Spa heater corrosion questions

[Squirtle]

My spa heater recently failed. The spa started tripping the breaker. A tech came out and determined the heater was the cause (he disconnected the heater and the breaker stopped tripping). After the $180 diagnosis fee and a quoted $350 for replacement heater (which I can find for $228 on first Google) + additional for labor, I decided to DIY a fix, hopefully by swapping out just the heating element. I have removed the heater assembly and am now figuring out next steps. Here is the situation (there are also attached images):

- According to the tech, the heater is a Watkins 77358. This does indeed look like my heater. I am a little confused because I don't see "77358" anywhere on the actual heater, which lists a model number of HEAT.WAV-YJ-4-240-2-INC-IF-T-V3 (see picture) and a brand of Gecko, but I guess this is just a different model #/brand slapped on the Watkins.
- The spa is 4 years old (purchased new Aug 2019) and was used for 3.5 years (was unpowered/unfilled for 6 months after a move).
- The last year of use it has been sanitized with a SmarterSpa salt water generator. Prior to that, the first year was trichlor followed by 1.5 years of bleach.
- Haven't been incredibly conscientious about water chemistry but haven't let things go to extremes. Typically refilled every ~4 months with average of maybe 30-person-minutes a day of use.
- When pH was out of the ideal range, it was almost always higher (>=7.8). Almost never had low pH.
- I observe light corrosion on the inside and outside of the heater tube and on the heater element. There is much heavier corrosion on the heater terminal bolts.

Here are my questions:

- Does the corrosion inside/outside the heater tube mean that I need to replace the whole tube + element, or would it be OK to just replace the element?
- What would happen if I just cleaned the corrosion off the terminals with a wire brush? Seems like this could be a viable fix if it's just "normal" slowly accumulating buildup over years of use?
- The tech told me that heaters regularly fail every 5ish years. Is this always due to corrosion? Any other causes of heater failure?
- Is the level of corrosion on my heater "normal wear and tear", or does it indicate something unusual?
- Amazon has various heating elements advertised as universal. I see they vary in length (7" and 9") and wattage. My heater is 4kW and I haven't yet removed/measured the element. In general, can I expect any of these 4kW elements to work with my heater? Is it important to match the length of the existing element or any other parameter?

 

[ajw22]

@RDspaguy may help when he wanders by.

 

[RDspaguy]

Ok, pressed for time so here's the short version.
You are corroding stainless steel. Even acidic trichlor struggles to do that, and you claim high ph. It's your salt cell most likely. People here will argue who have no problem with theirs, but I follow clues. Salt systems, aspecially aftermarket, toss-over types, can do some serious damage in the right circumstances.
3-5 years is average, but bad chemistry can take out a heater in a few months, and I've seen them last well over a decade. Corrosion is #1, but many things can damage a heater. Corrosion also has multiple sources, and in your case I suspect leakage current off your salt cell going to ground at your heater.
Most spa manfs are not equipment manfs, and buy parts from multiple vendors. That heater could have 20 different spa brand names associated with it if they all buy from the same manf.

 

[JoyfulNoise]

Spa heater elements come in different material types - Teflon coated steel, Incoloy, and Titanium. For a saline environment you want to use Incoloy at a minimum but titanium is the better choice. The higher salinity of your spa is leading to corrosion.

You can replace the element but you have to be absolutely careful when doing so. You must be careful to ensure the bolts holding it in the tube are tightened without cracking the seals and you have to make sure the element is precisely centered in the tube so that it is not touching the internal walls of the tube. If the heating element touches the tube, you will cause even more damage to the spa heater assembly.

 

[Squirtle]

Thanks @RDspaguy and @JoyfulNoise. Some follow-ups:

>You are corroding stainless steel. Even acidic trichlor struggles to do that, and you claim high ph. It's your salt cell most likely... in your case I suspect leakage current off your salt cell going to ground at your heater.

The corrosion on the washer between the terminal nut and the tube is by far the most extreme. One of them basically disintegrated when I removed the nut. Is that standard for all heater corrosion problems, or is it particularly indicative of your salt cell leakage current hypothesis?

Also on the leakage current question-- I don't fully understand what that means/how it works. To my (incomplete) understanding an SWG cell contains two electrodes. There is a voltage difference between the electrodes from the power source. The circuit is completed by positive dissolved ions flowing toward the cathode (electrode at lower voltage) and negative toward the anode (electrode at higher voltage). In an SWG setup, the positive ions are (mostly) Na+ and negative ions are Cl-. The voltage is such that the Cl- actually lose their extra electrons when they contact the anode and become oxidized to Cl2 gas. This process continues as long as the voltage difference is maintained between the electrodes and there is a sufficient concentration of chloride.

How and why would "leakage current" from the cell to heater occur here? Would it be because of something faulty with the cell, or the heater? Is it that the metal nuts securing the heating element to the tube are at high voltage and there is just enough moisture leakage at the site those bolts are attached that a conductive pathway (through the water from the cell, down through the pipes, to the heater) forms from cell electrode to the corresponding heater terminal? Since there is a large voltage difference between the heater terminals and cell terminals, wouldn't that cause a high current that would electrocute a spa user?

 

[Squirtle]

Also, in trying to remove the element I noticed that the heater bolts actually seem to be welded to the heater assembly power cord. This does not match any setup I've seen in YouTube "swap out your heater element" tutorials.

(1) I guess this means I probably need to replace the whole assembly... anyone swapped out just the element with this setup before?
(2) How tightly coupled are heater assemblies generally to the rest of the spa? Is it possible I could just use a different heater assembly?

 

[RDspaguy]

Electricity is not like a solid, but a liquid. It will always take the path of least resistance, but is by no means limited to one path. Salt water is a great conductor, and it only takes millivolts to corrode dissimilar metals in a salt solution. Make that solution acidic and it will make it's own dc voltage, since it becomes a battery (dissimilar metals in an acid solution). But maybe it's just acidic corrosion left over from your trichlor days, or damaged o-rings from an overheat or airlock causing leaking at that point.
I am unsure of the danger in using one of these, but I suspect it is quite low, likely using DC voltage to generate chlorine. Though I bet the instructions probably tell you to remove it during use.
You can replace the whole assembly if you like, or just the element. The welded wires are just that brands way, you can use a bolt terminal type, you just need the wires with soldered terminal connectors. Crimp terminals on these tend to overheat.