We have a Joule sous vide which recently developed an issue - the motor which drives the impeller started sounding like it was struggling, and then one day it just stopped in mid cook.
We do not have particularly hard water, but I tried the standard "run it in a vinegar / water solution" - this made a tiny improvement, but not enough to make a useful difference. I looked some online, but wasn't able to find any repair instructions - there were a number of posts showing that bits are glued in, and so it cannot be disassembled easily.
Anyway, I was not happy throwing it away, so I decided to try fix it - this worked for me, it may or may not work for you as well. Obviously, do this at your own risk, I take no responsibility for, well, anything...
Firstly, get some thin silicon oil - I use "Super Lube 56104 Silicone Oil 100 CST" - you are looking for something thin, silicone grease won't work for this.
Flip the Joule over, and use some tape to block off the water outlet - I used Kapton tape because it was handy, and knew that the adhesive would survive the oil.
Remove the impeller - it has a small hole in it specifically for using a fork to pop the impeller off.
Squirt a little bit of the oil in (another advantage of Kapton tape is that you can see the liquid level) - I filled it to around 1/2 way up the water exit hole. Now, leave it to sit for a few hours. Every now and then wiggle the shaft - it is remarkably stiff, and so can take a fair bit of force, but don't push too hard or you might bend it. Basically, you want to try get some of the oil to slide down the shaft so it lubricates the seal.
After a few hours I was getting impatient, and so I bent a paperclip into a small hook, chucked it in a drill, and used this to spin the impeller for a while. After I'd done this the shaft was noticeably easier to turn, so I flipped it over and tested it -- and it now runs like new...