How do I shock the spa?

[surfhawaii]

Looked around the site and can't find info on chlorine shock levels for the spa. Are there some posts somewhere I can read? Water is cloudy and trying to figure out the cause but thought it best to shock it first but not sure what the correct amount of bleach to use.

 

[chem geek]

Assuming your CYA level is in the 30-40 ppm range, shock the spa to an FC of at least 10 ppm assuming the water is hot (95-104ºF). If the water isn't hot, then 12-16 should be your target FC.

Are you using the Dichlor-then-bleach method?

 

[surfhawaii]

Uhm....I am using bleach but don't know what the 'dichlor-then-bleach' method is...is that addressed in pool school?

 

[chem geek]

You shouldn't be using bleach without any CYA in the water. It will be far too strong, will get used up too quickly, will oxidize skin and hair too quickly, and oxidize hot tub components including covers too quickly. Read Using Chlorine in a Spa, but I'd target 50 ppm for the TA and use 50 ppm Borates for additional pH buffering.

 

[surfhawaii]

Ok, sorry, didn't make the connection of dichlor being CYA and good point, I'll check it with other tests. When I refilled spa I used pool water and was assuming water came with the same CYA % in the pool...which is ~ 45 ppm.

I have noticed that pH level needs attention more frequently than the pool because it rises much faster...assuming that's because of more aeration. On a related track, I'm using the pH reducer the Spa people gave me...for some reason I'm a little nervous about putting in muriatic acid to adjust it like I do the pool.

Thanks again for your input...very valuable.

 

[chem geek]

Oh, when using pool water as a starting point, you will get CYA initially so that's good. The TA is probably higher than you'd want for a spa because as you've noticed between the hotter water temperature and the greater aeration, there is more carbon dioxide outgassing so the pH rises faster. If your spa as its own separate circulation isolated from the pool, then you should be able to manage it just as you would a separate spa.

Note that unlike your pool, with a spa you have to dose with a good deal of chlorine after each soak. The bather load is far higher in a spa because of the smaller water volume. Without an ozonator, each person-hour in a hot (104ºF) spa requires around 5 fluid ounces of 6% bleach (3.6 fluid ounces of 8.25% bleach) to oxidize the bather waste, though you should just use whatever amount gets you to around 1-2 ppm FC for the start of your next soak. You usually target a lower starting FC to reduce chlorine/chloramine smell during your soak. You can go higher if you want to, but no lower, and you must dose right after your soak.

 

[Iowamod20]

Just wanted to clarify the "per person hour" math. If we have 2 couples in the spa for the evening, does that mean 4 persons x 6 hours x 3.6 oz. = 86.4 oz. of bleach I would have to add?

Thanks.

 

[chem geek]

If the spa were 104ºF, then that would be the correct math. However, there's no way you could soak in a spa at 104ºF for 4 hours. Since the spa temperature must be lower, you will sweat less. So the amount of bleach needed will be less. It's probably closer to being in a pool and perhaps is half or less the amount you calculated. If you have an ozonator, then it would be at least half that again (so 1/4th the original calculation).

The real rule is to add whatever is needed such that you still have some chlorine residual 24 hours later. The per person-hour rule is just a rough guideline and applies to a typical person soaking in a hot 104ºF tub which they can usually only do for 20 or 30 minutes. Going for 1 hour at 100ºF would use somewhat less chlorine than predicted by the per person-hour formula.