Main Drain - How can blowing it out leave it water-free?

3MM

New member
Sep 4, 2021
4
NY
Hello All,

Just about to attempt to close my pool myself for the first time shortly so am researching my method and technique. To that end this forum has been invaluable.

I'm relatively confident and clear about what I'm going to do - except for one aspect:

How does blowing the main drain achieve anything useful?

The closest I came to an answer - which basically suggested there was no point was this thread:

... but I have seen time and again methods which always include blowing the main drain ... What am I missing?

For all the other lines I see the point, but the main drain is never above the lowered water level - and doesn't get plugged - so any air put into that line will just come out the line due to air being lighter than water - so the moment you stop blowing, water just replaces it - doesn't it?

I plan on blowing the other lines from the pump end, which is lower than the pool drain and I have a valve on the main drain line ... but even when I close that, there will be a pipe full of water which goes back under the pool. Isn't this the weak point in terms of freezing and bursting? Why is there never a conversation about lagging this exposed water-filled line and using heat tape etc?

What am I missing?
 

JamesW

TFP Expert
Mar 2, 2011
28,291
If you have a clear straw in a glass of water and you blow out the water and put your finger over the end, you can see that the air will be trapped in the straw.

Same principle as long as you can trap the air in the main drain line.
 

ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
29,670
Northern NJ
Pool Size
35000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-60
Welcome to TFP.

All you have to do is get water out of the lines until below the frost line for your area. You do not have to completely clear the line. The main drain at the bottom of the pool will never freeze so can stay full of water.

Frost_Line_Map.jpg
 
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3MM

New member
Sep 4, 2021
4
NY
If you have a clear straw in a glass of water and you blow out the water and put your finger over the end, you can see that the air will be trapped in the straw.

Same principle as long as you can trap the air in the main drain line.
That's because your finger is over the TOP of the straw!

Turn the straw upside down while submerged - which is similar to a pool situation - and the air will leave the straw and water will replace it!
 

3MM

New member
Sep 4, 2021
4
NY
Welcome to TFP.

All you have to do is get water out of the lines until below the frost line for your area. You do not have to completely clear the line. The main drain at the bottom of the pool will never freeze so can stay full of water.

Frost_Line_Map.jpg
Thanks for the map that's useful.

The only way I can envision the air blowing working is if there is some kind of P trap in the main pool drain plumbing line - similar to a sink - so that 'a gas lighter than water, in this case air - gets trapped in the downstream leg and can't escape. Anyone know if the drain line line is normally installed this way?
 

JamesW

TFP Expert
Mar 2, 2011
28,291
If the line goes into the ground, the air should hold the water back to at least the first straight down pipe.

You can plug the main drain from the drain side but that is a lot more difficult.
 

ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
29,670
Northern NJ
Pool Size
35000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-60
The only way I can envision the air blowing working is if there is some kind of P trap in the main pool drain plumbing line - similar to a sink - so that 'a gas lighter than water, in this case air - gets trapped in the downstream leg and can't escape. Anyone know if the drain line line is normally installed this way?

You "airlock" the drain pipe under blower pressure. The blower pushes the column of water down the pipe, and below the frost line. You put a plug on the line or close the valve so the pressurized air keeps the water level in the pipe down. Some people will pour RV antifreeze into the line before they airlock it. That all keeps the air or antifreze above the frost line and water below the frost line.
 
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vig1116

Well-known member
Jan 4, 2017
56
matawan NJ
as stated earlier, you blow the lines out, and while the blower is still blowing out the main drains, you close off the valve going to the main drains. The air stays locked in the pipes. some water is at the very end of the drain, but that will never freeze
 

Angie7089

New member
Aug 6, 2021
3
Hutchinson, Kansas
Pool Size
27500
Surface
Vinyl
Chlorine
Liquid Chlorine
as stated earlier, you blow the lines out, and while the blower is still blowing out the main drains, you close off the valve going to the main drains. The air stays locked in the pipes. some water is at the very end of the drain, but that will never freeze
What if the valve was accidentally re-opened after this process (pool pump & all equipment was off)?
 

vig1116

Well-known member
Jan 4, 2017
56
matawan NJ
What if the valve was accidentally re-opened after this process (pool pump & all equipment was off)?
That's a tough call, and one I'm not knowledgeable enough to make. I will say though, use a little physics to determine what you think you should do. If the main drain is 8 ft down, water will only go up to a certain point, not the whole way to the pump. They say, as long as there is 10% air (space) for water to expand when freezing, you should be good. I am not willing to tell you what you should do, but it all depends on where the water fills to.
 

Newdude

Gold Supporter
TFP Guide
Jun 16, 2019
11,543
NY
What if the valve was accidentally re-opened after this process (pool pump & all equipment was off)?
Even if the valve is only slightly cracked open, the air will rush out and the water in the pipe will equalize to the pool level in a fraction of a second. Once the air is locked in there, it needs to stay that way. (y)
 
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DorsalSpine

Gold Supporter
Silver Supporter
Jul 8, 2013
863
Columbus, Ohio
You do need to airlock the drain line. However, it takes more than "a fraction of a second" for the water to equalize. I blow mine down with a shop vac. Pull the nozzle with one hand and insert the rubber plug with the other. I wanted to make sure I had air in the line so I pulled the vac and just waited. It took several seconds to equalize. This will vary depending on the length of the line. Try it and see. That will give you a better idea of how much time and air you have in the line.
 
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Diggy77

In The Industry
Sep 3, 2021
46
Glen Carbon IL
Pool Size
11000
Surface
Fiberglass
Chlorine
Salt Water Generator
SWG Type
Hayward Aqua Rite (T-15)
Air locking the main drain with a solid valve has always worked for me. Any pool that I close and reopen in the spring is testament to that as when I open the drain valve you can hear the rush of air coming into that plumbing as the water is now pushing the air out.

After doing an airlock, with pump lid open, listen for air trying to creep into the pump. That will let you know if you have a solid airlock.

As far as an accidental opening of that valve, the best practice is to remove the handle of that valve for the winter if you have the type that is removable. If you don't currently have these type of valves and this accidental opening worries you, think about switching to a different valve
 
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