How do you blow out pool lines for winter?
Blow out the lines after you have balanced the water and lowered it, and after you have drained the pump, filter, and heater, not before. Clearing the plumbing is one step in closing the pool, so start with the full sequence in the winterizing guide and do the blow-out when the water is already down below the returns and the equipment is open and empty.
The core method is the same for every inground line: force air through the pipe from the equipment side until you see it bubbling out the fitting inside the pool, then seal that fitting while the air is still blowing. Sealing under air is the trick that keeps water from running straight back into the pipe the moment you stop. You work through the lines one at a time: the return jets first, then the main drain, then the skimmer.
Connect your air source at the pad, usually at the pump after you pull its drain plugs, or at the open skimmer line, and direct the flow toward the pool. Setups vary, so if your plumbing has valves at the pad, close off the equipment loop so the air goes out to the pool rather than back through the filter. Work calmly, keep plugs and a wrench within reach, and have an extra set of hands if the pool side and the pad are far apart.
What do you use to blow out pool lines?
The best tool for most pools is a wet/dry shop vacuum set to blow (exhaust), because it moves a large volume of air at very low pressure, which is exactly what you want. A 5-horsepower shop vac clears the returns and skimmer on a typical residential pool without any risk of over-pressurizing the plumbing. Pros often use a dedicated pool line blower (sometimes called a cyclone or Mighty Vac), which is the same idea with more airflow for long pipe runs.
An air compressor also works, but only with care, because a compressor makes high pressure and low volume, the opposite of what the job needs. If a compressor is all you have, run it through a regulator turned down low and let it fill a line steadily rather than hitting it with a hard blast. Volume clears a line; pressure just builds up behind the water until something gives.
Whatever you use, a few threaded and rubber expansion plugs sized to your returns and skimmer, a pair of channel-lock pliers, and safety glasses are the rest of the kit. Water and debris come back out of those fittings with real force when the line clears, so keep your face out of the line of fire.
How much air pressure is safe for pool lines?
Keep the pressure low, generally no more than about 10 to 20 psi, and never anywhere near a compressor's full output. Pool plumbing is thin-wall PVC with glued joints, and the filter's internal laterals and manifold are brittle plastic. Too much pressure does not clear the line faster; it pops a glued fitting apart underground or cracks the filter internals, turning a free afternoon into an expensive repair.
This is why a shop vac or a blower is safer than a compressor: they physically cannot build up damaging pressure. They just move a lot of air. A line is clear when air bubbles steadily out the pool-side fitting, not when a gauge reads some target number, so watch the water, not a pressure number.
Two more rules protect the equipment. Leave the filter's air relief open and the equipment drained so you are never pressurizing a sealed, water-filled filter, and never blow air through a closed system. If you are unsure whether your filter should be in the air path at all, the safe default is to route air from the pump or skimmer straight out to the pool and leave the filter drained and open on the side.
How do you blow out and plug the return and skimmer lines?
For each return jet, start the air at the pad and watch that jet inside the pool. When it changes from pushing water to blowing a steady stream of bubbles, the line is clear, and you thread an expansion plug into that return while the air is still flowing. The plug stops water from refilling the pipe, so the line stays empty and protected all winter. Repeat for every return before moving on.
The skimmer line gets the same treatment, blown clear until it bubbles, but the skimmer itself needs one extra piece of protection. Water sitting in the skimmer body freezes and cracks it, so after the line is clear most owners drop a hollow expansion plug (a Gizzmo, or skimmer freeze plug) into the skimmer's suction port. It seals the line and, because it is hollow and flexible, it absorbs the expansion of any ice instead of letting that force crack the skimmer wall.
A common backstop on the skimmer and any line you cannot fully clear is to pour non-toxic pool antifreeze into it after blowing it out, so a little residual water cannot freeze solid. Antifreeze is a backup for the lines only, not a substitute for blowing them out, and it never goes through the pump, filter, or heater, which you drain completely instead.
How do you blow out the main drain?
