normal rates of "infiltration" of outside air. A super-insulated, super-tight space/building may require outside air for the AquaStar even though the volume/area of the space meets this application.

Caution: In addition to the above, the AquaStar Water Heater must be properly vented to ensure safe and efficient operation.

For more information download an
installation manual in pdf format.


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