Source
r/hottub•
ragzilla

How much muriatic acid is safe to add at once?

In about 2 weeks we should have our 375 gallon hot tub finally in place, and on the advice of this subreddit and TFP I went ahead and got the Taylor test kit and just tested my fill water last night, and learned some more things about the wonderful limestone aquifers our ground source water comes from.

7.6 pH | 350ppm TA | 475ppm CH (unsoftened)
310ppm TA | 50ppm CH (softened, skipped pH)

To deal with the CH, I'm planning to have a plumber run a new outside spigot off the hot water (continuous, gas) heater which is downstream of the softener, I have an external bypass on the softener so I can mix it appx 35/65 (forget the specific ratio, need to recalc it, using a hose gallon counter to verify volumes) hard/soft to get it down into a manageable range (and as a plus, I can do the initial fill with the water heater dialed down to 100f so it won't take forever to heat up), but I'm struggling with how quickly I can bring the TA down so I can get the rest of the chemistry done. I think the only real practical way to do so is muriatic acid but right now I'm wondering how much is safe to add as to swing TA approximately 280ppm, PoolMath is telling me I'll need to add around 27 fl. oz of 31.45%. So, I'm hoping someone else here has a similar situation as mine and has a good routine for quickly bringing TA into check on the initial fill.

Posted May 10, 2023, 1:50 PM UTC

312
McBillicutty•

I typically add between 0.5-1.5 ounces of muriatic at a time (to drop my pH when it's high). This is enough to bring my tub from something approaching 8.0ph to something around 7.2. I believe I'm about 400 gallons as well. If you do plan to add this amount of muriatic acid I strongly suggest adding it in very small doses and keep an eye on your pH as you go.

2
bacontrees•

Agreed here. Only use enough to lower pH to 7.2. Aerate to 8.0 (should happen fairly quickly at first with such high TA), and repeat process until TA is in an acceptable range. Might only take a few hours.

3
tjcanno•

OMG, that is a HUGE amount of acid!!!

It will really lower your pH.

I don’t use PoolMath. I use the booklet that came with the Taylor test kit to figure out how much acid to add to lower the TA.

Start out with small amounts and let that stabilize. It is easy to overshoot. And let the pumps run with the cover off. Adjust in steps 24 hours apart.

1
ragzillaOPBullfrog A7D•

I don’t use PoolMath. I use the booklet that came with the Taylor test kit to figure out how much acid to add to lower the TA.

Sadly, Taylor's acid demand test is only good up to 180ppm TA (based on the documented interferences in the table). And yeah, it's a lot of acid because there's a lot of carbonate in the water that needs to be turned into salt, CO2, and water. The TA buffers the pH change to an extent hence why I'm looking for more info on how much you can safely shove it around at a time. Past posts by chem geek talking about someone else's high TA tub:

The total amount of acid you will need to add over time and with aeration to get the TA down from 300 to 80 is about 20 fluid ounces, though obviously not added all at once.

1
tjcanno•

Here's a thought: I have not had to do it, but I have heard of others getting a delivery of water to fill their hot tub, where the water is not really hard and will not have the high TA. Have you looked into that? It might not cost that much and be a whole lot less hassle.

1
ragzillaOPBullfrog A7D•

I’d have to truck it in from at least 1 state over which would be prohibitively expensive. All the local places use local groundwater sources which will have the same TA levels. Running the jets for a couple of hours while I adjust it 10-25% of the way at a time, or just dripping in acid (or peristaltic pump) seems much more practical.

1
bacontrees•

I started typing a long reply to this, messing around with different numbers in Poolmath, and realized this situation is pretty complicated!

Have you posted in TFP? Do that, summon chemgeek in a comment (or however that works over there), and I bet they get a plan rolling for you.

Have you tested numbers when running through an RV filter? $20 might be worth testing, and I'd test both with your softened and unsoftened water running through that. This works quite differently than your water softener and may very well pull out some of that TA. Maybe, maybe not, just guessing here and would be worth the $20 test, at least for me.

EDIT: This actually isn't all that complicated, and Nitro specifically addresses this. He says lowering TA by 100ppm should only take an hour, not too bad.

EDIT2: Also, unless you already have muriatic acid (I do from a concrete project and want to use it up), dry acid is much safer to use in a tub. If you need or want to use muriatic, make sure you dilute it in a jug first with some spa water, and slowly pour it into the tub. 14.5% Muriatic is strong shit for a spa and can easily damage components if not used properly.

1
ragzillaOPBullfrog A7D•

Sadly, chem-geek hasn't been active (publicly) on TFP since 2015.

1
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1
piehore•

Read your manual on adding chemicals some will break warranty

1
EliIceMan•

I have similar water with CH is the 400s and TA out the tap is very high as well. For starters, don't bother trying to mix your hard/soft with counters and such. Just start with soft, fill it ~40% of the way by eyeball and then switch to hard. It always come out almost perfect for me.

The next thing I have found that really seems to contradict everything I have read is that TA does not stabilize ph for me. It determines the ph drift. Initially I have to add acid almost every day. Again, I want to handle it as little as possible so I just eyeball it straight out the jug. Probably 2oz if I had to guess. Once my TA gets to around 40, which is lower than many recommendations, my ph gets really solid and only rises with aeration. I used to add baking soda all the time to keep TA up closer to 100 and I was constantly adding acid every other day then too. I accidentally let TA get down to 25 or so once and notice my ph actually go down and everything clicked for me. If my ph is not very slowly rising then I know to check my TA and if it's less than 40 I just throw a handfull of baking soda in.

On my next fill, I'm going to be trying boric acid for the first time and see what that does. (I think it stabilizes TA?)

1
jaswest2754•

Generally the safe limit is dropping by no more than 100ppm TA per an hour. For me that is 200ml in my hot tub your will be slightly different depending on your hot tubs volume. Go slow once your below 150 TA as it is easy to overshoot.

Also as an add on the safest way I’ve found to lower alkalinity by large amounts is to take a sanitary 5 gallon bucket, dip it into hot tub, add the muriatic acid to bucket, mix around a little then dump back into hot tub.

1