Swimming Pool Blog

The Many Kinds of Pool Plugs

The Many Kinds of Pool Plugs
by Rob Cox, August 28, 2012

The Many Kinds of Pool Plugs

pool plugsSwimming pool plugs come in all shapes, sizes and colors, and are used for many different purposes around the pool and for the pool equipment.

This simple blog post is a discussion on the various types of plugs used on swimming pools. Many you will be familiar with; others perhaps not.

Let's look at the many types of plugs and some of the uses of them.


1. Winterization Plugs

The most typical types of plugs used for winterization of pools is a tapered rubber plug with two stainless steel washers and a bolt passing through the center. Most commonly referred to as Expansion Plugs, because they expand as you tighten the stainless steel or nylon wing nut on top. Some pool guys call these Freeze plugs, although the more common use of that term refers to an engine block plug used for freeze protection on automobiles. These pool plugs are commonly available in 14 sizes, from 00 (fits 1/4in. holes) to 13 (fits 2-2.5in. holes).

expansion plugs and gizzmosExtended 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 3/4in. 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 too, 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 can also be used to 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.

Gizzmos are a combination plug used for skimmers. Long plastic tube that threads into 1.5in. and 2.0in. 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 rain water 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, 12in. and 16in. and also have one called the Blow-Out gizzmo, that allows you to insert into the skimmer without lowering the pool water level, pop the top, and blow air through the one-way valve contained in the bottom, for the purpose of blowing the water out of the skimmer lines.

2. Filter Plugs

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.

butterfly plug, used on Pentair pumps and FiltersPentair sand filters use a two stage plug - with their common 1/4in. Butterfly Plug, as I call it, to allow the filter to drain water for winterization or repairs. This 1/4in. plug has a small o-ring that is important to maintaining a proper seal, however 3 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. You may also have trouble getting the Spigot (as they call it) back in tightly, without cleaning the threads carefully and using silicone to improve the seal.hayward threaded drain cap for sand filters

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 3x 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 3/4in. cap to a 2in. cap for larger sand filters.

D.E. and Cartridge pool filters most often use a 1.5in. or 2in. threaded plug to drain the tank. They can be positioned on the back of the lower tank body, or on a Tee fitting on the pipe that comes out of the bottom of the filter tank. Hayward SP1022C plug (1.5in. threaded) can be used as a replacement filter plug for many filters, with the exception of those filters using a 2in. 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.

Some DE or Cartridge filters don't have a drain plug at all, such as older Anthony or Wet Institute filters which can drain out of the push pull valve. Some homestyle plumbing jobs seem to have forgotten to install a Tee fitting into the underdrain assembly and you will usually find that a small hole has been drilled and tapped, with a 1/4in. plug used as the filter drain plug.

3. Pump Plugs

hayward pump plug with gasketAlmost all swimming pool pumps use a 1/4in. threaded plug, some with an o-ring and some without an o-ring. Those without the o-ring should use 3 wraps of Teflon tape to create a good seal. Old Teflon tape should be removed first; if years of Teflon tape builds up, this can increase the size of the plug too much, which may crack the pump housing. You can substitute most any kind of 1/4in. 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 1/4in. plug without an o-ring.

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

4. Heater Plugs

test plugsPool heater plugs are most often 1/4in. 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 1/4in. brass plug or butterfly valve plug, whereas the modern thermoplastic headers will use a plastic plug, usually with an o-ring. Brass plugs should 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 20-1/4in. tap, if internal rust becomes a problem.

5. Test Plugs

test plugsTest 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

manual hydrostatic relief plugI recently wrote a blog post all about hydrostatic relief plugs. 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 inground gunite pool is drained.

7. One-Way plugs

duck plugsThe 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.


Did I miss any pool plugs that you use? Leave a comment below.