Adding Acid to pool - How and why?

rollymon

Member
Jul 18, 2007
10
I have read in this forum that acid should be added to the pool in the stream of a return line. What is the reason for this and is there a reason why acid should not be added to the skimmer?

Thanks,

Rollymon
 

ktdave

LifeTime Supporter
May 8, 2007
888
Katy, TX
It could damage your pump/filter/heater/salt water chlorine generator without being first diluted in the body of the pool.
 

MikeInTN

TFP Expert
LifeTime Supporter
May 27, 2007
1,335
Middle Tennessee
The reason to slowly add it in front of a return is so the acid will dilute much more quickly and go into solution with your water. Muriatic acid is denser than water, and if you just dump a bunch in in one spot, it will go to the bottom and pool there until it goes into solution. Having this acid pooled up on the bottom of your pool is not a good thing for your liner.
 

rollymon

Member
Jul 18, 2007
10
Everyone,

I appreciate your responses however, I am also aware that in the not so distant past it was considered by many pool pros taboo to add chlorine or for that matter anything at all (other than DE) to the pool via the skimmer for similar reasons as listed in the post by ktdave [/b]"It could damage your pump/filter/heater/salt water chlorine generator without being first diluted in the body of the pool."

I recently read a post on another forum where calculations were generated by the writer based on adding a gallon of chlorine to the skimmer. In the post the writer calculated results based on flow rates and velocity and volumes that by the time the chlorine reached the equipment the level of the chlorine had been diluted to the point that it was not a threat to the equipment. I wonder if this would not be a similar result if calculated for muriatic acid. Unless there is another reason that I am not aware of for distributing via the return stream method I would be interested to see the results of such a study in order to determine whether the acid would really pose a threat to any of the equipment. Seems to me that if the threat level to equipment is low that this would be the most effective method of distributing the acid equally in the water. Also it is a much easier method as well since my returns are in the floor and the closest one to the wall is approx 5 out into the pool.

I would enjoy learning any other reasons or thoughts on this.
 

JasonLion

TFP Expert
Platinum Supporter
LifeTime Supporter
May 7, 2007
37,879
Silver Spring, MD
I know that some people add acid to the skimmer and never have problems. Part of the chalenge is that the kind of problems you would have from too much acid in the skimmer would take years to develop and could easily be blamed on other causes. Thus it is difficult to know if adding acid through the skimmer has actually caused them any problems.

I am reasonably sure that acid would cause more damage than chlorine. But more doesn't nessessarily mean significant.
 

Poseidon

Well-known member
May 24, 2007
148
Houston, Texas, USA
rollymon,
You don't nesessarily have to pour it in near a return, but it should be poured slowly into water that is moving. I remember reading the posts you referred to. I would think that muriatic would be far, far more corrosive than chlorine.
 

AnnaK

TFP Expert
Platinum Supporter
LifeTime Supporter
Jul 15, 2007
1,138
Eastern Pennsylvania
In my mind, there are two systems. The 'closed' system, whereby the acid goes into the skimmer, through the filter, and back into the pool. And the 'open' system, where the acid goes directly into the pool and is dispersed by the flow of the return. To me, it makes more sense to add it to the return stream directly.
 

chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
12,083
San Rafael, CA USA
rollymon said:
I recently read a post on another forum where calculations were generated by the writer based on adding a gallon of chlorine to the skimmer. In the post the writer calculated results based on flow rates and velocity and volumes that by the time the chlorine reached the equipment the level of the chlorine had been diluted to the point that it was not a threat to the equipment. I wonder if this would not be a similar result if calculated for muriatic acid. Unless there is another reason that I am not aware of for distributing via the return stream method I would be interested to see the results of such a study in order to determine whether the acid would really pose a threat to any of the equipment.
I've done the calculation for the chlorine pouring (though I don't think you are referring to my post) and will give you the results for acid pouring as well, but I can't tell you if either is damaging since the time of exposure is relatively short. I can just tell you what the chlorine ppm will be and the pH level for the acid pouring.

I will assume a flow rate through the skimmer of 25 GPM which is probably reasonable if there is also a floor drain. If one pours slowly at the rate of 1 fluid ounce per second so 1 cup every 8 seconds (which is a slow pour rate -- not excruciatingly slow, but not fast either) then the 25 GPM flow rate is 53.3 fluid ounces per second so the dilution is about 1/50th. Adding one part 6% bleach to 50 parts "standard" buffered pool water (pH 7.5, TA 100, CYA 30) will result in a pH of 9.3 and an FC of 1200 ppm with a disinfecting chlorine level of about 15 ppm which is very high.

The same computation using acid is far more distressing. Muriatic Acid has a pH of around -1 (yes, it's negative due to the logarithmic scale of pH) and the addition of acid completely overwhelms the pH buffering (TA) of the water so the net pH result from the 50:1 dilution is a pH of around +0.7 which is HIGHLY acidic! Even though the exposure may be brief, I would never add Muriatic Acid into the skimmer and I would be very, very careful (i.e. add it very, very slowly making sure it is getting thoroughly mixed) adding it even to a return stream, especially in a vinyl pool.

Richard
 

giulietta1

In The Industry
Mar 29, 2007
289
Knippa, Texas
I've never added acid thru the skimmer, but I do pour it in somewhat quickly. I dilute it first, tho!

I'll get a gallon or two of water in a bucket, stir the acid into it (anywhere from 8 up to 20 oz) and pour it into the pool. Either I pour it in front of the returns or walk it around the pool. Sometimes I'll pour it in near the skimmer but not directly into it.

Should I do this differently?
 

chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
12,083
San Rafael, CA USA
I think diluting it in a bucket and then pouring in front of a return flow is fine. I'd still try and do it somewhat slowly (that would prevent splashing as well), though you don't need to be as slow as with concentrated acid. I was surprised with how quickly the pH buffer in the water gets overwhelmed, but practical experience from pool users would indicate that adding acid to the main body of pool water is fine as we haven't heard of problems from users doing it this way.