The main drain is cleared with an air lock rather than a plug, because the fitting sits at the bottom of the deep end where you cannot reach it. Blow air down the main drain line from the pad and watch the deep end: when a steady column of bubbles rises from the main drain, the line is clear of water from the pad out to that point.
At that moment, close the main drain valve at the equipment pad while the air is still bubbling. Shutting the valve traps a column of air in the line. That air column holds the water back and keeps it from rising into and refilling the pipe, so the upper section of the main drain line stays empty even though the drain itself is underwater. The trapped air is the whole protection, which is why you close the valve under air, not after.
If your pool has no shutoff valve for the main drain, this is a reasonable point to have a pro close the pool, since an uncontrolled main drain line is one of the easier things to get wrong and one of the harder things to fix. The returns and skimmer are straightforward to do yourself; the main drain air lock is the step that trips people up.
Do you have to blow out pool lines, or can you just use antifreeze?
Whether you blow out the lines at all comes down to your climate. If your ground never freezes, you may only need to lower the water a few inches and can skip the whole blow-out; the line clearing exists to beat a hard freeze. Decide by temperature rather than the calendar, the same way you decide when to close the pool, and follow local practice for how hard your winters actually get.
Where it does freeze, blowing out the lines is the real protection and antifreeze is only a backup. Antifreeze cannot push out a line that is full of water; it just floats on top. The sequence is to blow the line clear first, plug it, then add pool antifreeze to cover any water you could not reach. Use non-toxic antifreeze made for swimming pools (propylene glycol, usually dyed pink) and never automotive antifreeze, which is toxic ethylene glycol and has no place in a pool.
Above-ground pools are simpler, because they usually have no underground plumbing. You disconnect and drain the hoses, drain and store the pump and filter, and plug the wall fittings, with no lines to blow out at all. Either way, the payoff comes in spring: lines that were cleared and plugged mean no cracked pipe surprises, and you can move straight to opening the pool. Do not panic if the returns spit a little air for a day after you open and prime; that is the plumbing refilling, covered in air bubbles from the jets.
Frequently asked questions
Do you have to blow out pool lines to winterize a pool?
Only where the ground freezes. In a hard-freeze climate, blowing out the underground lines is what keeps water from freezing, expanding, and cracking a pipe or fitting you cannot reach, so it is the key step in closing an inground pool. In a mild climate that never freezes hard, you can often skip it and just lower the water a few inches. Decide by how cold your winters actually get, not by a date.
Can you use a shop vac to blow out pool lines?
Yes, and for most residential pools a wet/dry shop vacuum set to blow is the best tool. It moves a large volume of air at very low pressure, which clears a line safely without any risk of popping a glued joint or cracking the filter internals. Connect it at the equipment pad, blow each return and the skimmer until it bubbles out the pool side, and plug the fitting while the air is still flowing.
How much air pressure do you need to blow out pool lines?
Very little. You want high air volume at low pressure, generally no more than about 10 to 20 psi, which is why a shop vac or a dedicated pool blower is safer than an air compressor. A line is clear when air bubbles steadily out of the pool-side fitting, so you watch the water, not a pressure gauge. If you must use a compressor, run it through a regulator turned well down; high pressure cracks fittings instead of clearing them faster.
Do you blow out the main drain line too?
Yes, but you protect it with an air lock instead of a plug. Blow air down the main drain line until a steady stream of bubbles rises from the drain in the deep end, then close the main drain valve at the equipment pad while the air is still bubbling. That traps a column of air in the line, and the trapped air holds the water back so the pipe stays empty all winter. If there is no shutoff valve for the main drain, consider having a pro handle it.
Can you use antifreeze instead of blowing out pool lines?
No, antifreeze is a backup, not a replacement. It floats on top of standing water and cannot clear a full line, so you still blow the line out first, plug it, then add non-toxic pool antifreeze to cover any water you could not reach. Use only antifreeze made for swimming pools (propylene glycol, usually pink), never automotive antifreeze, which is toxic. Antifreeze goes in the lines only, never through the pump, filter, or heater, which you drain completely.