How do you know when to change the water?

[999]

My water is just over three months old but still looks crystal clear. I do have problems with the bromine level going up high when I shock it but otherwise it is fine. My alkalinity and PH are easy to manage. I have to raise the alkalinity around every 10 days due to me using bromine tabs.

Should the water be changed at a set period because of a build up of chemicals in the water or do I just keep going until it starts to look and feel different.

Just because the water looks good, is it really?

[TwinCitiesHotSpring]

on average most people are changing water in the the 3-4month range, some a little more, some a little less...it will all depend on usage and the TDS of the water that went into the tub.  If you have a local dealer with water testing capabilities you can bring them a sample and have them test your TDS (total dissolved solids)....to understand what TDS is and how it effects your water check this link out: http://www.rhtubs.com/TDS.htm

[999]

Unfortunately i live in the UK where hot tubs are not that popular. To my knowledge, my local dealers do not have facilities to test customers water. I have read the article on TDS but it s difficult to know whether i have high TDS or not.

[TwinCitiesHotSpring]

in your case then I would change just based on the age of the water, and yes once you become experience with the tub you will notice based on feel, look, and fluctuating chemical levels....or you could buy this:

http://www.amazon.com/gp/product/B0002T6L5M/ref=pd_lpo_k2_dp_sr_2/182-0586731-8071058?pf_rd_m=ATVPDKIKX0DER&pf_rd_s=lpo-top-stripe-1&pf_rd_r=1GDD9FSR5HSA4ZJSTKZN&pf_rd_t=201&pf_rd_p=486539851&pf_rd_i=B0027NOWMU

[chem geek]

The standard formula for Water Replacement Interval (WRI) = (1/3) x (Spa Size in U.S. Gallons) / (# of Bathers per Day) where the soak time is presumed to be around 20 minutes or so.  If you keep track of bather-hours per day (on average), then the formula would be (1/9) x (Spa Size in U.S. Gallons) / (Person-Hours per Day).  This formula works well when using Dichlor-only, but for Dichlor-then-bleach and possibly for Bromine, you may be able to go about twice as long so (2/9) x (Spa Size in U.S. Gallons) / (Person-Hours per Day).

This also depends on whether or not you have an ozonator or advanced oxidation system (e.g. HotSpring® ACE® Salt Water Sanitizing System) as those help break down additional chemicals letting you go longer between water changes.

[999]

Fortunately I do keep records of when the hot tub is used and what chemicals are added.
The water has been in the tub for 93 days.
It has been used 32 times.
There have been 71 people use the hot tub (2.2 people on average per use)
We use the hot tub for 40 min - 1 hour
I do have an ozonator.
Bromine is kept at 3-6 ppm
It has been shocked 12 times with Non Chlorine Shock when bromine goes up to 10-20ppm
The hot tub holds 1700 litres (not sure what that is in US gallons)

What does that work out as?

[999]

Just had a go at working this out;
71 bathers over 93 days each spending on avaerage 50 min each (50/60 hours) = 0.634 bather hours per day.
1700 litres = 448.8 US gallons

Dichlor : (1/9) x (Spa Size in U.S. Gallons) / (Person-Hours per Day)
             (1/9) x 448.8/0.634 = 78 days

Bromine : (2/9) x (Spa Size in U.S. Gallons) / (Person-Hours per Day)
               (2/9) x 448.8/0.634 = 157 days

How does this sound? Change the water after 78 days when using dichlor but double the time using bromine, 157 days, which is 5 months?

[wmccall]

All good answers so far, but here is a simple indicator. Run your jets for 30 seconds. Shut them off, how long does it take before you can clearly see the bottom of your tub?  Repeat this test on the day you change the water.  Your doing a good job when the water looks good after 3 months. So much so you forget what "new" water looks and feels like.

[chem geek]

How does this sound? Change the water after 78 days when using dichlor but double the time using bromine, 157 days, which is 5 months?

Your calculations seem right, but as wmccall notes, you can just look at your water quality directly.  The calculations are to just give you a rough idea of what to expect, but your actual results will vary depending on how clean you are when you get into the tub and how effective the ozonator is in handling bather waste.  Not everything you introduce into the tub gets oxidized by chlorine, bromine, ozone or MPS so eventually such substances can build up and make the water dull.

Note also that 0.634 bather-hours per day would require around 2.2 teaspoons of Dichlor to oxidize the bather waste if you didn't have an ozonator.  Let's figure that the ozonator cuts this down by half so 1.1 teaspoons of Dichlor per day on average.  In 449 gallons, that's 1.7 ppm FC and 1.5 ppm CYA so an increase in CYA of around 1.5*30 - 5 = 40 ppm per month.  So the first couple of months should be fine and basically because of your ozonator you should be able to go longer than 78 days with Dichlor-only though maybe not to 5 months.

Because the ozonator reacts with chlorine, your chlorine demand during the 3 days you aren't using the tub might be about twice as high as normal, so perhaps 1 ppm FC loss per day instead of 0.5 ppm FC loss.  That is the most challenging part of having an ozonator, using chlorine, and not using the spa every day.  You will likely need to add extra chlorine after your last soak before your 3 day "off" period.

[999]

I have ordered a TDS meter. They are quite inexpensive from ebay.

I have done the test that was suggested, turning the jets off and seeing how quick the water clears. It only took a few seconds for everything to go crystal clear and i could see the bottom like it was new water.

I hear what you say about dichlor consumption, and that is helpful.

As a good rule of thumb 3-4 months seems about right to change the water.

[TwinCitiesHotSpring]

I have ordered a TDS meter. They are quite inexpensive from ebay.

I have done the test that was suggested, turning the jets off and seeing how quick the water clears. It only took a few seconds for everything to go crystal clear and i could see the bottom like it was new water.

I hear what you say about dichlor consumption, and that is helpful.

As a good rule of thumb 3-4 months seems about right to change the water.

someone should have mentioned that 3-4 month timeframe in the first post of this thread....oh wait, I did  j/k good luck with the tub

[Tman122]

3-4 months and there is nothing like fresh water.

[999]

TwinCitiesHotSpring ..... Took your advice and bought a TDS meter. Tried it today and found my water to be 1650 and fresh tap water to be 197.

If nothing else, it is interesting to see the difference between new and old water.

[TwinCitiesHotSpring]

TwinCitiesHotSpring ..... Took your advice and bought a TDS meter. Tried it today and found my water to be 1650 and fresh tap water to be 197.

If nothing else, it is interesting to see the difference between new and old water.

sounds like its time for a change...even if your in a cold part of the country a "partial" drain and refill will help and should get you through to spring...good luck

[999]

Finally got round to changing my water, even though it easy below freezing. New water is at 7 degrees C and will take a while to heat up.

One thing i have noticed is that the ozonator bubbles are now big again, popping on the surface. The had gone really small. Maybe tests a sign that the water needed changing.