HVAC / Water Heaters
User Manual for Bosch AquaStar 125HX Tankless Water Heater
Quick guide for the Bosch AquaStar 125HX tankless water heater. Includes technical specifications, installation requirements, venting options, and operational features.
Table of contents
Manual images
Click an image to enlargeQuick Guide from the Manual
The Bosch AquaStar 125HX is a tankless water heater featuring a hydro-generated ignition system, which eliminates the need for electricity, batteries, or a standing pilot. The unit activates automatically when water flows through it (minimum 0.5 gallons per minute). It is designed for wall-mounting and provides an endless supply of hot water for one major application at a time.
Technical Specifications
The unit operates with a modulating gas valve to control temperature. Key specifications include:
- Power Input: 28,000 – 117,000 Btu (Natural Gas or LP Gas).
- Minimum Flow to Activate: 0.5 gallons per minute (gpm).
- Flow Rates: 3.6 gpm at 50°F rise; 2.0 gpm at 90°F rise.
- Energy Factor: 0.77 (NG) / 0.84 (LP).
- Thermal Efficiency: 80% (NG) / 83% (LP).
- Dimensions: 29.75" Height x 18.25" Width x 8.75" Depth.
- Weight: 44 lbs.
Installation Requirements
Proper installation is critical for safe operation. Ensure the following requirements are met:
- Gas Connection: 1/2" Male NPT.
- Water Connections: 1/2" Male NPT.
- Natural Gas Pressure: Minimum 7" WC, Maximum 14" WC.
- LP Gas Pressure: Minimum 11" WC, Maximum 14" WC.
- Water Pressure: Minimum 18 PSI, Maximum 150 PSI.
- Electrical Supply: None required (hydro-generated).
Venting Options
The unit requires proper venting to operate safely. The standard configuration is a vertical vent using 5" double-wall type B pipe. For installations where a vertical chimney is not available, an optional Power Vent (Part #AQ1) can be installed to allow for horizontal venting through a side wall.
Contact Information
For sales, service, installation, and technical information, contact Controlled Energy Corporation:
Toll-free: 866-642-3199Fax: 802-496-6924Email: [email protected]: www.ControlledEnergy.comAddress: 340 Mad River Park, Waitsfield, VT 05673 USA
Official resources from the manual
Manufacturer information
Bosch
Practical help
Common problems
Unit fails to ignite
Ensure water flow is at least 0.5 GPM. The hydro-generated ignition system requires sufficient water flow to create the spark.
Insufficient hot water or temperature issues
Verify that gas supply pressure is within the required range (7-14" WC for Natural Gas, 11-14" WC for LP) and that the unit is not being over-demanded beyond its capacity.
Before use
- Verify the gas type (Natural Gas or LP) matches your supply.
- Ensure water pressure is between 18 PSI and 150 PSI.
- Confirm gas supply pressure is within the specified range (7-14" WC for NG, 11-14" WC for LP).
- Check that the vent size is 5 inches (double-wall type B).
- Ensure the unit is securely mounted to the wall using the included hooks.
Specs in practice
- Minimum Flow to Activate
- The minimum water flow rate (0.5 GPM) required to trigger the hydro-generated ignition system.
- Modulating Gas Valve
- Automatically adjusts the gas input to maintain a consistent water temperature regardless of flow rate changes.
- Energy Factor
- A measure of the overall efficiency of the water heater; higher numbers indicate better efficiency.
Images and diagrams
- Closet installation diagram shows the unit mounted in a closet with proper clearance and venting.
- Power vent option diagram illustrates the side wall vent configuration using part #AQ1.
- Natural draft option diagram shows the standard vertical chimney venting setup.
Model compatibility
- Requires 1/2 inch Male NPT gas and water connections.
- Optional Power Vent (Part #AQ1) is required for horizontal venting configurations.
- Not compatible with electrical power sources; ignition is hydro-generated.
Manual page author
David Miller
Documentation analyst
Organizes user manual content into clear summaries, with attention to model details, product context, and everyday usability.