How to Blow Out and Plug Pool Lines for Winter

Why this matters

A pool line that holds water through a hard freeze cracks where nobody can see it: behind the return fitting in the wall, under the deck at the skimmer throat, or along the main drain line under the floor. Nobody finds out until spring, when the pool will not hold prime or the equipment area floods on startup, and the repair is a deck cut or a dig. 'Pool Closing Service' calls improper blowout the top cause of freeze-burst plumbing claims. The blowout itself is twenty minutes of attention per line; this card is that part of the closing visit, and the rest of the visit stays in the closing SOP.

Safety on the deck and at the pad

Use a low-pressure, high-volume blower, never a high-pressure compressor. A cyclone blower or a shop vacuum on its blow port moves a lot of air at a pressure the pool's PVC and fittings tolerate. A shop air compressor can push enough pressure into a capped line to blow out a fitting, crack a filter tank or launch a plug. If a compressor is the only thing on the truck, it stays on the truck.

Cords, water and a wet deck. Plug the blower into a GFCI-protected outdoor outlet, use an outdoor-rated cord with no splices, and keep the blower and the cord connections off wet concrete and out of puddles. If the GFCI trips, find out why before resetting it; do not move the blower to an unprotected outlet.

Plugs under pressure. A rubber expansion plug pushed into a return while the line is blowing is holding back air. Thread or tighten it fully before you let go, keep your face to one side of the fitting while seating it, and never stand directly in front of a return that is being blown.

Noise and cold water. A blower at the pad runs loud for a long time; wear hearing protection. Seating plugs means reaching into cold water at the wall; kneel on a pad, not on the coping edge, and keep one hand on the deck.

Start state

The closing work that comes first, per 'Pool Closing Service': final balance and shock, water lowered to the level that cover and climate call for (commonly below the skimmer mouth, and below the returns for some covers), equipment drained and plugs set aside together. Any drain plug on the path the air takes from the blower to a line (pump, filter, heater) goes back in while you blow, or the air leaves through that drain instead of clearing the line. You cannot blow a skimmer line properly with water still standing in the skimmer.

Map the plumbing before you blow. Walk the pad and label each valve: which line goes to the skimmer, the main drain, the returns, any cleaner line, spa or water feature. On a pad where several lines share one valve, note it; the air will take the easiest path, and you will need to block the easy one to clear the hard one.

Bring: the blower and an adapter that fits the pump's drain port or strainer pot (or a hose fitting the pad already has), rubber expansion plugs sized for every return and line port on this pool, threaded plugs where the fittings take them, a skimmer ice compensator for each skimmer, nontoxic pool antifreeze where the area calls for it, and a log sheet.

   BLOWOUT: PAD TO POOL, ONE LINE AT A TIME

   PAD                                 POOL
   blower -> pump port / strainer pot
     |
     +-- valve A -> return line ---> R1 (nearest) [plug]
     |                         ---> R2           [plug]
     |                         ---> R3 (farthest)[plug]
     |
     +-- valve B -> skimmer line ---> skimmer port
     |                                [plug + ice comp.]
     |
     +-- valve C -> main drain line -> main drain
                    close C while       (air trapped in
                    still blowing        line holds water
                                         out)

   only the valve for the line being blown is open

The procedure, line by line

1. Set the valves so air goes only to the line you are clearing. Open that line's valve, close the others. Air takes the shortest, lowest-resistance path, so with two lines open it will clear one and leave water standing in the other.

2. Returns: blow until the nearest return bubbles, then plug it, then keep going. The nearest return to the pad clears first and starts spitting air and then blowing steady air. Seat its plug while the blower runs, so water in the rest of the line cannot run back into it. The air then moves on to the next return. Plug each in turn, nearest to farthest. When the last return is plugged, blow a few seconds more and close the return valve at the pad. What breaks if skipped: a return plugged before it ran steady air leaves a slug of water behind the fitting, which is exactly where the wall cracks.

3. Skimmer: blow until the skimmer port blows steady air, then plug it and fit the compensator. Water rises out of the skimmer port first, then spray, then dry air. Plug the line port at the bottom of the skimmer while still blowing, then close the skimmer valve at the pad. (A line that gets antifreeze, per step 6, is the exception: close the pad valve as the blower stops, pour, then plug.) Fit an ice compensator in the skimmer: it is a collapsible plug or container that gives freezing water in the skimmer body room to expand, because a skimmer that fills with rain and freezes solid cracks at its throat. Any water left in the skimmer body is what the compensator is for; the line below it must be dry.

