Bosch Aquastar 250sx Problems

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Bosch Aquastar 250sx Problems

Error codes are the indicators of the problems inside the water heater. When the error code appears on the Bosch heater's LCD display, the reset button sometimes leads to an error removal, but if it doesn't work, the instructions below might help troubleshoot the problem: 
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Error code A2 - Problem with the flue gas limiter
If you are experiencing problems with your Bosch Aquastar 2700ES Tankless Hot Water Heater than put that money back in your wallet. This Troubleshooting and repair guide should help you isolate and solve whatever problem you may be faced with (hopefully). 250SX Installation Manual - French PDF 250SX Z-Vent Manual - English PDF Freeze Prevention Kit Installation Instructions.
The temperature of the flue gases is too high and is above 230 F. It is important to check for any gas leakage close to the gas combustion chamber and heat exchanger. Check if the limiter is operating and wiring is correctly linked to the control board and limiter. The element is located at the top-right corner of the unit.
Error code A3 – Wire harness fault 
Measure the resistance on wire harness; it must be between 115 and 120 ohms.
Error code A4 - Backflow temperature sensor
Check the backflow temperature sensor connection; it might be damaged, or it got short-circuited. It is located below the heat exchanger and ignition electrodes. Look for the diagnostic menu 1P and nC code.
Error code A7 – Outlet temperature sensor fault
The outlet temperature sensor reacts when the temperature is either below 36 F when the unit is protected from freezing temperatures, or above 210 F in a case of overheating due to the lime-scale deposits. Check the sensor and wiring. You might have to flush the unit to remove the mineral build-up.
Error code A9 – Outlet temperature sensor fault
Another error code associated with the outlet temperature sensor, but this time, the sensor is not sensing the expected output temperature. This is not an error code; it is a flashing status message that tells you to check; the wiring connection, gas pressure as it might be too low, voltage as it doesn't meet the required 120 VAC, the unit that might not be properly grounded or the issue with the control board.
Error code C6 – Slow-spinning fanThe primary fan spins slowly when the water heater is working. Check the wiring connection on the fan and control board. The voltage must be 120 VAC and the element correctly grounded. Does the venting system meet the code and manufacturer requirements? Ensure that the gas pressure is as required because the low pressure leads to fan rotation problems.
Error code C7 – Fan rotational speed sensorNo signal from the fan's rotational speed sensor. The sensor might be broken. Check the wire connection on the control board and fan. The voltage needs to be 120 VAC and adequately grounded. Ensure that a fan and control unit are not damaged.
Error code CA – Excessive water flowMake sure that the water pressure is less than 150 PSI and a flow rate below 10 GPM. Check the wire connections on the water valve and control panel.
Error code E1 – Extremely high temperature The temperature detected by the outlet temperature sensor is over 185 F. Ensure that the sensor is working correctly, and the wire connections are not loose. Another reason for the high temperature is the sediment build-up, so descaling is needed.
Error code E2 – Faulty inlet water temperature sensor
Check the sensor and wiring connections as this sensor is in charge of protecting the unit from freezing. 
Error code E3 – Problem with the exhaust temperature sensorThe unit will shut down if the temperature of the exhaust gases is above 194 F. Check and clean the heat exchangers if needed. Reduce the maximum power and water temperature.
Error code E4 – Problem with the backflow temperature sensorThe backflow temperature sensor reacts when the temperature is over 309 F. Check the vent pipes if they meet the code and requirements, look for damages and obstructions.
Error code E9 – Overheat sensor ECO has an open circuit Check the wire connections on the sensor and control board. Ensure the venting system is per codes and instructions. Descale the unit if necessary.
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Error code E3 – Problem with the exhaust temperature sensorThe unit will shut down if the temperature of the exhaust gases is above 194 F. Check and clean the heat exchangers if needed. Reduce the maximum power and water temperature.
Error code E4 – Problem with the backflow temperature sensorThe backflow temperature sensor reacts when the temperature is over 309 F. Check the vent pipes if they meet the code and requirements, look for damages and obstructions.
Error code E9 – Overheat sensor ECO has an open circuit Check the wire connections on the sensor and control board. Ensure the venting system is per codes and instructions. Descale the unit if necessary.
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Error code EA – No sparks for the electrodesThe ionization electrodes are not making sparks when water is flowing. There are many reasons for this problem and here are some; low gas pressure, the fan speed is low, wire connections are loose, the air in the gas line, no gas.
Error code EC – Electrode failureIonization electrode failure during water heating. The reasons might be; obstructed venting system, low gas pressure, and CO2 readings, the fan speed is too low.
Error code E0 hardware/software issuesThe issue with the hardware or software. First, try to reset the error code; there might be confusion inside the computer. Check if the wire connections are properly linked to the computer board or the board is not damaged.
