Threaded Pool Pump Connections
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Many swimming pool pumps now come with (or require) union connections which allow for a quick connect / disconnect on both the suction side as well as the discharge side of the pump. If you have a pump like this then you likely do not need any additional information about how to make this connection to your pump. However, if you have a threaded pump connection like female threads, which require that you use a threaded PVC fitting along with a thread sealant to connect to the pump...then this article is for you.
Female pipe threads used to be the most common configuration for pool pumps. This means that almost every pool pump required that you make a threaded connection in order to attach the pipe. Some new pumps are still like this and of course many older, existing pumps are still out there in operation today. These threaded connections are one of the single highest causes of pool pump failure. Making a poor connection, using the wrong fitting, using the wrong thread sealant, or simply overtightening the threaded fitting can all result in you needing to replace (or repair) the pump sooner than should have been necessary. Let's look at how you can make better, more reliable and longer lasting threaded pump connections.
High temperature unions VS. PVC male adapters - A threaded male adapter is the lowest common denominator for making a threaded pump connection. If you are not sure what you are supposed to use this will probably be the item that you end up using. The problem is that pool pumps can and will generate heat. Under some conditions the amount of head that they generate can be significant. PVC is not specifically great at handling high heat, which is why a much more expensive heat rated PVC is available, called CPVC. When you use a regular male adapter to connect to your pump it is highly likely that even if you achieve a true seal with a thread sealant, over time the heat from the pump will distort the male adapter and you can develop leaks on the suction side, but most especially the pressure side. A tiny leak from the pressure side of your pump would be hardly noticeable at all, other than white staining that you see starting to develop on the pump casing under this connection point. In addition to leaks in a closed loop plumbing system being bad, many pumps use ambient air to draw over the motor windings to cool. If you have a pressure side pump leak then chlorinated water will be drawn into and over the motor which will result in premature failure. All of this can be avoided by choosing the correct fitting to make threaded connections to your pump. The correct fitting to use is a high temperature union.
A high temperature union is a threaded male adapter but one made from more heat resistant plastic than regular PVC. This allows you to make a secure and leak free connection with thread sealants, and then have it stay leak free. It is more money for a set of high temperature unions versus a set of regular PVC male adapters but this is just the cost of doing the job right the first time. The correct thread sealant product to use will vary depending on who you ask. Some like teflon tape, some like silicone, some like pipe dope but be sure you have one that is specifically for use on plastic pipe threads as many are not and will cause damage to the female thread flange when the sealant expands over time. No matter which thread sealant you prefer to use, if you use it on a high temperature union then it should form a lasting, leak free connection. It is worth mentioning that some high temp unions come with an O-ring that should seal to the pump but I have found many cases where the O-ring failed to seal or developed a leak over time. For this reason I rely on the thread sealants I apply versus the O-ring for a leak free seal.
Tip - Never bury the threads all the way into the female adapter / pump receiver. A threaded male adapter or high temperature union should be threaded in by hand until it can be threaded no more. At this point 1/2 to 1 full turn more with channel locks is enough to seal the connection. With plastic flanges you can not simply crank the threads until they are all the way buried. This will crack the receiving flange. Hand tight plus one turn is the general rule to use when making threaded connections with PVC fittings.
To see examples of high temp unions for regular 1.5" threaded connections see this link: high temperature unions for pool pumps. Here is another example of larger high temp unions for Pentair Intelliflo and Whisperflo pumps.
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