Choosing the right filter cartridge for your spa or hot tub 

Recreational water has become a large industry, not only in the United States, but across the world. Spas, Hot Tubs, Swimming Pools, water parks, and splash pads have become commonplace everywhere. They can surely be a lot of fun, but the number one goal of all professionals and homeowners should always be safety. Dirty water caused by inadequate filtration and incorrect water chemistry can do a lot of harm. Bacteria, viruses, caustic, or acidic water can cause serious health problems.

Safe clean water only relies on 3 basic factors: Chemistry, Filtration, and Temperature. It is important to understand that filtration and chemical maintenance are both equally important. It is never advisable to compensate with more chemicals if filtration is lacking or vice versa.  As for temperature, when the water or ambient air temperature is colder, less filtration and chemicals are needed; however, when those temperatures rise like in a hot tub or spa, the rules change drastically. Hot tub water is generally kept between 90-104 degrees; couple that with the fact that these are very small bodies of water (300-500 gallons), the quality of the water can change drastically and very quickly.

Before we begin its important to understand the 3 major aspects of water quality. Each of the 3 affect each other and must be balanced.

Temperature determines how much filtration and chemical maintenance is needed. When water is cool or very cold, you will find less filtration times are needed, and chemicals like chlorine and bromine residuals tend to require less attention overall. The opposite is true when hotter temperatures are present. You may find that chemical maintenance requirements become much more frequent, and filtration cycles need to be set for longer run times.

Filtration is obviously a very important aspect of good water quality. Filter cartridges are designed to catch both large and microscopic debris. Keeping your filter cartridge clean, and replaced yearly is vital. Depending on usage, and water temperature, you will need to adjust filtering times to be sure its sufficient.  The main rule for normal hot tub usage is two hours of runtime every 12 hours (twice a day). If your hot tub is experiencing increased bather load, with very hot temperatures, that runtime should be increased as needed.

Good chemical maintenance is what keeps the water safe to use and free of harmful bacteria and viruses. Making sure to keep PH/Alkalinity in balance BEFORE adding sanitizer is key. You will find that with increased bather loads, and hotter temperatures, that more frequent tests and chemical corrections will be needed. For more information on why this is so important, see our guide “Understanding Chemical Maintenance for Recreational Water”.

With over 500 different types and styles of spa filter cartridges, it can seem difficult when attempting to replace a worn or missing cartridge. With so many different measurements, tops, bottoms, and thread types, it’s easy to become overwhelmed. This guide will walk you through the steps in identifying the 4 major specifications needed when it comes time to replace that filter cartridge.

Most spa filter cartridges are a cylindrical shape. The diameter is a measurement of the overall width of the filter. This is a very important measurement. If you choose a cartridge that is too large of a diameter, it will most likely not fit. If you choose a cartridge that is too small, the filtration ability may be insufficient. 

• Using a tape measure, run the tape across the entire surface of the top or bottom of the cartridge.

• Be sure to stay as close to the center as possible.

• Measure from one side through the center to the opposite side. (Fig.1)

• Write down the measurement and move to the next step.

The next step is measuring the length. Length is a measurement of the overall height of the cartridge.  As with diameter, the same types of problems can occur if the length is incorrect. Be sure to measure carefully.

• Using a tape measure, run the tape along the entire length of the pleated fabric only. 

• Be sure to only measure between the top and the bottom. Do not incorporate any part of the plastic top or bottom portions of the cartridge. (Fig.2)

• Write down the measurement and move to the next step.

The top of the filter can be one of a few different styles. It’s important to understand that sometimes manufacturers may style their tops a bit differently than their competitors. For example, a handle by one manufacturer may look semi-circular, while another may look more like a bar.  If it functions the same, it shouldn’t be a problem in your spa.

Some styles such as open holes, threaded, and keyed, open w/slots, etc. come in different sizes. With a tape measure or ruler, measure the specific item on your cartridge top that needs to be matched. Some measurements must be inside diameter (ID), and others outside diameter (OD). See below for how to measure each.

Bottom styles can vary almost as much as tops. Generally, the bottom is where the cartridge will connect to the plumbing of your filter or skimmer. You will find that many on the list below are also found as a top style.  This is because spa manufacturers can position cartridges in their spas any way they choose, which fits their design.

Some styles such as open holes, threaded, and keyed, open w/slots, etc. come in different sizes. With a tape measure or ruler, measure the specific item on your cartridge bottom that needs to be matched. Some measurements must be inside diameter (ID) and others outside diameter (OD). See below for how to measure each.

At this point, you should have a good grasp on what information is needed when replacing your worn or damaged filter cartridge.  How often you clean or replace your cartridge depends mostly on bather load and usage.  A good general rule for normal usage (2-4 individuals 3-4 times a week) is clean the filter every 2-4 weeks and completely replace yearly.  If you have just moved into a house that already has a spa, or you have purchased/received a used spa, replace the filter immediately before usage. Spa Parts ProShop carries virtually all filter cartridges, as well as filter wash, and garden hose attachment tools to help make the job of keeping that cartridge clean much easier.  We also have professionals on hand to help with any questions you might have.