Error code F7 – Ionization rod problemThe problem is with the ionization rod at standby. Check connections to the electrodes; it should be securely connected. Semp toshiba drivers ni 1401. Other reason might include damaged element or control board.
Error code FA – Gas valve circuitThe problem is with the gas valve circuit. Check the wire connections on the gas valve and control board. Measure the voltage on the wires when a water heater is ON… it should be 24 VDC. If the measurement is as specified, the gas valve is malfunctioning.
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Bosch and minimum water flow
Had a Bosch heater for 15 years. In the last 10 years, keep getting error code A2 - flue gas temperature too high. Sometimes I have to reset several times …
PLEASE NOTE: The Bosch 250SX has been replaced by the Bosch AquaStar 2400E
 Bosch 250SX NG (Natural Gas) Bosch 250SX LP (Liquid Propane) 
 Certainly the engineers at Bosch have outdone themselves — the beauty of this unit is in its simplicity. The AquaStar 250SX requires no special tools to install or maintain. And its compact size and sealed combustion enables installation in small spaces. A built-in power vent makes both vertical and horizontal venting possible while the electronic ignition maximizes energy efficiency. This unit yields enough power to supply two showers simultaneously and features an adjustable temperature control for improved comfort.
With an average life expectancy of more than 20 years, energy savings of up to 50%, and an endless supply of hot water, your family will thank you for years to come. Easy to install with multiple venting options, the Bosch AquaStar 250SX LP is the right choice. 
The Bosch AquaStar 250SX features the following: 
Delivers up to 6.4 gallons per minute, enough to run two showers at once 
Direct vent room-sealed combustion 
No standing pilot! (Electronic ignition) 
Built-in power vent allows for vertical or horizontal venting 
12-year warranty on heat exchanger – 2 years on parts 
Computer-adjusted fuel and air mixture optimizes performance 
Optional remote thermostat (TSTAT2) available to digitally control output temperature 
State-of-the-art Bosch quality and engineering 
Bosch Aquastar Replacement PartsBosch AquaStar 250SX Specifications
Bosch Aquastar 125b Ng Troubleshooting BTU Input 175,000 (51.2 kw) 
 Energy Factor .85
 Average Efficiency 87% (recovery efficiency)
 Ignition Electronic
 Voltage 120 VAC 50/60 Hz
 Exhaust Vent Size 3'
 Combustion Air Pipe Size 3'
Venting (Exhaust)Diameter: 3 inch
Material: Stainless Steel (AL29-4C) sealed
Minimum length: 3 ft.
Maximum length: 26 ft. (8m) with one 90 degree elbow. Less 2.5 ft. for each additional 90 degree elbow
 (1.25 ft for each 45 degree elbow)
Venting (Intake)
(Room-sealed installation)Diameter: 3 inch
Material: PVC, Aluminum or Galvanized
Maximum length: 26 ft. (8m) with one 90 degree elbow. Less 2.5 ft. for each additional 90 degree elbow (1.25 ft for each 45 degree elbow)
 Minimum Flow Rate (gpm) .8 gpm (3 l/m)
Capacity 
 Activation .8 gpm (3 l/m) 90° F rise / 3.1 gpm
 80° F rise / 3.5 gpm
 70° F rise / 4.0 gpm
 60° F rise / 4.7 gpm
 50° F rise / 5.7 gpm
 45° F rise / 6.35 gpm
 Weight 47 lbs. (21 kg)
 Optional Accessories Horizontal Vent Kit
 Wireless Remote control
Bosch Aquastar 250sx PartsIMPORTANT 250SX Venting Information
 NOTE: This appliance's exhaust must be vented to the outside with a sealed 3' stainless steel vent pipe (AL29-4C). The appliance's flue gases are under positive pressure and must travel through a stainless steel 3' pipe that is sealed gas tight. Stainless steel vent pipe is equipped with sealing gaskets for ease of installation, proper safety and durability. The heater shall not be vented in combination with any other appliance and the appliance must only be vented with a dedicated sealed ventsystem.COMBUSTION AIR
Because the Bosch AquaStar 250SX has direct vent room-sealed combustion, it can obtain make-up (combustion) air from inside the home (ambient air) or from outside the home. It is recommended to pipe the combustion air from outside the home as this is generally cleaner air, allowing for less maintenance on the water heater and a longer life expectancy of the unit.
Horizontal VentingVertical Venting
If you will be terminating the vent pipe from the Bosch AquaStar Model 250SX horizontally out a sidewall, the AQ3 Vent Kit is required. Please click herefor additional informationIf you will be terminating the vent pipe from the Bosch AquaStar Model 250SX vertically up a chimney, the AQ3 Vent Kit is not required. PLEASE NOTE: Additional venting components can be obtained by calling  888-726-9752.

IMPORTANT 250SX Venting Information
NOTE: This appliance's exhaust must be vented to the outside with a sealed 3' stainless steel vent pipe (AL29-4C). The appliance's flue gases are under positive pressure and must travel through a stainless steel 3' pipe that is sealed gas tight. Stainless steel vent pipe is equipped with sealing gaskets for ease of installation, proper safety and durability. The heater shall not be vented in combination with any other appliance and the appliance must only be vented with a dedicated sealed ventsystem. COMBUSTION AIR Because the Bosch AquaStar 250SX has direct vent room-sealed combustion, it can obtain make-up (combustion) air from inside the home (ambient air) or from outside the home. It is recommended to pipe the combustion air from outside the home as this is generally cleaner air, allowing for less maintenance on the water heater and a longer life expectancy of the unit.
Horizontal Venting	Vertical Venting
If you will be terminating the vent pipe from the Bosch AquaStar Model 250SX horizontally out a sidewall, the AQ3 Vent Kit is required. Please click herefor additional information	If you will be terminating the vent pipe from the Bosch AquaStar Model 250SX vertically up a chimney, the AQ3 Vent Kit is not required. PLEASE NOTE: Additional venting components can be obtained by calling 888-726-9752.