Left Hot Tub Unattended For 2 Nights. Should I Drain & Refill It?

[rob.montreal]

Hi everyone.

Newbie hot tub owner from Montreal, Canada here. I've had my 450 gallon Trevi hot tub for 3 weeks now and loving it. I am also really glad to have found this forum and the wealth of information you have posted here.

We went away for 2 nights this weekend and I just want to pick your brains on whether I did enough to keep the hot tub water sanitized or if I should decontaminate and refill.

Here's some background info and what I did before and after the weekend:

The tub is 450 gallon, with an ozonator. I keep the water at 94 degrees. PH is 7.2 and ALK is 80. I sanitize it with chlorine (lithium), but without CYA. I only found this forum and the dichlor+bleach tutorial after I had bought my chemicals, and I haven't had the time to go back to the store and buy CYA. I plan on starting that next week.

The night before we left, I chlorinated to 10ppm. The tub had not been used that day. The next morning I still had 5ppm of free chlorine in there. I didn't add any more chlorine that morning, but dropped 4 bromine tabs in a floater, opened it to the max and put in in the tub. Then we left and the tub was alone from Saturday morning to monday evening. The power was on, so the ozonator and filtration cycle (8 hours / day) ran during our absence.

Last night when I got back, there was zero FC (which I believe is normal since there's no stabilizer), zero bromide and the bromine tabs were barely showing any sign of use. Combined Chlorine was at 1 ppm. The water was crystal clear as always and smelled clean. I ran both pumps on full speed for a few minutes and there was no weird stuff coming out of the pipes and no foaming in the water. PH was still 7.2 and ALK was down to 60.

I removed the bromine floater and added 4 tablespoons of lithium, which shocked the tub to 17ppm FC. Kept the cover up for about half an hour and ran both pumps full speed for about 10 minutes.

I forgot to check the FC this morning to see how much was consumed, but I'll check it tonight and report back.

Based on all of the above, do you think that was enough to keep my tub water sanitized or should I drain and refill? Like I said, the water looks and smells great.

On a different topic: before I found this forum I took a mandatory spa maintenance class with the manufacturer. They recommended that we sanitize with unstabilized lithium: 1 tablespoon per user each evening, up to a max of 4 tablespoons a day. On days when the tub is not used, just add 1 tablespoon. They specifically told us to not use any stabilizer because "it will damage the headrests". Does that sound like a valid reason, or are they just trying to make us use more lithium and thus buy it more often? (The spa manufacturer has their own store and sell their own chemicals at a pretty steep price). I honestly would like to go the stabilizer+bleach method as it is not only cheaper, but will also allow me to measure FC accurately. The way it is now, FC is always 0 the next day and I never know if that's only because of the ozonator, or because I didn't put enough to oxidize all human waste.

Many thanks!

Robert.

[rob.montreal]

Update: so i just tested the water again 22 hours after I chlorinated and both FC and CC are at 1ppm. That means the chlorine I put in last night oxidized / killed everything without running out and that my water is safe for use, right?

Btw: i finally bought some CYA today. Dissolving it as we speak and hope to be able to pour it in before the end of the week and start using bleach.

Thanks!

[chem geek]

I'd add more chlorine to see what percentage of the FC you lose over 24 hours. With an ozonator running during circulation cycles, you will likely lose around 50% of the FC over 24 hours with hot water. Without an ozonator it would be more like a 25% loss. This is because ozone reacts with chlorine. So you are probably OK to use the spa -- so long as you have measurable FC in the water and the water is clear, you should be OK.

[rob.montreal]

Thanks! That's reassuring. The water is crystal clear and smells great (i.e. doesn't smell )

The night I wrote my previous post, I added 1.5oz of lithium without having used the tub (triple the dose the store recommended), which brought FC up to 9ppm. ~22 hours later FC and CC were both at 3ppm. This is without CYA.

The CYA is still dissolving in a jug. Damn, it takes long!! Hope to be able to pour it into the tub this weekend and measure Chlorine Demand.

The 50% loss you mentioned above, is that with or without CYA? I assume it's with CYA, right? Because I've been running without CYA so far and FC usually drops to zero by the next evening, even when I don't use the tub, unless I over-chlorinate the way I did this week. The specialist at the store told us that this is normal without stabilizer and that we should only worry about pouring in the recommended dose of lithium in the evening. (.5 oz/person).

But I just never know if the zero FC is normal and due to the ozonator and lack of CYA or if it means that I didn't put enough to oxidize our waste. But it also drops when we don't soak, so that must mean it's the ozonator that depletes it.

Hopefully starting on CYA this weekend will help me properly measure what's going on.