High CH: Do I really need to drain and refill?

[salinda]

First, I want to post from a thread I had last year with Chem Geek at PF:

(me): " . . . my calcium is also high at 520-530. It is actually hard for me to get it to blue with the kit because the water in the tube starts globbing up as I add more reagent. I recently successfully lowered my alk from 130 to 90. My ph is hanging out in the mid 7's right now for much longer than it ever has. I have had high calcium for years now and my plaster is very smooth. I do get flakes from the returns sometimes, but the cleaner sucks those into the filter, except in the spa where I use the PoolBuster to remove them. It actually seems like there are fewer flakes this year than in previous years..."

(Chem Geek): "When you had CH of 525, TA of 130, and the pH was rising and hit 7.7, then your saturation index was around +0.6 so slight scaling was possible. With your current TA of 90 and a pH of 7.5, your saturation index is a much more manageable +0.21.

If you use a non-acidic source of chlorine such as chlorinating liquid or bleach, you can lower your TA even more if you want to, even to 50 ppm, and this should further reduce the tendency of your pool's pH to rise. You'll probably find that something like 70 ppm will be fine.

Think of this as an advantage to having a higher CH pool -- you can have a correspondingly lower TA and have calcium carbonate saturation in balance and be even better off in terms of having less pH rise."

Then I reminded Richard that I have an swg and he said:

"Sorry I didn't see your signature line and notice that you had an SWG. No, you don't need to move away from your SWG -- it's fine. But you can lower your TA a little more to around 70 ppm if you want to. It will lower the pH rise a little bit more. Up to you -- if the pH rise isn't annoying where your pool is at now, then there's no need to change it"

What I can't remember is if I should just leave my crystal clear pool water at a high CH level or if I should drain some with the new season. I want to make this decision now before I add salt, cya, and any energy for heating into the pool for the spring "opening." I live in CA so the pool is open all winter. These are all just considerations when the pool starts warming up and the rain is over for the year.

Here were my numbers this week:

FC 1.5 (I know this is low, but so is the water temp.)
CC negligible
pH 7.5-7.6
TA 120
CH 580, but the end point was really violet from 520 or so.
Temp 58

didn't measure CYA or salt because I want to decide whether or not to dilute the water first. I don't use borates because I see warnings about pets and my dog does love to "catch" water from water guns that the kids shoot around the pool for her and I read that borates could be harmful to pets who drink the pool water.

We are about to embark on a remodel in the house and there might be some construction debris (sawdust) getting into the pool this year. I'm thinking this would just mean frequent filter cleanings, but if it will effect the water chemistry a lot too, then I would want to hold off on the drain and refill until next year. Water has gotten a little $$$ around here along with gas & electricity.

This winter, about 6 inches of fresh water were drained due to heavy rainfall. That is around what it is every winter, give or take an inch. Not much winter dilution, as the CH levels show. About 3 years ago, I drained out a couple of feet, but the pool is 10 feet at the deep end, so not much was really done then either.

If I don't drain the pool this year, can I lower the TA successfully even when the water temps are lower or do I need to wait a few weeks?

 

[JasonLion]

If you can continue keeping the CH from going up any further and lower the TA to the 70 to 90 range you should be fine.

I believe that you can lower TA when the water is cold. It will probably go more slowly however.

 

[mas985]

I have SWG and am constantly battling high CH. I have very high CH fill water in the summer months but the fill water CH drops in Jan-Feb because more surface water is used. During these months I usually do a refill. If I didn't refill, the CH would probably increase by 400+ ppm per year. So I would check your fill water, although this time of year in CA, most cities are pulling off of surface water which makes the CH pretty low. Now is a very good time to refill if that is the case.

Anyway, I usually end up with a CH of over 600 ppm by end of summer and so far have only had scaling once when the PH got above 7.8 for a period. So, I think you can manage it but you really need to watch the PH. Lower TA helps keep your PH in the zone but PH is probably the most important parameter for the scaling index since it is a 1:1 change. If you don't want to worry about it so much then I would refill.

 

[salinda]

I live close to you Mark, in the South Bay. I appreciate these comments. I tested the tap water and here are the numbers:

TA 210
CH 120
ph 7.8

Wow! I can't believe how high the TA is! This is using Ben's test kit. I guess, even after I do the drain and refill, I will be struggling with TA numbers.

 

[mas985]

Yes the price of water is high but for me at least it is worth not worrying about scale.

Also, if you can remember, try testing your fill water in mid summer as well. This will give you a good indication if your situation is the same as mine. The city pumps a lot of well water in summer which makes the fill water CH well over 300 ppm. Add lots of evaporation and the pool CH will rise rapidly. Solar makes this worse.

 

[salinda]

I edited my post and saw that you had posted to the old one.

My water is balanced right now. I guess, according to Poolcalculator, that it wouldn't be balanced if the temperature went higher, right?

 

[mas985]

Your fill water is pretty close to mine which probably makes sense given your location. Unfortunately, to get a CH close to 200 ppm will require replacing close to 75% of the water. 60% for 300 ppm.

Temperature has a small effect on CSI so it takes a fairly large change in temp to push it out of balance.

 

[JasonLion]

Your water is fine right now. As it warms up the calcite saturation will go up. You can compensate for that by either lowering the TA, or keeping the PH on the low side, or both.

 

[salinda]

I decided to do a smaller water change now and cut my losses.

 

[chem geek]

Your water is fine right now. As it warms up the calcite saturation will go up. You can compensate for that by either lowering the TA, or keeping the PH on the low side, or both.

Though it is still true that the Calcite Saturation Index (CSI) will go up as the water warms, it turns out that this doesn't happen as much as one would think because the pH also goes down as the water warms (and vice versa, it goes up as the water cools). I see this happen in my own pool which has an opaque safety cover so is normally very stable in pH. When the temperature of the water drops from around 88F to 50F, I've measured pH rise from around 7.5 to 7.7 or so (my spreadsheet says it should get to 7.74). When the water warms back up, the pH drops back again. The net result is that the CSI doesn't change very much -- only dropping by around 0.1 when it cools and rising by 0.1 when it warms back up.

Richard

 

[JasonLion]

Although the PH falls as the temperature rises, the overall range the PH is maintained in does not change. Various things will change the PH over time other than temperature. One wants to adjust TA and CH so that the CSI is still plausible throughout the normal rage of PH variation, say 7.2 to 7.8 or 7.0 to 8.0, at the higher temperature. The TA adjustment I suggested allows for these future PH variations at warmer temperatures, so it is still recommend. Given how high CH is it wouldn't hurt to lower TA even at lower temperatures, though at lower temperatures it is also fine where it is now.