Cusser

I've used bromine tablets in my 7 ft CalSpa since it was new in 1988. Why bromine? Because CalSpa manual said to use bromine. I'm not sure why. But that's all I've used.

Ridiculous - they have a defect, and want to make money from it !!! Reminds me of the time my mom had a recall on her used Cadillac, during which the dealer told her she had a coolant leak and the dashboard would need to be removed to fix it, her $$$. I asked her how big was the leak on the garage floor, and how often she had added coolant - NEVER. She took it to a mechanic I've used, and he tightened a hose clamp to fix it - AT NO CHARGE !!!

I set at 104F, Mrs. Cusser doesn't like hotter. I have a cheap thermometer in the spa that I use to determine this, and set the rotary dial accordingly. We rarely use the jets or the air bubbles (due to noise), and I use brominating tablets.

My guess - and just a guess - is there likely is a calibration screw somewhere, to get to that 104F as each sensor/thermostat/heating has a specification range like automotive connecting rods, not all are EXACTLY the same. My own 1988 CalSpa has a dial with a line which I turn clockwise for higher temperatures, and this can get too hot if turned too far, but this was way before regulations. If I needed to go hotter still, I could pull the sensing tube farther away from its tube/cylinder holder in the heater manifold and it would read lower than actual, so would produce more heat to compensate. Maybe the modern spas with all the complicated electronics have stuff that cannot be over-ridden.

I wouldn't use boiling water, too dangerous to handle and a hassle.

Ever think about plumbing in a small electric water heater in-line with the spa plumbing ? https://www.homedepot.com/b/Plumbing-Water-Heaters-Point-of-Use-Water-Heaters-Tank-Point-Of-Use-Water-Heaters/N-5yc1vZbqo7

OK, have a 1988 model CalSpa, since new, 220 VAC. The first 2 decades, due to a demand electric rate surcharge for entire month based upon highest HOUR of usage, the spa was on load controller, which wouldn't deliver electricity for heating unless the demand was below our set limit. Then the load controller died,over $1000 to fix, and electric company was due to change rates to demand/higher costs in certain afternoon - early evening hours. So I started just leaving it "set" to 104F 24/7 so any "demand" during those peak hours would be low. To my surprise, the spa only actually heats less than 30 minutes a day in winter, and winter here can be 35-40F. So now I just leave it 104F all the time, except in summer, when we don't heat it because it gets that hot anyway (Arizona).

I use a small vacuum that looks like this on a pole, with a plastic vacuum hose, for my above ground pool with plastic liner. I've had the pool since 2000, on its second liner.

My assessment: neither the Ace or the FlexSeal really did a decent seal applied under the water, I returned both to Ace, product did not perform as stated in writing on the packages, both curled up starting at the edges, the Ace patch floated away. Maybe these would've worked fine if I had drained the pool to apply, but with temperatures like our 116F here in Phoenix yesterday, just not the time of year to be without use of pool for a half day or day.

Air pressure increases in a sealed system as temperature rises. However, doubt that your system is perfectly sealed, like one needs to add air periodically to vehicle tires, bicycle tires, etc. So my guess is just needs air addition each month. However, I think that you should call your manufacturer's technical line and ask there.

Disclaimer: I'm not a professional electrician, shocking news. Consult an electrician.

Your "stone age jacuzzi" is 17 years newer than my 1988 CalSpa !!!

If I chose to drain the pool, then sealing it/repairing it is less of an issue.

Funny - just last night I tried a piece of FlexSeal tape underwater to the liner of my above-ground pool over a seam where I'd patched about 4 years ago. So it's too soon to tell for me. An underwater patch kit from Ace Hardware pool aisle (about $4) tried a week ago did not stay attached and will be returned as not performing as to claims/satisfaction.

Ha !!! My 1988 CalSpa did not come new with a GFCI for the tub, just one for the light !!! Mine's 40-amp, 240 VAC hard-wired, done by professional electricians who also ran the 240 VAC wires through my attic. I read about GFCI for spas in Popular Mechanics magazine a few years later, and added my own 40-amp GFCI for the tub/heater/pump etc. So I bought an appropriate-sized circuit breaker and a circuit breaker box and added my own; mine happens to be inside the spa cabinet. I'm not a professional electrician, but this offers me more protection than I had before. Plus, the non-GFCI circuit breaker is on the complete opposite side of my house, outside the fence.