Main drain problem

[Debra Ann II]

Problem started upon opening of pool. 20% is all I am getting out of the total flow coming from the main drain ball valve going into the basket in front of pump. Turned off skimmer ball valve and not enough water comes from main drain ball valve to sustain pump. Skimmer ball valve works with enough water to sustain pump. Used the skimmer drain and vacuum all season to circulate water, chemical levels did fine, pool did not drain, water level went down due to evaporation.
Both valves worked in 2013. Did change ball valves in 2013 as handle came off old ball valves couldn't work the valves.
Time to close pool and I am concerned with line coming from main drain freezing.
There is a butterfly valve buried 2 feet in ground, I believe comes up from the main drain; turned that each way watching nothing changed.
Pool was built in 1994, I have no plan of pipe layout and the company who built it couldn't help me as it was to long ago and the other person who knew past away this January.
Recommendations from local pool supply companies of pressurizing the line with a compressor going into the basket in front of pump and closing skimmer valve and seeing what happens sound risky to me, maybe could explode the pipe.
Surly there is a method to rule out which valve or what other situation it could be with out changing the two related valves.
The very least, a sure method to blow out lines and be worry free from them cracking this winter.
Thanks to all you who have experienced this situation and survived to tell it.
Debra.

 

[Texas Splash]

Hello Debra and welcome to TFP! I think it's fair to say that your low flow must be related to either a valve or obstruction in the line. Before doing any kind of plumbing repairs to remove a valve, you may want to look for a blockage. Since your issue seems to be isolated to the main drain line, and you state you are confident there is no water loss other than evaporation, then perhaps you can try to see if there's an obstruction in the main drain line. Are you able to remove the main drain cover? If so, remove it and go back to your pump basket with the pump off and remove the lid. Instead of using air like the pool company wants to try, simply insert a garden hose where water would enter the pump basket from the drain. Close the valve to the skimmer and leave the main drain open only, then turn-on your garden hose while trying to keep a good seal around the hose (maybe with a rag). See if you are able to get enough force to go backwards. Perhaps something is plugged in that line and just needs to go backwards or get moved around a bit. Maybe you will see some debris come back out from the main drain into the pool. If that butterfly valve is defective in the ground, the water from the garden hose may come right back up at you in the pump basket. Some folks will use air pressure to do the same thing if they can maintain a good seal where they insert the air line. You don't need a ton or air pressure to know if there's an obstruction. If one of the valves was defective preventing air from escaping, you should know if fairly quickly.

When it comes to closing, naturally you'll want to try and remove as much water as you can from that line, and/or add an approved antifreeze to the line as insurance. So even if you can't blow the lines out with air and have to rely on suction to remove some of the water, at least the combination of the water line being 6' deep and antifreeze should help protect the line.

Hope this helps a little. Others may reply as well who have other tricks and ideas to assist. Good luck!