How often to change water?

[im45us]

How often do you change your water? 3 , 4 or 6 months or different.

[Sam]

Water change intervals are affected by bather load.  As a rule of thumb, most places just say every three months.  There is a formula that I cannot recall.

[chem geek]

The standard Water Replacement Interval (WRI) that I've adjusted to account for person-hours (the original formula assumed a fixed time of probably 20-minute soaks) is the following:

WRI (days) = (1/9) x (Spa Size in Gallons) / (# of Person-Hours Per Day)

This formula works well when using Dichlor-only.  When using the Dichlor-then-bleach method you can go at least twice as long between water changes so the factor in the formula would be (2/9).

[Dr. T.]

The tub is new (as am I to hot tubs), so the water is only about 3 weeks old - not sure how often I'll need to change it.  Although there are approximations (and calculations), I've read a good rule of thumb is:  If it smells and looks good, then don't change it.  Since it's new, we're getting quite a lot of use out of it - so I think 3 mo. is a good guess.

The Enhanced Shock (SpaGuard brand) is only about half dichlor - the rest is MPS (I think), and my understanding is that you can break the chloramine bond through high Cl concentration or by oxidation.  So my weekly dose of shock is to accomplish this.  I'm adding Dichlor (98%) after each use - about 1 to 2 TBSP, depending on number of bathers on that day.  The next day, my Cl usually reads zero.  Seems like I'm adding a lot of Cl, but maybe not.  The water is nice and clear. 

I've read alot of threads where people use their shock as a daily treatment instead.  I'm sure there are mixed opinions on that one, but my point of view is that after adding shock, the chemicals/gases generated should have some time to dissipate before the cover is put back on.  I don't see this as being an optimal scenario, especially in the winter when I want to jump out and get into the house :-)

[chem geek]

The reason Dichlor-only doesn't last as long is that it builds up Cyanuric Acid (CYA) that makes the chlorine less and less effective because the active chlorine level is proportional to the FC/CYA ratio.  So your use of a mix with Dichlor and MPS should build up the CYA more slowly.  This is how the Dichlor-then-bleach method works where the CYA is initially build up and then bleach is used so that the CYA no longer builds up (Dichlor is used about once a month to bring the CYA back up since it's slowly oxidized by chlorine).

Now unless you already purged your spa using Ahh-Some, you may find that you need to change your water sooner because you didn't get rid of the greases, oils, and biofilm often present in new spas.  After that first change, it will be better, especially if you use Ahh-Some just prior to the change.

[Brian3248484]

Sorry this is a little off topic but I saw you said that you shock weekly?  I just got my hot tub about two weeks ago and we have been using it a lot too.  I've been throwing in the shock every time we get out and running the pump on high for 30 mins.  Am I wasting the chemical?

[chem geek]

Brian3248484,

If you are dosing with chlorine properly, then you should never need to "shock", but what exactly are you adding that you are calling "shock"?  You should add enough chlorine after your soak so that you still measure a residual 24 hours later.  If you aren't soaking every day, then shoot for a background 2 ppm FC chlorine level in between soaks, but not more than 4 ppm.  As for the target before your next soak, most people prefer 1-2 ppm FC to minimize noticing chlorine/chloramines during their soak.  If one is dosing after a soak and measuring zero 24 hours later, they aren't adding enough.

Note that ozonators consume chlorine.  Ozone reacts with chlorine and typically double the amount of chlorine consumed in between soaks so the daily (24-hour) chlorine demand with an ozonator is typically 50% or more whereas with no ozonator it's usually less than 25%.  On the plus side, an ozonator typically cuts down the amount of chlorine needed after a soak by at least half.  That is, ozone will oxidize some of the bather waste so that chlorine doesn't have to.

To prevent buildup of CYA from the Dichlor that makes the chlorine less effective over time, one can either switch to using bleach or can use some non-chlorine shock instead of Dichlor.

Dr. T.,

1-2 tablespoons of oxidizer is a lot if it were only Dichlor (but it's not so read on).  Normally it takes around 3-1/2 teaspoons so around 1 tablespoon to handle one person-hour of soaking in a hot (104ºF) spa so, say, two people for 30 minutes.  Your product, SpaGuard Oxidizer Enhanced Shock, according to this MSDS is 58.2% Dichlor and 15-25% sodium persulfate (note that this is not the usual non-chlorine shock which is potassium monopersulfate).  Usually people find persulfate to be irritating to the skin so I'm surprised they aren't using monopersulfate (MPS) that is more commonly used as non-chlorine shock.  In any event, you'd need more of this product to handle your bather load.  Your 1-2 tablespoons counting primarily the Dichlor would handle 30 person-minutes to one person-hour.  Is that your usual bather load?  Do you have an ozonator?

Note that if you have an ozonator you can usually add roughly half these amounts, but in between soaks the chlorine demand would be roughly doubled.  However, the real rule is to add whatever it takes to measure a small chlorine residual 24-hours later.  So you clearly aren't adding enough to handle the bather load.

[Brian3248484]

I have the marquis the spirit with a ozonator and the spa frog system (mineral stick and bromine)  I added the bromine booster when I filled the tub like my local dealer said and have plenty of bromine per the test strips.  I have been throwing in marquis I think it's called nonchloirne oxidizer whenever I get out.  Perhaps I'm not using the terms correctly.

[chem geek]

No, what you said is correct.  You are adding non-chlorine shock to oxidize the bromide back to bromine.  That's fine.  You aren't really "shocking" the water so much as regenerating more bromine by adding oxidizer.

[Brian3248484]

Oh ok great I'll keep doing it like I am then, thanks for clearing that up for me.

[wt21]

"Ozone reacts with chlorine and typically double the amount of chlorine consumed in between soaks so the daily (24-hour) chlorine demand with an ozonator is typically 50% or more whereas with no ozonator it's usually less than 25%.  On the plus side, an ozonator typically cuts down the amount of chlorine needed after a soak by at least half.  That is, ozone will oxidize some of the bather waste so that chlorine doesn't have to."

So, use more chlorine because the ozonator eats it up but use less chlorine because you don't need as much?

I'm really confused by this...

[chem geek]

You use less chlorine after a soak because ozone will oxidize some of the bather waste but you use more chlorine in between soaks (i.e. starting 24 hours after a soak) because ozone reacts with chlorine depleting it.  So an ozonator work fine with chlorine if you are using the spa every day, possibly every other day, but it does not work so well if you use the spa infrequently such as only on weekends.  With an ozonator, you'd need to add chlorine to the spa every day or two while with no ozonator you could add enough chlorine to add it perhaps twice a week or even raise it enough to last a week to go on vacation (especially if the water temperature is lowered).

An ozonator works well with a bromine spa because it generates more bromine from bromide.