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Useful Bulb Testing Method To Solve Household Instant Water Shower Heater

By on March 7, 2016
water heater diagram







A friend  has asked me to analyse this 3.6KW Joven water heater for him which cannot turn on. Water heater is a common household electrical applicance but surprisingly not much informations can be found in the internet The unit has an electrical diagram sticked on to the casing as shown in figure 1

joven water heater repair

Water heater after disassembled from the water supply cannot be turned on or test due to  safety built-in feature. In order to test or trouble-shoot the fault you need to over-ride the safety function.

You need to use the bulb method i.e the heater need to replace with a 100w bulb  and the flow  switch sensor or reed switch need to over ride with a jumper wire as shown in figure 2. Alternatively, you may use a small magnet & place on top of the sensor to close the reed switch.

water heater repair schematic

water heater repair

Figure 2

Now the water heater can be tested without water and it will function like a normal light dimmer when the pot switch is rotated (temperature control switch) as shown in figure 2.It is very much convenient and easier to test  in this mode rather than trying to trouble-shoot or repair  the unit hanging on the bath room wall. When power on the  led is not lighted indicating the voltage is being cut off completely. There is a resetable thermal cut-off switch mounted on top of the heater which is at cut-off position.After resetting it the power is able to turns on.

When the heater thermal fuse is triggered and cut-off , I suspect the heater is shorted but when I measured the dc resistance it showed normal reading i.e 16 ohm.( power rated at 3.6KW).Using the power formula V2/R=3.6KW which is correct. Since there is no other faults can be found I decided to test the water heater in the normal working condition i.e with water supply and electricity connected.

Now the real fault appeared, there is no water coming out of the shower head indicating the pressure switch is having a problem. This is the reason why the thermal switch tripped due to over heating as insufficient water is flowing inside the heater. When I disassembled the pressure switch ,the rubber pressure stopper was observed to be  deteriorated  and sticky as shown in figure 3 which prevent the pressure from releasing through the centre hole. Normally, the flow sensor reed switch will not close and the heater will be cut-off or non operative if the pressure is insufficient. I suspect the pressure sensor sensitivity is having problem.

water heater parts

Pressure switch is not easily available as it is normally a customised part and expensive to replace, so I replaced the rubber stopper with a pencil eraser rubber which is cut to almost the same size and diameter as the original stopper (see figure 4). It works perfectly normal when I tested the water heater again.

Any brand or model of water heater can apply using the bulb testing method. I have tested and repaired a few thousand units of water heaters with this method without any problem.


This article was prepared for you by SK Wong from Malaysia and he graduated from London with a degree in Electronics and Electrical Engineering. He has more than 40 years of experience in the consumer product manufacturing. With the passion to repair electronic products from young , he has been doing it part time until now.

Currently, he can provide turnkey solutions for consumer products including services from idea or concept to design , prototyping , finished products (including electronic,mechanical and plastic parts) and manufacturing.

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Note: You can check his previous repair articles below:




  1. Gerald

    March 7, 2016 at 1:47 pm

    Well done SK,

    thanks for sharing. Excellent repair tips and well documented.


  2. beh

    March 7, 2016 at 3:22 pm

    Hi SK
    thanks for this article and very good trick to solve the problem
    i have seen this machine all around the THAILAND and this is a common problem there .

    • beh

      March 9, 2016 at 3:22 pm

      SK : rubber stopper require heat and pressure resistance let us to see how long this eraser rubber stand against these two bad factors in this part of the machine

      • skwong

        March 10, 2016 at 4:34 pm

        Heat will not be a problem due to the water inside the switch . Pressure resistance may be a problem if you use inferior eraser rubber.Silicon rubber will be a better choice but not easily available.

      • Peter Lim

        August 19, 2021 at 4:14 pm

        Joven water heater 830i
        Behind the flow switch, there is a magnet flow bracket was broken.
        Do you have the spare part?
        If you have, please contact me.
        Thank you

  3. Yogesh Panchal

    March 7, 2016 at 3:27 pm

    Well done! and thanks for sharing your experience.

