When to change water?

[Curmudgeon]

How will I know when I need to change the water?

My water has been in the tub for a little more than 3 months. The only difference I have noticed is that very small islands of larger bubbles collect in the corners when the air and water pumps are on. The borax induced froth is unchanged. The water is clear and has no odor.

The pH slowly drifts up to 7.8 in about a week and has to be adjusted down to 7.5. Chlorine is kept between 0.5 – 3 ppm with 12 ppm every two weeks. I add an appropriate amount of bleach twice a day. I wash out the filter with the kitchen sink hose when I superchlorinate. Total alkalinity is 45. Calcium hardness is 100.

Thanks
Curmudgeon

350 gal Infinity using BBB

 

[chem geek]

I assume you initially added Dichlor when you started out. Since it's been 3 months, you might check the CYA level and if it's low then switch to Dichlor temporarily to bring it back up (to around 20-30 ppm if that's what you had before). The CYA can get slowly oxidized by chlorine over time.

As for how long before changing the water, that's up to you. With the Dichlor-then-bleach method, most people are able to go twice as long between water changes compared to Dichlor-only, though really it's a personal preference. Some people still change the water at 3 months but find the water to be far better with Dichlor-then-bleach hardly noticing a difference when they change the water whereas those using Dichlor-only notice a big improvement after changing the water because their water had degraded in quality.

The official Water Replacement Interval (WRI) used by the spa industry is the following formula:

WRI (in days) = (1/3) x (Spa Volume in U.S. Gallons) / (Number of Bathers per Day)

The formula doesn't give soak time, but it is likely to be around 20 minutes at high spa temps (100-104F). For a 350 gallon spa and one person soaking 20 minutes every day, that would be a WRI of 116 days or somewhat under 4 months. However, you don't have to follow this formula if your water is still clear, doesn't smell, and there are no other issues such as a significant increase in chlorine demand.

You might find a little less frothing or bubbles if you raise the CH somewhat, say to 150 ppm. Your saturation index is low due to the low TA so there should be no problem raising the CH to 150 ppm since your saturation index (at pH 7.8) would still be only -0.2 and in fact you could probably raise your CH to 200 ppm with no problems (i.e. minimal risk of scaling) though there's not much need to do that.

Richard

 

[Curmudgeon]

Thanks for your suggestions Richard.

You might find a little less frothing or bubbles if you raise the CH somewhat, say to 150 ppm.

The frothing I refer to is like the head on beer, but much less persistent. I didn't get frothing until I added borax several months ago and I think it is the frothing that contributes to the impression that the water feels "softer". Frothing is good I think.

The islands of larger bubbles (which are more persistent and are new) are similar to the bubbles I used to see in the wake of a canoe or boat before the EPA cleaned up our waterways.

I seem to have a bigger problem. The submerged surface of the tub has just recently developed a sticky feel. The surface is textured but only recently have I noticed that I can no longer slide across a seat smoothly or rub my finger tip across the surface without feeling it "vibrate" as it alternately slides and sticks etc. The surfaces above the water line do not have this feel.

Any thoughts about what is on the surface and more important, how best to clean it when the tub is drained?

Thanks,

Curmudgeon

350 gal Infinity using BBB