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I have a 215L Dux branded hotwater electric tank. 240V, single 4800W element. Its about 3 years old and about to have its 4th thermostat installed.

What is causing the thermostats to blow?

What I do know:
The fuse box has a switch (lever: on and off) and a single 20A breaker that does not trip. I put it in myself replacing the old ceramic fuse that was installed. It had signs of getting hot but was not blown.

Themostat is a robert shaw (previous three also) 30A. The hot water system was moved about 2m by the plumber and electrical cabled extended by an electrician hired by the plumber. I have not looked at these wires. The system is on an offpeak meter controlled by a ripple control system. I dont know much about the ripple controller.

It seems that when we are away for a few days and its cold that the issue occurs. Or even after a longer cold spell.
Note its still warm when we get back but is dead the following day or soon after. Could be coincidental but seems like the use of the dormant system trips the cause frying the stat.

This last time we were away only overnight. Tank is outside. Temps have been down to close to zero deg C, but consistanly around the 5 mark.

Tank is set to 70 deg C, a tempering valve at the tank drops temp into the house.

The second time the thermostat blew the element was also replaced (just in case), though it tested fine.

What else. The reset button never appears to do anything, is u can push it, but feels like its not out, push and it doesn't go in.

Its getting to the point I need a new electrician that can help trouble shoot the actual problem and not just change the $40 stat and charge me $200 for a smile.

Sparky here.

How are the thermostats blowing? Are they melting or catching fire, or just ceasing to work?

Does the little red overheat/reset button pop out?

What temperature do you have them set at?

Is the thermostat hard up against the tank?

Take a photo and I'll see if I can spot something obvious

I would call the supply Authority (Ausgrid? Endeavour?) and get your ripple control relay looked at if the thermostat isn't obviously broken. Ask them if they could install a time clock as these very rarely fail. It will also be free if the fault is found to be with the relay and they provide a warranty for any of their work.

Gannon wrote:Sparky here.

How are the thermostats blowing? Are they melting or catching fire, or just ceasing to work?

Does the little red overheat/reset button pop out?

What temperature do you have them set at?

Is the thermostat hard up against the tank?

Take a photo and I'll see if I can spot something obvious

Stat just ceases to work. They look fine to the eye. Ohms tested this one, zero pos to right side element. New one reads a few ohms. Element reads 11 ohms. 0 to ground/tank.

Reset button has never poped.

Set at 70deg

Yes stat seated tight against tank. No signs of rust.

From memory. The original stat did melt and shorted out frying the stat

Is this like yours?

A thermostat has 3 separate thermo switches in it. 2 are over temperature cutouts that are re-settable by the reset button and the 3rd is a cycling switch that is adjusted by the temp dial

Now on the element above, it looks like active and neutral are the top 2 terminals, and the bottom 2 are connected to the element.
There should be a circuit between the 2 left terminals, 3 & 4, as this is the neutral side of the OTC.

There should also be a circuit between the 2 right terminals 1 & 2, as this side has the active OTC and the cycling switch.

11 ohms on the element sounds about right for a 4.2kW

555Ron wrote:I would call the supply Authority (Ausgrid? Endeavour?) and get your ripple control relay looked at if the thermostat isn't obviously broken. Ask them if they could install a time clock as these very rarely fail. It will also be free if the fault is found to be with the relay and they provide a warranty for any of their work.

Yeah had the ripple control meter checked the second time, but who knows what they checked. They came out ensured it was working and left.
It could be related, there is a problem, just need to locate it.

If your thetmostat is going open circuit, it won't have anything to do with your ripple receiver.

Sent from my new touchy phone thingy

Gannon wrote:Is this like yours?

Yep that's about it Gannon. Mine is the ST1205134 to be exact. The only thing I can think is that the plumber put a 4800W element in when the stat died the first time. Was a replace both just in case type event. But he broke the original element getting it out. Anyway, the fuse box has a 20A breaker. Is it possible the 4800w is too much for the wire/fuse from the box to the heater? The product specs show its original was 3600w.

Weird though that the breaker at the box has never tripped, nor has the thermostat. It just dies.

So I put the new thermostat in last night, water is nice and hot again this morning. But I still need to resolve the issue itself and stop bandaid fixing before the house burns down. The stats are frying for a reason!

So prob next course of action will be to get a 3600w element and change next time we go away for a few days (ie turn the heater off before we leave). If it dies again after that, get an electrician to inspect the entire run from the box etc

4800w is 20A so you are pretty much on the limit of your cable and the circuit breaker. But there is nothing in the thetmostat that will trip out from too much current. It only works off temp. If you want to send me the thetmostat that is broken, ill test it for you to find out why its not working. Pm me.

Sent from my new touchy phone thingy

Gannon and Smokey,

Gannon, are those thermostat switches Solid State Relays (SSRs)? Or mechanical ones?

Couple of ideas if they're SSRs (from experience)...
- If there's an occasional short to ground, through the water, they could melt
- It says resistive load. Is it possible the 4.8kW element is (at least partially) inductive and likely to back-EMF and possibly blow the SSR? May need a snubbing capacitor. Happens with PID controlled furnaces/ovens.
- If there's another inductive load somewhere on the circuit, such as a motor compressor (fridge, A/C, etc) the device may not like it either.

Also is it possibly exposed to steam/humidity from the hot water (or outside elements) due to poor design/placement?

Just some ideas,

Gannon, are those thermostat switches Solid State Relays (SSRs)? Or mechanical ones?

Mechanical. Hot water thermostats are 3 separate thermo switches in one that fail safe /open circuit. It would not be cost effective or safe to have a SSR controlled by an electric circuit.

- It says resistive load. Is it possible the 4.8kW element is (at least partially) inductive.

No

- If there's another inductive load somewhere on the circuit, such as a motor compressor (fridge, A/C, etc) the device may not like it either.

No, hot water systems are on their own circuit

Also is it possibly exposed to steam/humidity from the hot water

No, the tank is a sealed unit so there should be no steam on the outside of the tank. Even if there was, I have seen these thermostats continue to work with a constant stream of water flowing over them.

The issue will be that, for some reason, the otc trips prematurely.

Smokey,.. when the element was installed, did the plumber install it the right way up? There will be writing stamped on the metal plate part of the element that says "install this way up for bottom element"

Gannon,

re: no, Non, NEIN....

Ta for setting me straight- obviously full of my own hot air/steam . But your valuable info has been filed 'upstairs' for future reference!

Thanks,