HVAC / Water Heaters
User Manual for Bosch AquaStar 125X Water Heater
Quick guide for the Bosch AquaStar 125X tankless water heater. Includes installation instructions, gas and water connection steps, temperature adjustment, maintenance, and troubleshooting.
Table of contents
Manual images
Click an image to enlargeImportant Safety Information
If you smell gas, do not try to light any appliance, do not touch any electrical switch, and do not use any phone in your building. Immediately call your gas supplier from a neighbor's phone. Installation and service must be performed by a qualified installer, service agency, or the gas supplier. This heater is for heating potable water only and is not approved for space heating, mobile homes, RVs, or boats.
Installation
The heater must be mounted on a wall using the provided L-shaped hooks. Ensure proper clearances: 12 inches at the top, 4 inches at the front and sides, and 0 inches at the back. The unit requires a 5-inch vent. Do not install in bathrooms, bedrooms, or occupied rooms normally kept closed. Ensure an abundant supply of combustion air.
Gas and Water Connections
- Gas: The unit comes with a gas pressure regulator that must be installed. Use black iron pipe for natural gas or semi-rigid copper/black iron for LP gas. Check for leaks using soapy water; never use a flame.
- Water: Cold water inlet is on the right, hot water outlet is on the left. Use flexible connectors provided. Ensure water pressure is between 30 and 50 psi for private well systems.
Operation
To operate, set the On/Off switch to the 'I' position. The heater uses electronic ignition. If the red LED indicator flashes, replace the batteries with two D-cell 1.5V alkaline batteries. To adjust the temperature, turn the large knob beneath the main gas control. Turning clockwise increases temperature and decreases flow; counter-clockwise decreases temperature and increases flow.
Maintenance and Troubleshooting
The heater should be checked annually. Periodically clean the water inlet filter and inspect the vent system. If the pilot does not spark, check the batteries, water flow, and microswitch adjustment. If the unit fails to ignite, ensure the gas supply is open and the gas line is bled of air. If water is not hot enough, check the temperature setting and ensure gas pressure is sufficient.
Manufacturer information
Bosch
Practical help
Common problems
No spark at pilot
Check batteries, ensure water flow is sufficient (min 1/2 GPM), clean water inlet filter, or adjust the microswitch.
Pilot sparks but burners will not ignite
Bleed air from gas line, ensure gas cock is open, check for dirty pilot orifice, or check water valve assembly.
Water not hot enough
Increase temperature setting, reduce water flow demand, or check for insufficient gas pressure/supply.
Unit deactivates during use
Check for unbalanced water pressure or mineral build-up in shower heads/aerators. Do not add flow restrictors.
Before use
- Ensure the system is filled with water.
- Verify gas type matches the unit (LP or NG).
- Check for gas leaks using soapy water (never use a flame).
- Ensure proper venting and combustion air supply.
- Check water pressure (30-50 psi for well systems).
- Set On/Off switch to 'I' position.
Specs in practice
- Water Pressure
- Min 18 Psi at 4 GPM, Max 150 Psi.
Images and diagrams
- Fig 1: Mounting the heater using L-shaped hooks.
- Fig 4: Gas pressure regulator installation.
- Fig 12: Microswitch adjustment for ignition issues.
- Fig 14: Diagram of AquaStar 125X components.
Model compatibility
- Not approved for space heating.
- Not approved for mobile homes, RVs, or boats.
- Indoor installation only.
Manual page author
David Miller
Documentation analyst
Organizes user manual content into clear summaries, with attention to model details, product context, and everyday usability.