Iso Routine For A Small, Frequently-Used Hot Tub

[daystreamin]

Thanks to all the great posters on this forum - I've learned a lot over the last few weeks as I try to get my head around maintaining our new-to-us hot tub.

Obviously, hot tub chemistry is complex - there are so many variables to take into account. We screwed around following just the sketch instructions we received when we bought products. Then we found this forum and realized we were not sanitizing properly. Then we realized our water chemistry was really out of whack. So with a clean fill, we are now focusing on trying to get the water chemistry balanced and the sanitation levels right. But then I read about water replacement intervals.

Oi. My head hurts. ;-) So here's additional details and a list of questions - thanks for any help!

Tub Use & Info

• We have a 2002/3 Hot Spring Solana (equiv model Hot Spot TX) - holds 140 gallons
  
• Use: 2 adults, 30 min, 2x/day, 5 days/wk - 2*30*2/60*5 = 10 person-hrs/wk = 1.43 person-hrs/day
  WRI = (1/9) x (Spa Size in Gallons) / (person-hours per day) = (1/9) x (140/1.43) = 10.5 days
  
• We are using BaquaSpa products (ph Decreaser, Metal Control) and Dichlor.
• We have readily available: QB Brite MPS, BaquaSpa pH increaser, Borax 20 Mule, baking soda, and household bleach.
• The tub has a new continuous silver ion sanitizer and a new filter cartridge.

Water Quality

• New fill from well (run through garden hose and Systems IV Exterior Water Filter, which needs to be replaced soon) testing with Taylor K-2006 kit - fill water reads:
  TA 200 PPM | pH 7.5 | CH 190 PPM | Temp 103
• From well water quality test in April 2015, we know the unfiltered water is really hard:
  Calcium 70.9 PPM | Iron 1.1 PPM | Manganese 107 PPB
  Total Dissolved Solids 227 PPM | Hardness 12 Grains

What We've Gathered...and What We Want to Know

We are interested in dichlor / bleach method for sanitation. We have read that it might extend the WRI. Can we reasonably assume that the dichlor/bleach method will extend our WRI to 21 days?

Using Nitro's Lowering Total Alkalinity method, we got it down to TA 50, but still showing pH 8 (or more...constrained by the K-2006 upper limit). CH 190 remains. Not knowing the pH prevents us from calculating the saturation index, but if we assume pH 8, the Taylor Water Balance Calculator reads SI 0.3.

Based on a couple of uninformed fills and empties, where we focused on chlorine but not water balance, we suspect the chlorine demand is going to be high, and with a heavy bathing load, we expect a lot of dichlor in the first few days. Will dichlor bring the pH down enough to leave TA/pH alone at this time and move forward? Should we add borates after the pH drops with the dichlor or do we need to bother if the tub is going to be refilled in 10.5 (or 21) days?

Ideally, we would remove some of the iron and manganese to prevent cleaning problems. Are there other reasons to apply metal control? When should we apply metal control in the fill process?

As you can tell, I'm obsessing about this just a little. I really want to understand the underlying principles, even if I can't grasp the details at the molecular level. So thanks in advance for any help - and again, thanks to those of you who have unknowingly helped me already with your previous posts. I really appreciate it.

Christy

[chem geek]

Wow, that's an extraordinarily high bather load due to soaking twice every day with 2 people and in such a small spa.

So you are doing things right to start out with the Dichlor-then-bleach method. When you are lowering the TA by adding acid, if the pH is high stop using the jets. Aeration will raise the pH faster. If you add some acid now, the pH should drop. And yes, if you start using Dichlor it will slowly lower the TA but as to whether it will lower the pH it depends on the aeration since carbon dioxide outgassing will raise the pH. So I'd try adding pH decreaser to calm spa water and test after a few minutes of circulation mixing (not spa jets). If you get the pH closer to 7.5, then add boric acid which should help slow down the rate of pH rise.

When you have the TA low and a lot of aeration you really need the Borates to help prevent the pH from getting high as quickly. This is especially true in your case because your CH starts out higher so you want to prevent calcium carbonate scaling by keeping the TA low (which you are) and the pH from getting too high (too much above 8.0). The good news is that with such a small spa the amount of chemical like boric acid you need to add is fairly small in quantity so not expensive.

The metal control is just to prevent metal staining from the fill water. Given your high pH, you are at risk for metal staining if the metal level is high enough and with well water that can certainly be the case.

If you've never used Ahh-Some it would be a good idea to do so, if not now then at the next water change (you use it before you dump the water).