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Thread: Need to do a partial Drain - worried about hydrostatic pressure

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    Need to do a partial Drain - worried about hydrostatic pressure

    Hello All,

    As the title states I need to do a partial drain and I have googling the issues with hydrostatic pressure and I have been hesitant to do so thus far. Just FYI, I am very new to pool ownership. I have been in the house for one month and my pool is turning into the green monster.

    I have the TF-100 kit and ever since I moved in, my CYA levels have consistently been in the 90s -100s if not more. I have tried to go higher ends of FC levels, however it's not successful.
    Attached are the pics of my setup, I just need to know step by step what I need to do.
    So, The first picture is my main drain and the two skimmer returns...Currently the "off" is pointing towards the plant and valve handle is in line with the skimmer returns. When draining which should the handle point.
    The second picture shows my drain pipe --- it's underground and ends at the tree line at the end of the yard.

    Please give step by step directions as I am a novice...Also, with my multiport valve, do I point to waste or Backwash.

    Thanks in advance. Looking forward to doing my first SLAM

    pool valve.jpgpool line.jpg
    20'X40' Inground Vinyl Pool
    3ft to 8 Ft deep
    Filter: Hayward Pro Series High Rate Sand Filter
    Pump: Hayward 1.5 HP Super Pump
    Pool Cleaner: Dolphin Nautilus Plus

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    DaninFLA's Avatar
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    Re: Need to do a partial Drain - worried about hydrostatic pressure

    have you dug a hole next your pool to the depth you want to lower it to? if you do that and it doestn fill up with water, you are fine. you only get hydrostatic pressure when you lower your pool's water level lower than adjacent groundwater. you can lower your pool water lower than the groundwater a little bit without any concerns. just dig a small test hole first and see where you are at.

    set your multivalve to waste and make sure you are drawing from your main drain, not the skimmer
    Dan
    9,000 gallon in-ground Saltwater pool, plaster, cartridge filer, 1.0 hp pump, Hayward T-15 SWCG, TF-100
    Pool School - Chlorine / CYA Chart; Pool School - Test Kits Compared;

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    borjis's Avatar
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    Re: Need to do a partial Drain - worried about hydrostatic pressure

    If you drain down to winter levels (just below return jets) you shouldn't have any chance of the pool popping.

    I was able to lower my cya from 300+ to about 90 doing 3 partial drains / refills to winter levels

    You really shouldn't be draining all the way anyway it could damage the liner.

    and speaking of which, if you know for a fact the bottom of the pool is vermiculite, then it won't pop up at all.
    16x32 IG Vinyl, 13,000 Gals. Hayward S-244T sand filter, SP2810X15 pump
    (1.5 HP motor) Raypak PR266AEN (266k btu) gas heater. TF-100 Test Kit.
    Dolphin E10 Cleaning Robot.

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    Re: Need to do a partial Drain - worried about hydrostatic pressure

    Yeah unfortunately this is my first summer with the pool, thus have never winterized the pool. I have no idea what the bottom of the pool is.

    20'X40' Inground Vinyl Pool
    3ft to 8 Ft deep
    Filter: Hayward Pro Series High Rate Sand Filter
    Pump: Hayward 1.5 HP Super Pump
    Pool Cleaner: Dolphin Nautilus Plus

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    Re: Need to do a partial Drain - worried about hydrostatic pressure

    Quote Originally Posted by DaninFLA View Post
    have you dug a hole next your pool to the depth you want to lower it to? if you do that and it doestn fill up with water, you are fine. you only get hydrostatic pressure when you lower your pool's water level lower than adjacent groundwater. you can lower your pool water lower than the groundwater a little bit without any concerns. just dig a small test hole first and see where you are at.

    set your multivalve to waste and make sure you are drawing from your main drain, not the skimmer
    How do I make sure I am drawing from main drains and not the skimmers.. Sorry for dumb questions, just want to make sure I do it right

    20'X40' Inground Vinyl Pool
    3ft to 8 Ft deep
    Filter: Hayward Pro Series High Rate Sand Filter
    Pump: Hayward 1.5 HP Super Pump
    Pool Cleaner: Dolphin Nautilus Plus

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    borjis's Avatar
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    Re: Need to do a partial Drain - worried about hydrostatic pressure

    take the skimmer baskets out, run the pump and put your fingers near the suction, then adjust the valves,
    when you no longer feel suction on the skimmers then it should be pulling from the main drain in full since
    both go through that valve. If you somehow cut off the water coming while the pump is running DO NOT
    re-open the valve quickly, do it very slowly. and or turn the pump off then open it. otherwise it could "water hammer"
    and blow a seal or the pump basket lid off.


    just draining down below the return jet then refilling is all you should need to do.

    Make sure you close the other skimmer valve too when you do as it will draw air if you don't.

