Hot water system element - short life - Page 1
Forum section: electronics / general

author: DTJ
author_rank: Super Contributor
timestamp: July 01, 2017, 01:40:59 am
content: I've got a Rheem solar HWS and the heating element blew a couple of weeks ago.I replaced it with a genuine Rheem element and two weeks later it's failed again and I'm wondering if it was just a dud element or if there's an underlying reason.The previous element lasted 2 years, this one only 2 weeks.The element has a split open section where it failed. It appears to have a coiled heating element surrounded by white ceramic and then the sheath.Crappy photos below.It's a 2400W 240V element (Australian system).The thermostat seems to be working (ie not stuck closed) as the tank temperature reaches and stays at the 60°C thermostat setting.The earthing all looks to be intact. Although probably not to be relied upon the plumbing to the system is all copper which should provide a secondary earth.Any ideas as to what could accelerate the heating element failure?

author: Cubdriver
author_rank: Unknown
timestamp: July 01, 2017, 01:45:37 am
content: Similar failure to the other element? I suppose it's possible you might have gotten a defective replacement that suffered an infant failure.-Pat

author: DTJ
author_rank: Super Contributor
timestamp: July 01, 2017, 01:55:44 am
content: The prior failure (lasted 2 years) had gone open circuit but the stainless casing seemed to be intact.

author: Brumby
author_rank: Unknown
timestamp: July 01, 2017, 02:01:55 am
content: Open circuit is pretty normal - but, 2 years seems to be a rather short life.To my eye, this failure is clearly the result of pressure buildup at that point of the element. The outer casing has expanded to the point of fracture at which point, water will end its days very quickly.It would seem the normal construction with the actual current-carrying wire being suspended and insulated from the casing by a ceramic medium must have included a contaminant of some sort that, when heated (probably), expanded significantly. Remember too, that the temperature of this wire is going to be much higher than the surface temperature of the outside of the casing ... and also, that the internals are sealed, like a pressure vessel.A drop of water, oil or any number of things in the ceramic material would be enough...If the manufacturer got a bad batch of ceramic, there will probably be a lot of early failures like this - or you may be the lucky one who received that one special one.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 01, 2017, 02:13:38 am
content: Is the mains voltage high, even if still within spec? Remember that power dissipation in a resistive device is proportional to the square of the voltage.One possible workaround is to install a 277V element, if you could even find one that would fit. Another solution is to add a series diode, although that would greatly affect performance. Maybe the best solution is a series diode bypassed with a relay during peak times?

author: DTJ
author_rank: Super Contributor
timestamp: July 01, 2017, 03:48:44 am
content: It's looking like I might have just got unlucky and forms dud unit.The mains is around 239V (inner suburban) so over voltage shouldn't be an issue.One thing I've been wondering about is when the element sheath fails one side of the element is still connected to active.The water will provide a path to neutral and also back to Earth via the plumbing.The element is supplied by a circuit breaker but not an ELCB.I thought I'd see more electrolytic corrosion from current flowing after the sheath failed.

author: SeanB
author_rank: Super Contributor
timestamp: July 01, 2017, 09:10:53 pm
content: You do not see much till the element starts to arc over and trips the breaker. Looks like you just got a faulty one, and should return it under warranty asa it is defective in manufacture. Just a question, have you changed out the sacrificial anode in the system, it should be there, but is probably a pile of white crud and bits sitting in the bottom of the cylinder, and a very corroded plug it screws in to on the base of the unit.

author: floobydust
author_rank: Super Contributor
timestamp: July 01, 2017, 09:20:52 pm
content: It might be stress fractures from the U-tube bending process, during manufacturing. The mandrel can apply sharp point stress and stretch. The tube wall thickness looks much thinner close to the bend.It might be heat stress if the U is mounted vertically, not horizontally. The lower tube heats the upper tube and localized boiling perhaps.

author: DTJ
author_rank: Super Contributor
timestamp: July 02, 2017, 12:02:27 pm
content: Quote from: SeanB on July 01, 2017, 09:10:53 pmInterestingly it did not trip the circuit breaker. Unfortunately I have to wear the cost of the failure as I purchased it from a trade outlet and they will only provide warranty if the element was fitted by a licenced electrician.My system does not have a sacrificial anode. It's a stainless tank system.My previous system had an enameled steel tank with an anode that required periodic replacement.

author: DTJ
author_rank: Super Contributor
timestamp: July 02, 2017, 12:05:18 pm
content: Quote from: floobydust on July 01, 2017, 09:20:52 pmYep I'm hoping it was just a manufacturing defect. The element is installed with the tubes one above the other, I assume this is correct as this is the orientation of the original factory installed element,.