CYA Testing and Depletion

First off i would like to say man what a forum. Even though I have never posted on the forum you guys have already helped me a ton.This is the first year of having a pool. My family purchased a home in December of last year that came with the responsibilities of keeping up a 18x36 inground vinyl liner pool. After having an algae bloom already this year and taking the advice on this forum to successfully get it cleaned up I am at yet another crossed road and confused about my CYA level. I have done some reading up on the forum and am kinda familiar with how to take care of the problem but want to make sure I fully understand the process. First off let me say I have a high CYA level. The previous owner of our home had been using stabalized chlorine tablets. When i took the water to have it tested (before i found the forum and bought a test kit) they told me it was at 75ppm. I then proceed to check with my newly bought kit (5-way HTH) and checked to be over 100ppm. How can i manage to bring this level down to where it needs to be. I know I can drain the pool half way and do a refill but have also heard that this is dangerous for a vinyl liner. Second does burn off and splash out reduce the CYA level. I am adding water on a daly basis as we have not had rain in a while. Also I have read about how inaccurate you CYA reading can be unless down correctly. Could someone point me in the right direction of how this is accomplished. Any help is appreciated and thanks for the space.

Test Results
FC= 8
TC=8
PH=7.5
ALK=180
CYA=75 (by pool store) 100+ (by HTH 5-way test)

My first suggestion would be to immediately order one of Ben's test kits, so you can properly test your own CYA, as well as all your other levels, that the other store kits can't do. Plus, I never trust pool store readings, as I feel like they smudge the results in their favor to make you "need" to buy more products from them.

Someone else can help you lower your level properly, as that is never my problem, as I always seem to drop to ZERO CYA over the winter. Strange, but it lets me start with a clean plate each year in the CYA dept.

Good luck,
John

If you are worried about damaging the vinyl liner you could use a very large plastic sheet (the clear drop cloth kind you can pick up at any hardware store). You'll probably have to duct tape a few together to get the size you'd need to cover your pool surface plus 5-10 feet on all sides. That would act as a membrane between the old water and the new water and you could do a 100% water replacement without your pool ever actually being emptied.

Splash out/backwashing/etc. will lower your CYA...but not enough to make any kind of difference at all in your number.

Thanks all for the replies. The next thing on my list is actually Ben's kit. I was in the decision of it and and automatic cleaner and the hold up times on the test kit actually made me opt for the cleaner first. Just wanted to make sure before i do the drain down though. Does the sun have any effect on CYA level in a pool. I have had to keep the pool full with the hose for awhile now since we have had no rain. I was just thinking if it would lower little by little that way that i would just deal with it like that cause my #'s really are not out of the park. Just run the FC level a little higher around 6-8 ppm.

No, the sun does not deplete CYA. The only way to lower is drain/fill or let your pool turn into a swamp over the winter...and even for some people the swamp thing doesn't change their CYA levels...CYA is a strange creature...;)

Oh, there is a chemical you can add to reduce the CYA, but it's very expensive and it takes a lot to do it...it's way cheaper to drain/fill.

Luckily, my pool's cya drops over the winter and the pool is crystal clear and not nasty. I haven't "closed/winterized" my pool for the last 2 winters and luckily, have not had any problems, knock on wood. However, I have been battling a very small amt of mustard algae this season for the first time, but my CYA seems to go to zero over the winter every year, whether I open to a clear pool, or keep it open and clear all year. I had it tested for nitrates which I believe Ben said could cause that drop, but there were no nitrates, according to the pool store. I believe that test was sent out for testing, and not done at the store, but that was a good few years ago.
John

OK, let's clear up a misconception:
You CAN dump half the water in your vinyl pool safely. You just need to make sure there's at least 1 foot of water in the shallowest area, then refill. You don't need the Poconos Plastic Sheet method for that.

A CYA of 50ppm is the high end of normal, but still normal--and a good number for a VERY hot,sunny area and lots of usage. That's where you get to if your CYA is 100 and you replace half your water.

Hope that helps.