Fixing an Overheating / Controlless Instant Water Heater shower
Tower Instant Water Heater shower – RWH808E (Made in Malaysia)
I was using the above model of ‘Hot water shower’ since 10 years in my bathroom. A few days ago, when the hot water shower was turned-on, I experienced that:
- It’s temperature couldn’t be controlled by rotating the Temp_Controlling knob (Variable resister) on the top. Though it was rotated to any possession, heater worked with it’s maximum temperature.
- There is a water flow sensor, if water doesn’t come inside the heater, heater won’t ‘On’ and will be at the standby mode till a water supply is detected. This is a safety mechanism to protect the heater from ‘over heating’. However, now, the heater was ‘On’ regardless of the presence of water supply.
- There were two buttons for ‘Testing & Reset’. But the water heater responded to neither of those functions.
I turned off the unit, removed it from the wall, opened the front cover and did the following basic test:
- Water heater comprised two small circuit modules: Front panel & Controller circuit. As a rule of thumb, I searched for a burnt component or a burnt smell on the circuit board. But there wasn’t such a mark and all parts looked really well.
- Then I checked few semiconductors (such as IC, Transistors and rectifiers) on the two PCBs with a DMM, but no any faulty was identified.
- Then I saw a Current controlling Isolated Triac and a Reed switch of flow control was attached to the ‘Water inlet’ tube. Metal body of the Triac was directly attached to the water line for preventing it from overheating.
- Here, I made a Hypothesis: ‘The circuit could have been passing the total current to the Heater-Coil without controlling it’. However the controller circuit is checked already, and found Ok. Then I checked the Reed switch with a DMM and saw no issue with it.
- Then I removed all the connected wires from the Triac and tested it’s terminals. (Mr Yong had described how to test a Triac in his early articles and Books, please find them). As I expected, terminal 1 and 2 of the Triac was “Shorted” ! At normal working condition it should show nearly 20MΩ resistance, however as you see in the Pic, it saw only 1.2 Ω !!!
- Then I bought a new Triac (Same part number : TG25C60) from a local electronic shop and replaced.
- Now my Water heater is working perfectly with it’s all the functions.
- As a best practice, I dry-solded all the joints pass high current through the PCB.
Root cause: At a normal condition, Triac pass the current from ‘In-terminal’ to ‘Out-terminal’ by regulating it according to the reference signal receive by the Gate terminal. However, here, T1 & T2 was shorted. So, the total current was flowing through the Triac to the Heating coil regardless of the reference signal received by the Gate terminal. So it was heating to it’s maximum temperature.
This article was prepared for you by Kushan Ranatunga who works as a Network Engineer in Sri Langka. He has been doing electronic experiments since age 11.
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