Draining a Fiberglass Pool

kirkl

LifeTime Supporter
Aug 18, 2013
5
#1
I could be your poster child for what 10 seasons of pucks and Cal Hypo shock look like. But I've found your site, done lots of reading, am ready to covert to BBB, and it looks like my first step is to lower my 100+ CYA (Taylor test, dot disappeared far below 90 line). I've also read about the dangers of draining a fiberglass pool and the precautions that must be taken. If I wanted to try a 50% drain, does anyone know if that is as dangerous as a full drain?
 

ping

TFP Expert
LifeTime Supporter
Jun 24, 2011
2,932
Long Beach, CA
#2
When draining any pool completely empty, you should know what the water table is. If the water table is high enough, then the water under the pool will push the empty pool out of the ground. By only draining half the water, you will be safe as that should be more than enough weight remaining in the pool to prevent the pool from being lifted out of the ground.

The only problem with changing half of the water at a time is that it will use more water to remove the CYA if you need to drain the water more than once. That is a small price to pay to prevent a pool from lifting out of the ground.
 

kirkl

LifeTime Supporter
Aug 18, 2013
5
#4
Not sure, and my builder is long gone. Someone also asked if I had a "site tube" that would enable them to insert a light and camera to assess the extent of water and the answer is no. I may be able to contact the on-site manager of my job who may know what their standard was.
 

diyindux

Well-known member
Jul 6, 2013
187
#5
Your bottom drains may have relief valves in them as well. They will keep the pool from "floating" but they will also let ground water flow in. On my drains, there are ports in the bottom. I have no relief valves so they are capped, but if yours are open you have the valves.

One way to see if you may have a ground water issue is to look for standing water or streams nearby. If there are and the water surface is higher than the bottom of your pool you will likely have an issue. Another thing to check is the depth of your basement in relation to the bottom of the pool. If your pool is 6 ft. deep and the top is within couple feet of the elevation of your first floor, and your basement is dry, you are probably ok.
 

kirkl

LifeTime Supporter
Aug 18, 2013
5
#7
Suburb of Baltimore, clay soil, and yes...the property slopes. Perhaps another relevant fact. There is a retaining wall at the outside of my pool deck, then a few steps up to a patio. That upper patio had a sink hole issue two years ago. A professional opened it up, diagnosed it as a failed buried drainage line, installed new drainage elsewhere, and filled it with cement. No issues since.
 

diyindux

Well-known member
Jul 6, 2013
187
#8
I would say it sounds like a 50% drain wouldn't be that risky as the groundwater must be at least a few feet below the top of pool elevation. Basically, the pool can only float if it weighs less than the water it displaces. As long as the water level inside the pool is higher than the water level outside the pool, you are ok.