Hot water shower unit faulty... Manufacture?

[Motorcar]

Hi all, I read a lot on TV forum and seldom post anything.. Anyway something just concern me and I thought other might be interested in it too.

Anyway, here is the story....a friend of mine had to replace her 1 year old hot water shower unit as it wasn't working properly... ( heating system stay on even thou after turning off the tab, so resulting hot steam come out of the shower head!!) very dangerous!, so have to turn off the power to disable it. So, she got someone to replace to a new unit as it might not worth fixing and out of the 1 year warranty. She told me about this and I told her I will have a look for her as I like to fix stuff being an engineer..

Checked all the sensors and they all working fine until I come across the wires to the heating element... To my horror, it stuck together!! I pull it apart and here is the pictures...

Point is, I believe the manufacture have used an undersize wire (14AWG) for this 4500Watt unit causing it to overheat over time and the insulator melted and fuse the LIVE and Neutral wire of the heating element!! Did a bit of google for wire size, it should have minimum of 12AWG (and recommended running 10AWG) , which they did use 12AWG for the terminal screws to circuit breaker. After the circuit breaker, is all 14AWG.

So yeah, thats my rant .... I might be wrong, please do correct me as I'm not a sparkie.... Just watch out if you got one this unit at home... Is just fire waiting to happen!

[CharlieH]

MOVED to Electrical forum

[Crossy]

14 AWG is about 2mm² so probably good for about 18A, 4,500 Watts at 220V is 20A or so so wire is a little under-size but I would be more concerned that it does not appear to be a heat resistant wire (only 105^oC).

I wonder if it being allowed to overheat melted the insulation (the short wouldn't have kept the heater on, it would have opened the breaker).

Worth an inspection if you have one to ensure there's nothing similar going on.

[Motorcar]

So, the melted wired might not be the main course of the problem then... The reed switch is working fine, nothing on the PCD look burn or anything. All the thermostat have same resistant... So might have to manually heat up each thermostat with a lighter or something to check to make all working at 100% you think? Diagram might help..

Edited May 4, 2015 by Motorcar

[RichCor]

A sticking water Flow-switch would allow the heater element to remain on, causing the heating vessel to superheat and then conducting heat up and back through the metal wire, creating conditions that allowed the jacket to melt and the conductors to partially fuse.

Are you sure the Flow Switch is working properly. They can have issues with hard water (or if the filter screen isn't installed).

Edited May 4, 2015 by RichCor

[Motorcar]

Diagram...

[Motorcar]

Yep, the flow switch in this case is a reed switch and the flap inside the pipe is moving freely... Tested with multimeter and seem to work flawlessly ...

[Tywais]

Possibly the triac is faulty/shorted in the unit. Even when the heater is on it doesn't pull full power but uses a triac controlling phase to vary the power to the heater element. If shorted, it will put the full power to the heater all the time and overheat marginal wires. When working normally, the average current through the wires will be below a level to heat them like that. A faulty flow switch will not do that.

[RichCor]

Possibly the triac is faulty/shorted in the unit.

Even when the heater is on it doesn't pull full power but uses a triac controlling phase to vary the power to the heater element. If shorted, it will put the full power to the heater all the time and overheat marginal wires. When working normally, the average current through the wires will be below a level to heat them like that.

A faulty flow switch will not do that.

Yep. A faulty triac constantly passing current would also explain why the supposed fail-safes of a reed water-flow switch and temperature cut-off didn't work and allowed the water in the containment vessel vaporize. Nice.

[Motorcar]

Make sense... Will get a new Triac to test it out later on.. So the wire size is fine? Thinking of replacing them with 12AWG since they a bit too short if terminating at where it got short out... Or might see if the shop got the wire with fibreglass sleeves..... Thanks guys

[Crossy]

Possibly the triac is faulty/shorted in the unit.

Even when the heater is on it doesn't pull full power but uses a triac controlling phase to vary the power to the heater element. If shorted, it will put the full power to the heater all the time and overheat marginal wires. When working normally, the average current through the wires will be below a level to heat them like that.

A faulty flow switch will not do that.

Yep. A faulty triac constantly passing current would also explain why the supposed fail-safes of a reed water-flow switch and temperature cut-off didn't work and allowed the water in the containment vessel vaporize. Nice.

Indeed, I'm most surprised that a mainstream brand does not have a foolproof overheat cutout, particularly as triacs do tend to fail short (on).

[lopburi3]

Is this unit rated multi-point? If not it is most likely designed to be turned off at unit and after each usage (not using water flow switch). Can not find the manual but it should be mentioned in it - many of these point of use heaters are designed to be turned off after each usage (indeed some have water flow/turn off combined control). Understand this is not mentioned as much as previously (better flow control switches I guess) but using upstream turn off has always required the multi-point type of unit AFAIK.

[RichCor]

When Tywais mentioned the triac and I looked at the logic of the posted schematic diagram... oh my.

Really surprised the flow switch and thermo-cutoff aren't controlling a power relay prior to the heating element (this design seem to think the triac can do that job as well).

Replacing the triac may get the unit running in spec again, but there's still the direct danger of the same issue repeating -- leading to an electrical fire.

This isn't a safe product.

[Tywais]

When Tywais mentioned the triac and I looked at the logic of the posted schematic diagram... oh my.

Really surprised the flow switch and thermo-cutoff aren't controlling a power relay prior to the heating element (this design seem to think the triac can do that job as well).

I've repaired a few units across different brands and what is shown is standard in them. The thermal cutoff you see in the OPs photo is a small low current sensor and connects to the controller and disables the triac gate driver. I suspect it is manufacture cost cutting the precludes the inline bimetallic thermal switches being used as 20-30 amp thermal switches can be expensive to implement. The example below costs about 2500 Baht which is the price of some entire units.

[Tywais]

Also, it would be better to just test the triac before replacing it. Make sure the breaker is off to the unit, disconnect one power wire on the triac and disconnect the wire on the gate terminal. Use an ohm meter and check across the triac for a short. Then check between the gate and each of the power points on the triac. This will verify if it is the triac or a problem in the logic PCB.

[Motorcar]

Also, it would be better to just test the triac before replacing it. Make sure the breaker is off to the unit, disconnect one power wire on the triac and disconnect the wire on the gate terminal. Use an ohm meter and check across the triac for a short. Then check between the gate and each of the power points on the triac. This will verify if it is the triac or a problem in the logic PCB.

Zero ohm between the power terminals which mean the contact is closed and get 88ohm with either power terminal to the gate. I guess this mean the Triac is short circuit?

[Tywais]

Also, it would be better to just test the triac before replacing it. Make sure the breaker is off to the unit, disconnect one power wire on the triac and disconnect the wire on the gate terminal. Use an ohm meter and check across the triac for a short. Then check between the gate and each of the power points on the triac. This will verify if it is the triac or a problem in the logic PCB.

Zero ohm between the power terminals which mean the contact is closed and get 88ohm with either power terminal to the gate. I guess this mean the Triac is short circuit?

Yep, it's toasted.

[Motorcar]

Triac toasted... Wires also toasted as can be seen on the picture... No real fail safe eh???