Installation and Operating Instructions for Bosch AquaStar 125HX Water Heater

Quick Guide from the Manual

The Bosch AquaStar 125HX is an instantaneous, tankless water heater designed for potable water heating. It features automatic variable power modulation and electronic ignition. Important: This unit is not approved for space heating, mobile homes, RVs, or boats. Installation must be performed by a qualified installer or service agency.

Installation Requirements

Proper installation is critical for safety and performance. Ensure the unit is mounted on a wall using the provided L-shaped hooks, spaced 13 1/4 inches apart. If mounting on a stud wall with plasterboard, use support boards (1x4 or 1/2 inch plywood) to prevent movement. Maintain minimum clearances: 12 inches top, 4 inches front, 0 inches back, 4 inches sides, and 12 inches bottom.

Venting and Combustion Air

The unit requires a 5-inch vent. Ensure the vent pipe is not restricted and has a minimum height of 6 feet (if no elbows are used). The vent must terminate 2 feet above any obstruction within a 10-foot radius. Ensure adequate combustion air supply; in confined spaces, provide two permanent openings (one within 12 inches of the top, one within 12 inches of the bottom) to allow sufficient air intake.

Gas and Water Connections

The unit is supplied with a gas pressure regulator that must be installed before the gas supply line. Use black iron pipe for gas lines, sized according to the distance from the meter. For water connections, use 1/2 inch copper or brass fittings. Install full-port valves on both cold water feed and hot water outlet lines to facilitate servicing. A pressure relief valve (PRV) must be installed on the hot water line.

Operating Instructions

Before operating, ensure the system is filled with water. Open the cold water inlet supply fully and open a hot water faucet to purge air from the system. To start the heater, flip down the cover plate on the front panel and set the On/Off switch to the 'On' position. Use only your hand to push the button; do not use tools. The green LED indicates the main burner is on; a flashing red LED indicates insufficient water flow to ignite the burner.

Setting Water Temperature

Adjust the temperature using the knob located beneath the main gas control. Turning the knob clockwise increases the temperature and decreases flow; turning it counter-clockwise decreases the temperature and increases flow.

Maintenance and Service

Perform maintenance annually. This includes checking the vent system, inspecting the pilot assembly, and cleaning the water inlet filter. The water valve should be lubricated every two years and rebuilt every 3-5 years. If the pilot flame appears soft or yellow, the pilot burner orifice may need cleaning or replacement. Do not use wire or sharp objects to clean orifices.

Troubleshooting

If the unit fails to ignite, check the following: ensure the On/Off switch is 'On', verify water flow is sufficient (minimum 0.5 GPM), check for clogged water inlet filters, and ensure there are no plumbing cross-overs. If the pilot sparks but burners do not ignite, check for air in the gas line, ensure the gas cock is open, and verify the gas regulator is not locked or damaged.