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The Many Kinds of Pool Plugs

Swimming pool plugs come in all shapes, sizes and colors, and are used for many different purposes around the pool and for the pool equipment. Let's look at the many types of plugs used on swimming pools and some of their uses.

1. Winterization Plugs

winterization pool plug

The most common winterization plugs are rubber plugs with two stainless steel washers and a bolt in the center. These are most commonly referred to as expansion plugs, because they expand as you tighten the stainless steel or nylon wing nut on top. These pool plugs are commonly available in 14 sizes, from 00 (fits 1/4-inch holes) to 13 (fits 2-2.5 inch holes).

Extended plugs are expansion plugs with an extra long bolt to allow easier plugging of non-threaded pipes and pipes that are cut off at an angle, like the common 3/4-inch return line in the wall. A double plug, used on old style Sylvan pools and some Anthony pools, is used for winterizing a combination skimmer, when the main drain pipe is tied into the pipe beneath the skimmer. These are also known as combination plugs.

Used as pipe plugs to plug skimmers, returns, cleaner lines, to prevent water from entering the pipes during winter, expansion plugs have other uses, such as pressure testing pool lines or other methods of leak detection, or stopping water flow from the pool when equipment repairs are made.

Expansion plugs 

These plugs can isolate certain lines or abandon certain lines known to be leaking until repairs can be made. They also come in handy to allow pressure to build up and release in a pipe suspected to be clogged, creating a water hammer effect to help move debris stuck in skimmer lines.


Gizzmo plugs are a combination plug used for skimmers. They are long plastic tubes that thread into 1.5-in. and 2.0-in. skimmers to serve as a plug to keep water out (recommended to wrap Gizzmo threads with Teflon tape). Their second purpose is to absorb ice expansion inside the skimmer. As rainwater builds up inside the skimmer body and freezes during the winter, it can crack the skimmer body. The gizzmo breaks up the ice pack in the skimmer and collapses inward to absorb the strong force of expanding ice. Gizzmos come in two lengths, 12-inch and 16-inch.

2. Filter Plugs

butterfly plug

Different manufacturers use different plugs on their pool filters. Many don't use a plug at all, instead using a cap. The difference between a plug and a cap? Plugs have male threads, whereas caps have female threads.

Pentair sand filters use a two stage plug with their common 1/4-inch Butterfly Plug to allow the filter to drain water for winterization or repairs. This 1/4-inch plug has a small O-ring that is important to maintaining a proper seal. However, three wraps of Teflon tape around the plug threads in place of the O-ring will usually seal the plug. Remove the entire assembly from the tank by mistake, and you'll start to drain sand too.

Most other sand filters use a plastic cap that threads onto a drain tube. A cap gasket or O-ring is used on these caps to make a seal. Missing the gasket? You can use silicone, wrapped over three times with Teflon tape to substitute, in most cases. Filter drain caps vary in proportion to the drain tube they attach to and can vary from a 3/4-inch cap to a 2-inch cap for larger sand filters.

D.E. and cartridge pool filters most often use a 1.5-inch or 2-inch threaded plug to drain the tank. Hayward SP1022C plug (1.5-inch threaded) can be used as a replacement filter plug for many filters, with the exception of those filters using a 2-inch threaded plug on their DE filters. Some filters use a tapered threaded plug as opposed to a gasketed plug, and with these, be careful not to overtighten the plug, which may cause the underdrain assembly or tee fitting to crack.

3. Pump Plugs

1/4" threaded plug

Almost all swimming pool pumps use a 1/4-inch threaded plug, some with an O-ring and some without an O-ring. Those without the O-ring should use three wraps of Teflon tape to create a good seal. Old Teflon tape should be removed first; if years of Teflon tape build up, this can increase the size of the plug too much, which may crack the pump housing. You can substitute most any kind of 1/4-in. threaded plug for another; they are fairly universal, as long at they are NPT, or National Pipe Thread design. If your pump plug uses an O-ring, however, you may have trouble getting a tight seal when using a 1/4-in. plug without an O-ring.

Pool pumps usually have two pump drain plugs: one in the front, at the base of the hair and lint strainer or pump pot, and another plug further back toward the motor to drain the volute, or impeller housing.

4. Heater Plugs

Pool heater plugs are most often 1/4-inch plugs, and you will find one on both sides of the heat exchanger or on the front header, where the pipes come in and out and on the rear return header. Cast-iron headers or bronze headers use a 1/4-inch brass plug or butterfly valve plug, whereas the modern thermoplastic headers will use a plastic plug, usually with an O-ring. Brass plugs can be lubed with green lube, like Aqua-Lube, to keep the rust from building up on the threads of the cast-iron headers. These headers can be tapped, with a number 20-1/4-inch tap, if internal rust becomes a problem.

5. Test Plugs

Test plugs look similar to winterizing plugs, except that they are not tapered, the sides are straight, and they have oversized wing nuts to allow for strong tightening. Some are smooth, some are ridged and some are inflatable. They are used primarily for pressure testing plumbing, because their straight sidewalls allow for a more secure plugging of the line; tapered plugs can push out of the line under high pressure. Another type of test plug has a hole that runs through the center of the plug, with a valve stem style threaded top. These plugs are used to push compressed air into the pipe for the purpose of pressure testing or for blowing out the plumbing lines for winterization.

6. Hydrostatic Plugs

hydrostatic pool plug

These are available in two types: spring loaded automatic and manual. Spring loaded plugs are referred to as hydrostatic relief valves and are commonly inserted into the main drain pot, directly in the bottom center hole. The manual type of hydrostatic plug is commonly a Hayward SP1022B plug. Both manual and automatic hydrostats are meant to allow water from a high water table to enter the pool. This prevents floating or popping of the pool shell due to intense hydrostatic pressure, when an in-ground gunite pool is drained.

7. One-Way Plugs

The Anderson Duck Plug or the Winter One-Way Plug are examples of plugs that are used during pool closing winterization. Air is blown in one direction through the plug, which pushes the water in the pipe out through the plug and seals up to prevent water from reentering the pipe after the water is expelled. A plastic cap (not shown) is then pushed over the opening as added protection against water coming in.

In The Swim makes every effort to provide accurate recommendations based upon current ANSI/APSP/ICC-5 2011 (R2022) standards, but codes and regulations change, and In The Swim assumes no liability for any omissions or errors in this article or the outcome of any project. You must always exercise reasonable caution, carefully read the label on all products, follow all product directions, follow any current codes and regulations that may apply, and consult with a licensed professional if in doubt about any procedures. In The Swim assumes no legal responsibility for your reliance or interpretation of the data contained herein, and makes no representations or warranties of any kind concerning the quality, safety, or suitability of the information, whether express or implied, including, without limitation, any implied warranties of merchantability or fitness for a particular purpose.