How much shock can I safely add at one time to my pool?

Question

I have not been able to get my pool to hold any chlorine since opening it two weeks ago. Today I had my water tested and was told I had a high demand and needed to add 21 pounds of shock to rectify the problem. The pool store got upset with me for purchasing my shock somewhere else because it was less expensive and now they won't discuss the "how-to" of doing this. Before the got upset, they mentioned not to add a lot at once but I don't know how much I can safely add at one time and how often. It's a 24' round, 52" deep, vinyl liner.

Answer 1

It depends on which product you're using and your pool chemical levels (chlorine, ph, alkalinity, calcium, and temperature).  You want to get the pool to 3-5 ppm of chlorine.  Follow the manufacturer directions on the side of the chlorine and you should be okay.  

On a side note its possible to find cheaper pool supplies at some of the big box stores and online.  However those retailers don't generally have the knowledge base and/or the willingness to help the novice.  The head of maintenance at my pool made fun of my for shopping at the local hardware store for my home improvement needs saying they charge more.  I responded that while they charge more, they also have provided me with the knowledge I needed to solve problems myself and generally made my life easier.  Sometimes what's cheap in the short run gets real expensive over the long haul.

Answer 2

In the entire 15 years I spent as a pool tech, never, ever have I had to dump sixty plus dollars of shock into even a pitch black pool that has three times the volume yours does. Something isn't right there. Either the product you're using has virtually zero chlorine content and is made up almost exclusively of calcium or someone has dropped the ball in the chemistry department.

For starters, do you have a stabilizer level established in the pool water? You need to have that level at 40 at least to keep your chlorine from being zapped out by ultra violet from the sun. Secondly, the pool shop shouldn't be confusing acid demand with free available chlorine or total chlorine. All three of those are separate readings. Acid demand is the requirement of adding acid (or not) to vary pH. Total chlorine is the entire amount of chlorine in your pool...good ( working for you) and bad ( already used up and just wasting space) Free available chlorine is just that. The good stuff that can still do some work for you.

If the pool shop is giving you a total chlorine count, well..that's actually something you don't need to know for your current predicament. You should be concerned with the FAC though.

The only reasons you can't hold a chlorine residual in the pool would be:

1/ An out of range stabilizer level

2/ Your pool is green or cloudy and you haven't reached break point chlorination. This is the point at which there are more good soldiers ( chlorine) than bad guys ( algae etc) in a fight. If you haven't got enough good guys, you lose the battle and the war. Any bad guys left will propagate and you're back to square one.

3/ The rest of your water balance is out of whack. For your chlorine to work effectively, it needs to have the pH 7.2 - 7.6 , the total alkalinity should be between 80 -120.

I'd be checking the above first, before adding any shock just yet. You may be throwing a lot of money away. As for how much shock to add at one time. Whatever it takes to reach break point. If you don't reach it, you just wasted some cash.

Personally, if all your equipment is working fine ( clean filter, functioning pump) and if your pH and TA are fine, with a pool so green that you can't see bottom, zero FAC ( which is why it would be green) and your pool volume, you shouldn't need to put more than 4 kilos ( about 8 pounds) of decent shock in there. It's hard to go by what you've posted here as there may be other factors that you haven't posted about. For ecxample, I'm only guessing there's a water clarity issue. If it's in fact clear, then that 21 pounds is VERY wrong no matter how crappy the brand of shock.

Edit: Ok. Regrading the newest test results. The TA is in the basement. It needs to come up. It ought to be in the 80 -120 range. Did you have appreciable rainfall between tests? That will account for a bit of that drop but normally a drop of that amount is only due to something acidic going into the pool. Not chlorine being the cause, since chlorine is a base substance.

Your pH in both samples, is acceptable but if you're TA really is currently sitting at 69, then your pH can actually be swinging a bit, hour by hour.

Your CYA is cyanuric acid. That's your stabilizer level I'd mentioned. If it's zero, then your chlorine isn't working well for you or even staying in the pool as a usable form of it, very well. It needs upping to about 40. Your stabilizer level shouldn't be dropping like that. It's an aspect of water balance that usually only goes in one direction....up. The only reason it would go down is if you're constantly losing pool water, adding fresh and are not using stabilized pucks. Are you experiencing any of that? This would also be relevant to your FAC level as well. It would be a rather large amount of water, in the area of a drop of  over an inch per day.

As for test strips. I have a low regard of them as they can be quite inaccurate. You'll note some of the differences between their titration tests and your stip tests. Titration is far more accurate as long as the reagents haven't expired or frozen in a shed.

Regarding breakpoint. Yes, that's essentially what that is. Your army has to be bigger than the bad guys army, in simple terms or the surviving, un oxidized organics will thrive again in a now unchlorinated pool.

Besides possible water loss, I have one question. Scratch that, three questions.

1/ What is the type of shock they are pushing on you and what is it's chlorine content? It should be clearly stated on the bucket or bag label.

2/ Are you also using chlorine pucks, on a regular basis? If you are, they must be the unstabilized ones. Cheaper..but don't have stabilizer in them.

3/ You're sure your ( I'm assuming cartridge filter here since this is obviously an above ground) filter is ok? No holes in the element?

I still have a huge problem with that 21 pounds of shock. If you're pool is only slightly cloudy, that's still far too much to add for that water volume. The general rule of thumb is with that volume and a shock such as calcium hypo chlorite (30-35 % chlorine content), on a clear pool, you'd use only about 2-3 pounds once per week ( assuming average pool use, no parties) as a regular maintenance. If the pool is slightly cloudy, you double that. If you can't see the bottom you triple it. Also assuming your water chemistry is in balance.

I can't quite figure out why it is that they don't want to address your water balance issues first. Simply put, you can't fix water clarity chemically, without that being done. Heck...you can't even use a clarifier, algaecide or any other product ,let alone expect your chlorine to work.

Have you tried another pool shop? At this point, I would be. The best one's are usually the Mom and Pop outfits. They don't tend to have a high staff turnover so they usually have the best summer help kids or do it all themselves. I've worked as a tech for both types of shop and NEVER had time for the BS in a chain type shop. Kids with no clue as a rule.

Edit: Ok, sand filter and rural water source. If the filter is more than 6 years old or so, you may need a sand change. High calcium in rural water can cause the sand bed to actually turn into "concrete". Water still flows through the filter, but it does so through channels in the media. Filter pressure can read normal, but the filter isn't filtering. This would cause your water to look turbid and also explain your FAC dropping quickly.

If age isn't an issue, then I'd ask around to see who sells a product called "Filter Aid" or "Filter Plus". It's a product you add to the skimmer that temporarily turns your sand filter into a DE quality filter. Instead of sand's usual 40 micron filtering, it'll boost that to 1 micron. Your filter now filters out finer particles. It works until you backwash, then the powder is dumped from the top of the sand bed. A very quick method of clearing a pool that has a sand filter and relatively cheap. It won't work though if the sand bed has calcified.

Yes, your weather right now is going to knock your chlorine down. You're on the right track though regarding everything else. I wouldn't worry about your chlorine count dropping after a shock. It's meant to. Super shock ( calcium hypochlorite) is a 30-35% chlorine shock but it's unstabilized, as all shocks are. It has to be or you wouldn't be able to swim. :) Your Cl levels zoom up, oxidize organics and drop to acceptable again ( 1.5).