Hot Tub Pump Union Sizes Explained!

A hot tub pump union is basically a screw nut that attaches to both the front and either the top or side discharge of the pump so that you can connect it to your plumbing.

The idea is here is that you can actually isolate with a gate valve and take off the pump whether for service or maintenance.

Lets take a closer look at the different sizes to find which one is right for you.

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How do we know the size of the union?

This is very important when you’re changing your pumps. Given that models change or go out of production, you might have to go for a different pump and you need to know the hot tub pump union size to ensure proper fitting. 

Now, this industry doesn’t help at all with this. The size of your unions is not the physical size that you will measure. What does this mean? It means that the size of the union you need is dictated by the sized of the pipe it will connect to and not the physical size of the union.

This might sound confusing so I’ve created this diagram. You can see the physical collar sizes and the associated union sizes. So for example, if you have a collar measuring 3.1 inches you need a 2 inch union.

Its confusing yes but you’ll get there. As you can see in the diagram, I’ve listed the 3 most popular sizes for unions and their corresponding collar size.

Things are not easy in the hot tub industry but thats maybe part of the fun. I hope this article helped and now you have a better idea of selecting the right union.

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If I can help you in any way I would love to hear from you. You can get in touch using the form below.

Thanks - Andi

 

Hi, Andi here. I own Buildahottub.com and also write all of the articles and info pages on the site. Some years back now, I built my own hot tub but struggled to find the information I needed. So, once my tub was complete, I started this website to help others in their own pursuit of hot tub and plunge pools DIY building information.

Fast forward to 2025, I've helped over 1400+ DIY customers just like you all over the world build hot tubs and pools. Have a good look around the site, there are lots of resources here. Please do get in touch if I can help you. - Cheers, Andi