Blowing lines with combo skimmer and single ball valve setup


New member
Sep 28, 2019
St. Louis, MO
Hi, everyone! Brand new to the forum. I've tried to find other posts/threads about this issue and haven't found one yet, so here I am.

My issue is that I am unsure of how to blow out the main drain of my pool because I am unable to isolate that line due to my plumbing setup. I may just need some kind of adapter fitting for my combo skimmer, but here are the details. Any assistance is greatly appreciated.

I have crudely drawn the plumbing diagram for my IG pool, including the combo skimmer and single ball valve that is right in front of my pump. In other words, because it's a single valve, I am unable to isolate the Main Drain to blow it out by connecting my pancake compressor to the drain port on the pump. I can blow out the returns no problem by simply closing the ball valve and setting the MPV to recirculate.

I can also blow out the suction side from the pump and put a Gizzmo in the right-hand hole, but the left-hand hole (in my diagram) leads to the MD, and unless I get some kind of fitting for that hole and connect my compressor to it, there's no way to blow air into that line. Also, even if I do get a fitting for that port, once I blow the line and pull the fitting, won't water surge back into the line before I can get it plugged? I wouldn't have a way to create an air lock. Does water in the MD line at the bottom of the pool matter because the water won't freeze at a depth of about 5 feet? I'm sure the MD is below the frost line, but I need to remove the water from the line leading to the skimmer from the MD at least below the frost line, right?

This is my first attempt at closing my pool myself, but I'm tempted to pay the pros this time and just make all sorts of notes and maybe even video them so I can do it myself next year. Insurance against making a costly mistake is how I'd look at it. If someone can review my diagram and give me some confidence that I'm doing this right, I'll save about $300.

Thanks!Pool plumbing schematic.jpg


Platinum Supporter
TFP Guide
Jul 30, 2014
Monmouth County, New Jersey
Thanks Allen for the link to my thread.


I would suggest you look at the blow-thru plugs. If you lower the water below the skimmer (1") and then place the plug flush and use the compressor on low pressure, you will see the main drain bubbling. If you see it bubbling, keep it running as air will run out for at least a minute (again low pressure under 20 lbs.), and then you are done. The plug should hold and seal the MD. There are many thoughts on clearing the main drain, but there is enough for water to expand if became ice. It takes a long deep free before main drains can freeze.

Use a regular winter plug to determine which size you need and order extra ones for years down the road. has these plugs and very good prices on everything else. Shipping is free over $50. I always stock up for years to come.