    It might also be that those 2 pipes coming in to the one valve are in fact your skimmers and
    the main drain is the one by itself. That would make more sense to me. That's how mine are setup.
    16x32 IG Vinyl, 13,000 Gals. Hayward S-244T sand filter, SP2810X15 pump
    (1.5 HP motor) Raypak PR266AEN (266k btu) gas heater. TF-100 Test Kit.
    Dolphin E10 Cleaning Robot.

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    DaninFLA's Avatar
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    Re: Need to do a partial Drain - worried about hydrostatic pressure

    Quote Originally Posted by coolpoolnj View Post
    How do I make sure I am drawing from main drains and not the skimmers.. Sorry for dumb questions, just want to make sure I do it right

    Sent from my SM-G900V using Tapatalk
    you have two valves it looks like, should be able to open the one for the main drain and close the one for the skimmer. close the one you think is for the skimmer, then go over to the pool and confirm there is no suction from the skimmer and you should be good. if its not, then do the opposite thing and then confirm. you just don't want to be sucking air into your pump. do you have two main drains or two skimmers? the valve on the right in your picture shows two lines coming in. if you have 2 main drains and 1 skimmer, then that right one would be the main drains it looks like.
    Dan
    9,000 gallon in-ground Saltwater pool, plaster, cartridge filer, 1.0 hp pump, Hayward T-15 SWCG, TF-100
    Pool School - Chlorine / CYA Chart; Pool School - Test Kits Compared;

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    borjis's Avatar
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    Re: Need to do a partial Drain - worried about hydrostatic pressure

    oops double post
    16x32 IG Vinyl, 13,000 Gals. Hayward S-244T sand filter, SP2810X15 pump
    (1.5 HP motor) Raypak PR266AEN (266k btu) gas heater. TF-100 Test Kit.
    Dolphin E10 Cleaning Robot.

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    EVChargeGuy's Avatar
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    Re: Need to do a partial Drain - worried about hydrostatic pressure

    From the pic you posted it looks like the top portion of the pic shows the suction side of the pimp with 2 valves and 3 i puts. I'm guessing you have 2 skimmers and one main drain but this needs to be confirmed? It could be the other way around?
    IG Tear Drop, Dark Gray Gunite , 27,000 gallons -Age 50+-years, very old Hayward Split Egg Sand Filter with side mount Multi-port- Hayward SP2600VSP Pump- Hayward chlorine feed (not used since finding TFP) - Laars 315K BTU Oil heater- LoopLoc Safety cover- AquaBot RAPIDS 4WD- TF-100 Test Kit w/ Speedstir- 390 Gal Hot Springs Spa Envoy installed May 2017

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    Re: Need to do a partial Drain - worried about hydrostatic pressure

    I have two skimmers and one main drain....
    The pool depth is from 4 to 8 feet... I only plan on replacing about a foot of water or so at a time

    20'X40' Inground Vinyl Pool
    3ft to 8 Ft deep
    Filter: Hayward Pro Series High Rate Sand Filter
    Pump: Hayward 1.5 HP Super Pump
    Pool Cleaner: Dolphin Nautilus Plus

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    DaninFLA's Avatar
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    Re: Need to do a partial Drain - worried about hydrostatic pressure

    Quote Originally Posted by coolpoolnj View Post
    I have two skimmers and one main drain....
    The pool depth is from 4 to 8 feet... I only plan on replacing about a foot of water or so at a time

    Sent from my SM-G900V using Tapatalk
    ok, so the right valve is probably the skimmers. close that valve and you will be good to go.

    I would not recommend doing 1 foot drain/refills. much more efficient to do larger refills. check your water table like I explained and go from there. with vinyl, never drain more than 1 foot above your shallow end.
    Dan
    9,000 gallon in-ground Saltwater pool, plaster, cartridge filer, 1.0 hp pump, Hayward T-15 SWCG, TF-100
    Pool School - Chlorine / CYA Chart; Pool School - Test Kits Compared;

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    Mod Squad YippeeSkippy's Avatar
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    Re: Need to do a partial Drain - worried about hydrostatic pressure

    If you're totally unsure, you can always do it the easy way- turn your equipment all off and use a sump pump to drain the desired amount.

    Just remember that anytime you drain a vinyl lined pool, never drain below one foot in the shallow area so as to not disrupt the vinyl lining

    (sidenote: when my pool was new I found it very helpful to take a sharpie pen and mark with arrows and label the direction of the water/purpose of each pipe)
    My Amazon Smile for November12K Fiberglass IG, Infinity 4000 automatic cover, SWCG, Hayward Sand Filter, Hayward 1.5 Pump, Doheny Discovery Robot, Savi Melody LED pool lights, outdoor speakers and other assorted doo-dads. Sundance Altamar Hot Tub.
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