4. Main drain: blow until it bubbles, then close the valve at the pad while the blower is still running. You cannot plug a main drain from the deck without draining the pool or diving, so the line is protected by trapped air instead. Blow until a steady column of bubbles rises from the drain, then close the main drain valve at the pad (or cap the line there) with the blower still running. The air sealed in the line keeps pool water from running back down it. If you shut the blower off before closing the valve, the air escapes up through the drain and the line refills.

5. Other lines: cleaner, spa, water features. Same method: one line open, blow to steady air, plug at the pool end, close at the pad. A dedicated suction cleaner line plugs at its wall fitting like a skimmer port. Spa jets plug like returns.

6. Add nontoxic pool antifreeze where the area calls for it, never automotive. Pool antifreeze is a propylene glycol product made for this. Automotive antifreeze is usually ethylene glycol, which is toxic and has no place in a line that will discharge into a swimming pool in spring. Pour it into lines that can hold water at a low spot even after blowing (a skimmer line that dips under the deck, a long run to a far return) before the final plug goes in, in the amount the jug's label gives for that pipe size and length. In mild climates where lines do not freeze, many shops skip it; in hard-freeze climates it is the insurance on a line you could not get fully dry.

7. Pull the equipment drain plugs again, then log each line. The pump, filter and heater drain plugs you refitted for the blow come back out and go in the bag with the rest, or whatever water the blow left in the equipment freezes there. Log line, what came out (water, spray, dry air), how long it blew steady air, plug or compensator installed, valve closed, antifreeze added. 'Pool Closing Service' asks for a photo of each plug; the log is what tells the spring opener which line was the hard one.

Worked example: a pool with three returns and one skimmer

A plaster pool in a hard-freeze climate. Three returns on one line, one skimmer, one main drain, each on its own valve at the pad (A returns, B skimmer, C main drain). Water lowered to about 4 in below the returns for a safety cover. Equipment drained and plugs bagged, with the pump and filter drain plugs refitted for the blow.

Returns (valve A open, B and C closed). Return 1, nearest the pad, spat water then air at about 30 seconds and ran steady air at about 45; plug seated while blowing. Return 2 spat water and then ran steady air about 20 seconds later, plugged. Return 3, farthest, took over a minute and blew a second slug of water partway through, which is the low spot in that run showing itself. It was blown a further minute after it ran steady, then plugged. Valve A closed.

Skimmer (valve B open). Water rose from the port, then spray, then steady air at about a minute. The skimmer line dips under the deck to reach the pad, so it gets antifreeze: valve B closed as the blower stopped, the label amount of pool antifreeze poured down the port, then the plug seated, and an ice compensator went in the skimmer body.

Main drain (valve C open). Steady bubbles at the drain after about two minutes. Valve C closed with the blower still running. Blower then shut off; no bubbles resumed at the drain, which means the trapped air held. Pump and filter drain plugs pulled again and bagged.

Log. All five ports (three returns, skimmer, main drain) logged with times and plugs, photos taken of each plug and the closed valves. Return 3 noted as the hard line, with the second water slug, so the spring opener knows to check it first if anything leaks.

What changes the job

• Lines that share a valve. Plug the line that clears first at the pool end, then keep blowing to clear the next. Without that block, the air never reaches the harder line.
• A pool with no main drain valve at the pad, or one where the drain and skimmer are tied below the valve. You cannot trap air from the pad. Many shops plug the skimmer side and rely on antifreeze in the drain line, or the drain line is left full and protected only by being below the frost line; follow the installer's design and say what you did in the log.
• An above-ground pool. Hoses come off and drain; there are rarely buried lines to blow. 'Pool Closing Service' covers it.
• Mild climates. Where lines do not freeze, many pools are not blown out at all and run a freeze-protection pump schedule instead. That is a different closing.

How to know the line is dry

Steady air with no spray for a sustained stretch at the pool end is the test for each line, and it has to happen before the plug goes in, not after. A plug seated on a line still spitting water has sealed water in. For the main drain, the test is that bubbles stop when the blower stops and do not resume, which proves the valve closed on trapped air. If bubbles keep rising after the blower is off, the valve did not seal or the line has a leak; reopen, blow again and close faster.

References

• See related: Pool Closing Service (the full closing visit, water level, equipment drains, photos)
• See related: Pool Opening Service (removing plugs and compensators in spring)
• See related: The Equipment Pad Flow Path: From Skimmer to Return
• Pool antifreeze product label (amount per pipe size and length, nontoxic formulation)