  4. Robert Calk

    March 7, 2016 at 3:41 pm

    Nice job, SK. Thanks for the article.

  5. Arthur

    March 7, 2016 at 4:39 pm

    Good show SK, Solved sticky problem. Some times not in electrics.
    Regards, Arthur.

  6. Albert van Bemmelen

    March 7, 2016 at 4:59 pm

    Thank you for this nice repair article were we normally do not read often about!

  7. Mike Zook

    March 7, 2016 at 8:28 pm

    Way to go SK. Thanks for the article. Good job.

  8. Mason

    March 7, 2016 at 9:32 pm

    Thank you for sharing. Nice fix using an eraser, innovative. Love reading the articles. Thank you

  9. Imraz

    March 8, 2016 at 4:32 am

    Great Article SK Wong. I really like the usage of Pencil Eraser Rubber idea. Thanks

    • skwong

      March 9, 2016 at 5:47 pm

      I like to comment why I use the pencil eraser as some readers are impressed on the method. Getting replacement parts in Malaysia is not an easy task especially those which are not common part.After sales service center for most of the brands do offer it but normally at high price which makes the repair work non profitable or not feasible.
      In my personal opinion getting replacement parts are more challenging than trouble-shooting the fault.With a bit of thinking and innovation can save you a lot of time, money and inconveniences.

  10. Anthony

    March 8, 2016 at 1:04 pm

    Great repair tip SK with nice photos ! Thank you for sharing

  11. Suranga Electronics

    March 8, 2016 at 5:22 pm

    Mr- SK Wong,
    Nice and great repair work you have done.

  12. Parasuraman S

    March 8, 2016 at 7:17 pm

    Another insight into an unknown area of electric repairing! Thanks for sharing this very valuable info!

  13. Peter O

    March 13, 2016 at 7:27 am

    Clever repair tackled methodically!
    I wonder about the original design, seems foolish to rely on something which is almost guaranteed to deteriorate. Your eraser rubber will ultimately suffer similar fate but at least the owner now knows the "quick fix"?

  14. Scott Morton

    May 11, 2016 at 10:20 pm

    Greetings Mr. Wong

    I have a Joven EC802 on the bench with an inoperative magnetic flow switch. I have often seen this problem with these units, and obtaining a replacement part is impossible here in Laos.

    The problem seems to be the magnet within the moving plunger, which has become very weak. The reed switch appears to be OK, since bringing another small test magnet within 1 cm of it will activate the heater.

    I am considering disassembling the flow switch to remove the plunger and then re-magnetizing it.

    Have you ever tried this? If so, what was your experience?
    Thank you.

    Best Regards - Scott Morton
    Luang Prabang Lao PDR


    October 4, 2016 at 4:41 pm

    Great repair tip SK with nice photos ! Thank you for sharing.

  16. SK Yong

    January 17, 2017 at 9:38 am

    Hi SK,

    Good work in sharing the knowledge. I only dared took the step of dismantling the pressure switch after reading your article. Although the pressure switch in my case was intermittent due to a sticky operating plunger, you had certainly helped me to identify the problem.

  17. KC

    April 22, 2018 at 3:17 pm

    Good to read your sharing. Need your help Sir.
    My instant water heater Joven i70P. No light, cannot reset. Water flowing out of the shower is powerful. No heating up happen. Your advice please. Thank you

  18. boey kok soon

    September 22, 2020 at 9:02 pm

    Hi SK Wong Really enjoyed your article . I followed your instructions and managed to get my Joven water heater back again . This heater was giving intermittent operations so i suspected the fault could only be due to the pressure switch . Put in a new rubber stopper and its helped solved my problem
    thank you

  19. HaarshA

    January 9, 2021 at 2:10 am

    Good job thanks can u explain why ithas read switch


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