If you’re looking to replace an older pool filter or are in the midst of planning a new build, you may have heard about cartridge filters. This type of pool filter uses a cartridge to filter the water instead of other traditional filter media like sand or DE. Before you buy one, there is a lot to understand about how they work.
It’s essential that you spend the time to understand how cartridge filters work, how they stack up against other options and the ins and outs of servicing them. This time spent researching cartridge filters will ensure that you and your pool professional are on the same page.
And, if you’re installing it or planning to service it yourself, understanding the basics will help make sure you aren’t getting in over your head.
What Are Cartridge Filters And How Do They Work?
The design of cartridge pool filters centers around using spun polyester fibers as the filtration element that cleans the water. The fibers are folded tightly together and packed into a cartridge. The cartridge looks a bit like an air filter from a car, but they are taller and narrower than what you typically see in an automotive application.
Some filter systems load more than one cartridge into the tank for more filtering surface than a single cartridge.
This setup is a departure from the more traditional sand and DE filters you might be more familiar with as pool filters. They are very popular because they tend to be very efficient and easy to use and service.
Diagram: A) Pressure gauge, B) Dirt and debris gets trapped here as water passes through, C) Bulkheads, D) Air relief valve, E) Air relief tube, F) Cartridges, G) Lower manifold
A typical cartridge filter tank consists of a top lid and a separate bottom section, with a band, knob, or clamp holding the two halves together. Inside of the tank, the cartridge or cartridges sit on a pedestal or manifold that keeps them in place. Another manifold sits on top to further secure them, and they help force the water through them properly.
Your pool’s pump forces water into the tank, pushing it under pressure and making it flow through the membrane of the cartridge(s). The fine debris within the water remains trapped within the fibers of the cartridge element. Then the cleaned water is returned to the pool.
You can tell when the cartridge has become inundated with debris via the gauge on the top of the tank, just like traditional DE or sand models. When the pressure rises, the filter is getting dirty. When it’s up about ten pounds-per-square-inch (PSI) over normal, it’s time to clean the cartridges. But, to clean them, you don’t backwash. Instead, you open the tank to access and clean the cartridges.
This might seem like a lot more work than simply turning a multiport valve to backwash and clean your sand or DE grids. But, there are quite a few positives that offset having to open the tank. And a typical swimming pool cartridge filter will not need to be cleaned more than once or twice per season.
Pros and Cons of Cartridge Filters
There are pluses and minuses to owning a cartridge filter, just like with any filter system or any pool hardware for that matter. Let’s take a look at some of them.
- High-efficiency with large filtration surface area
- No multiport or backwash valve required
- No adding carcinogenic DE or replacing heavy sand
- Easy replacement and simple access
- No loss of pool water to backwashing
- Rare cleaning required
- New cartridges can seem a bit expensive
- Cartridges need to be replaced after a few seasons
- Cartridges need to be physically cleaned when the tank pressure rises
Cartridge Filters vs. Sand Filters
The biggest difference between cartridge filters and sand filters, aside from their choice of filter media, is that sand filters need routine backwashing. When the debris that is filtered by the grains of sand gets too thick, and the pressure rises in the tank, you have to act.
First, you shut down the pump. Then you reverse the slide valve or rotate the multiport to its backwash setting. When you re-energize the pump, the dirty water in the tank is forced back through the sand to rinse it.
That wastewater flows wherever the plumbing carries it. Every backwash removes water from your pool, and if you have treated it with chemicals or warmed it with a heater, you are literally flushing your money down the drain. And, if you aren’t backwashing into a drain, that wastewater is flowing out of the valve and into your yard, where the chlorine in it can damage plants and your lawn.
Backwashing a sand filter is a routine activity that should be performed any time the gauge rises by more than about eight pounds PSI. That means every time you vacuum, or there is a big storm or activity that dirties the water, you’re backwashing.
Cartridge filters tend to take much longer to need cleaning. And there is no backwashing. Open the tank, rinse the cartridge, close the tank, and you’re good to go. Occasionally, you’ll need to replace your pool cartridge filter as well. More on that below.
Cartridge filters are also much easier to service than sand filters. A typical sand filter is difficult to open, as the tank isn’t designed to be opened. Access is achieved through a small valve opening or access port. Then, if you are replacing the sand as is necessary every few seasons, you have to face a bit of an ordeal.
You need to use a small scoop, like a coffee can or a cup, to scrape up the many pounds of sand within the tank. Then, you have to add new sand inside. And, that sand should be purchased for this express purpose, not something you dig up from your kid’s play area.
The biggest drawback of a sand filter is that sand is heavy! Any servicing that requires messing around with the sand itself is a bit of a backbreaker.
Cartridge Filters vs. DE Filters
Cartridge filters and DE filters are very similar in terms of their mechanics. But, a cartridge filter doesn’t use DE.
DE filters use Diatomaceous Earth to coat the grids within the tank and filter the water. DE is a natural, fine powder made up of fossilized skeletons of tiny organisms called diatoms. DE particles are very porous because of the hollow nature of those structures. They are pretty well-suited for filtering water.
