DIY Series 40: DIY Repair Rubine IA991D DC Pump Instant Water Heater (No Hot Water, Red LED is OFF)

Symptoms:

• Heater LED is not ON. No hot water.

Findings: 

• Flow Sensor: Magnet misaligned, reed switch is not ON/OFF accordingly
• Thermistor: Thermistor at the top of tank need to be reset (NC: Normally Closed).

[Note: Turn OFF the power supply before any installation / checking / debugging] 

Videos:

1. Test the flow sensor using multimeter (flip it over to let the magnet touch the reed switch). Multimeter shows short circuit (with beep sound) means flow sensor is working fine.
2. Test the Heater is working or not by install it back with power and water supply. Red LED will be turned ON and hot water will flow out if flow sensor & thermistor is in good condition.

Photos:

1. Rubine IA991D DC Pump Instant Water Heater (with internal block diagram)
2. Dismantle of power supply wires (E/L/N)
3. Unscrew 4 pcs of screws to take it down from the wall
4. Checking on flow sensor (reed switch and magnet) functionality, remove the reed switch cover
5. Checking on the thermistor functionality, press the button on top (to reset) if it is not shorted
6. Power ON (without water supply) and test the flow sensor (refer 1st video above)
7. Power ON (with water supply) and test the heater function (refer 2nd video above)

Tools required:

• Screwdriver
• Multimeter
• Test Pen

 

Costs involved:

• NIL (New Flow Sensor costs about US$15)
• NIL (New Thermostat KSD303 costs about US$1 ~ US$2)
  

Action Taken:

• Dismantle the flow sensor, take out the magnet and clean it, then realign it back to work with reed switch correctly.
• Reset the thermistor if it is not shorted by default (NC: Normally Closed).
• Thermistor might need to be replaced if it is frequently disconnected in near future.

Parts Sourcing (if needed):

• Found the thermostat written ATT S102, try to look for the original parts, but not available for sale at online store.

• Found another thermostat, KSD303 having same shape and size. Rating at 250VAC/10A, 150°C.

• Use Google Gemini to check compatibility. For a 10A application at 250VAC, both the ATT S102 and KSD303 are technically suitable, as 10A is a standard rating for both series. At this current level, the choice depends primarily on whether you need a direct manufacturer-specified replacement (ATT S102) or a more widely available, generic industrial alternative (KSD303)