A DE filter has a set of grids inside the tank, held in place by a top and bottom manifold, pressure plates, and a gasket. The grids have fingers that need to be coated with DE for them to filter the water properly. Once coated, dirty water is forced through the DE-coated grids, trapping debris. When the DE has been filled with fine debris, it’s time to backwash, just like a sand filter.
But, before you can start filtering again, you have to add DE to the filter again by pouring some into the skimmer’s mouth. Doing so is a bit of a messy job, and it is inadvisable to inhale DE. You may even want to wear gloves to avoid irritating your hands.
Again, cartridge filters also get dirty and need to be cleaned. You need to wash them by opening the tank. But, you don’t need to mess around with the chalky white mess that is DE.
If you’re still not sure which type of filter to get, read my entire cartridge vs. DE vs. sand filter comparison.
Criteria You Should Consider When Looking for Cartridge Filters
When choosing a cartridge filter, you should ensure that the cartridge(s) size will be sufficient to clean your pool’s water. If the cartridges are undersized, they will require you to run the filter pump more to circulate the water through them and achieve adequate filtration.
That can lead to you running your pump more, which uses more electricity. That can hit your wallet when it’s time to pay the electric bill and wear down your pump more rapidly. And, if the cartridges are too small, they will also get dirty faster and require more cleaning. A single cartridge is sufficient for most smaller pools or spas and above ground pools.
If your pool is a bit larger, you may want a cartridge filter that has a two- or four-grid design. Some filters have large outer cartridges and taller, thinner inner cartridges. Other multi-cartridge systems have cartridges that are all the same size, arranged around the center of the filter tank.
Hayward, Blue Wave, and Intex are some of the most popular brands of cartridge filters, all of which have some good options. I get into more specifics in my research on the best pool filters, so be sure to check that out for the top cartridge options.
Tips for Cleaning Your Cartridge Filter
The biggest tip for cleaning your pool cartridges is a simple one. If you’re going to do it, do it properly. If you’re going to go to the trouble of opening the tank, make sure you’re prepared to spend the time to clean the cartridge thoroughly.
If it is filthy, you might need to soak it overnight in a cartridge cleaning solution. And you are definitely going to want to take the time to thoroughly rinse between all of the pleats of the cartridge.
But, the process is pretty simple. Shut off the pump, and bleed the pressure from the system by opening your pressure relief valve, usually on the top of the tank. Make sure all of the pressure is out of the system before you begin to loosen the belly band or clamp (or the retaining nut on the top of the tank).
Once you have released the system that holds the top of the tank on the base, lift the top off and set it down carefully. You might want help as they can be heavy and unwieldy.
Pop the top manifold off of the cartridge(s). Now you can remove the cartridge. Don’t be lazy and leave the cartridge in the tank. Lift it out and set it down someplace where you can clean it thoroughly. Use moderate water pressure, but don’t go crazy with a pressure washer and your most aggressive tip. That will only risk ruining the bands that hold the pleats in place.
You can also consider purchasing a wand with a brush attached that is specifically designed for cleaning filter cartridges.
While everything is disassembled, inspect the rest of the innards of your tank for signs of wear or impending issues. Finally, reinstall the cartridge(s), reassemble the filter, and turn the pump back on. Leave the pressure relief valve open during startup so the air trapped in the tank can escape. When water begins to spurt from the valve, close it off.
Some pool owners make the grid cleaning process a little easier on themselves by rotating cartridges. If you have a spare set, you can just swap them out quickly and clean them at your leisure, which also minimizes downtime.
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How Long Do Cartridge Filters Last?
Cartridge filters typically last for a few seasons before they need replacement. Some owners keep the old set as a backup or even to use specifically during startup when the pool is at its dirtiest. Instead of exposing your new, clean cartridge to everything in the water after the off-season, use the older grids to deal with the heavy debris. When the water clears up, remove the older cartridge(s), and install the newer ones.
The filter tank itself can last for decades.
When to Know It’s Time to Replace Your Cartridge Filter
It’s time to replace your filter cartridge when they can no longer filter the water properly. A primary indicator that they are going to need replacement is if the bands that secure the pleats begin to fall apart. That is going to loosen the pleats and reduce the efficiency of the cartridge to filter the water.
You might also want to replace the cartridges every three or four seasons prophylactically. It is almost always a good idea to retain the old cartridges as long as they are not falling apart. This way, you have an emergency set for backup that can also serve as a dedicated set for the dirty season.
When not in use, simply store them in a cool, dry place. Make sure that they are safe from critters like mice and other rodents who might love to build a nest within them. This way they’ll be ready to serve if you need them in the future.
And if you feel they are a little bit worn out, your local pool store probably sells products that can help rejuvenate the cartridge. Or you can shop online or ask your swimming pool pro for help.
And that’s about it for this popular type of filter. Questions? Let me know.