In the circulation system of a swimming pool, the filter is positioned after the pump. If there is one, the heater is then the next component the water travels through before the water reenters the pool. Every in-ground swimming pool needs a filter to remove unhealthy impurities like algae and bacteria and keep the water safe and hygienic.
Filters use one of three media – a cartridge, diatomaceous earth (DE) or sand. Of course, the type of filter installed in your pool is entirely up to you. Each filter media has unique benefits. Some are more efficient; others require less maintenance.
Regardless of the filter you choose, the most important part of your decision should be getting the right sized filter, which is determined by the volume of your pool and the flow rate of the pump. Undersize the filter, and your risk restricting the water flow and damaging the pump.
To give you a list of dependable filters for your in-ground swimming pool, we considered factors such as durability, efficiency and usability. If you can’t decide on the type of media or want to know how to size a filter, our buyer’s guide provides all the information you need.
Best In-Ground Pool Filters
Best In-Ground Pool Filter Buying Guide
Every pool needs a filter to remove the germs that lead to recreational water illnesses like diarrhea, E. coli infections, Legionnaires’ disease and swimmer’s ear. Cartridge, diatomaceous earth and sand filters are all capable of capturing dangerous germs and particles, but some media are more efficient than others.
When choosing an in-ground pool filter, you can’t just make a decision based on efficiency. You must also consider the amount of time you can spare to maintain the filter. A dirty, unmaintained filter isn’t doing your pool, or your health, any favors.
This guide will tell you how pool filters compare in terms of efficiency and maintenance. You’ll also find out how to do a few simple math equations, so you can get a filter that’s appropriately sized for your swimming pool.
Types of Filter Media
Outwardly, all types of in-ground pool filters look similar. Inside the filters’ housing, the media differs. The filter will either contain a cartridge, diatomaceous earth or sand/sand alternative. A cartridge filter contains one or four pleated, cylindrical media. As water flows through the cartridges, the pleats trap the particles.
Diatomaceous earth (DE) media is made from the fossilized exoskeletons of algae called diatoms. The DE used for pool filtration is heat-treated and formed into a powder-like substance. The DE powder is spread over fabric-covered grids that are secured by two manifolds. When water travels through the filter, the DE captures any impurities in the water.
Sand filters contain 20-grain silica sand, which are quartz granules approximately 840 microns in size. Water goes into the tank via a diffuser, traveling to the sand bed at the bottom. Particles in the water become trapped in between the granules while the now-clean water is returned to the swimming pool through laterals connected to a central standpipe.
People often gravitate toward sand because it’s the cheapest filtration media to buy upfront. However, it’s the least efficient of all three media, capturing particles from 20 to 40 microns. Sand lasts for about 3 to 5 years, which is longer than a paper cartridge’s lifespan of 1 to 3 years.
Synthetic and fiberglass cartridges have a slightly longer working life than paper ones, and many can trap particles as small as 10 microns. Cartridge filters come in single and quad configurations. A single-media cartridge filter is ideal for small or medium-sized swimming pools. Quad-filter setups are better suited for large pools that get a lot of use.
The initial price of a DE filter costs the most. Still, its lengthy lifespan of 10 years makes it a money-saver in the long run. DE filters are also able to trap the smallest particles, often down to the size of 1 micron.
Alternatives to Sand
Other granules are available for use in place of sand. In comparison to sand, filter glass, pollyballs and zeolite are able to capture smaller particles. All but zeolite have a longer lifespan. Filter glass, pollyballs and zeolite aren’t as dense as sand; therefore, less of these media are needed to achieve the same performance as sand.
Filter glass is made of recycled glass that’s crushed into fine pieces. This media holds a negative charge that attracts minerals and water. It can trap particles down to about 5 microns and boasts a lifespan of approximately 10 years or longer.
Pollyballs are simply small balls of polyester that you can use in the place of sand. It only takes 1 pound of pollyballs to equal the efficiency of 100 pounds of sand. While they achieve a filtration rate of 10 microns, pollyballs aren’t compatible with some clarifiers.
Zeolite can be natural or synthetic, but natural zeolite is derived from volcanic rock, and the natural type is often used for pool filtration. Depending on the specific zeolite media, the filter can capture particles anywhere from 2 to 6 microns. It’s also lighter in weight compared to sand.
Maintenance comes with the territory of pool ownership. DE filters are the most tedious to maintain. You have to backwash it yearly, but some are also equipped with a bump handle that shakes off the powder from the filter’s grids. You must mix the DE powder with water and add the solution to the filter after each cleaning.
Sand filters require less maintenance than DE versions. Cleaning a sand filter involves replacing the sand once it begins to lose its efficiency. To replace the sand, you must first drain the water, take off the valve and suction or scoop the old sand. Since there aren’t many internal components in a sand filter, breakdowns don’t happen often.
Cartridge filters are the easiest type to clean; however, they must be cleaned often. These filters aren’t backwashed, but routine cleanings do consist of rinsing the pleats and blowing off dust after the cartridge dries. About once per year, a filter cartridge must be deep cleaned by soaking in a solution of muriatic acid.
Sizing an In-Ground Pool Filter
The right sized filter maximizes the pool’s ability to trap debris. While you never want to undersize the filter, we do recommend getting one a little larger than your calculations indicate. You’ll need to know the pool’s total volume in cubic feet – Multiply its length, width and depth (3.14 times depth for circle-shaped pools).
Then, multiply the cubic feet times 7.48. Take the volume and divide it by 8 (the average number of hours for a pool to effectively recirculate all the water it holds) to calculate the gallons per hour. Finally, divide the final figure by 60 to calculate the flow rate in gallons per minute.
You also have to consider the filter’s surface area. As far the surface area goes, there is no firm rule. However, a good guide is 3 square feet for a sand filter, 100 square feet for a cartridge filter and 36 square feet for a DE filter – These numbers are based on 10,000 gallons of pool water.
How Do I Know When It’s Time to Clean or Replace My Filter?
Over time, the particulate matter in between sand granules causes pressure to build inside the filter. Similarly, particles accumulate on the grids of a DE filter, eventually increasing the pressure. Sand and DE filters are equipped with a pressure gauge. When the gauge reads at 10 psi above its starting pressure, it’s time for a cleaning or a replacement.
Cartridge filters also have a pressure gauge, but particles are captured within pleats, so pressure building is an unlikely occurrence. If the cartridge is stained or worn even after cleaning, you need to replace the media. When cleanings become more frequent or you start having problems with algae blooms, the cartridge should be cleaned.
What Type of Valve Is Better?
The valve lets you control the filter’s flow of water. Valves come in two types – side and multiport. Side valves are specifically designed for side-mounted filters where space is limited. These valves only have a filter and a backwash setting. The filter setting is the standard cleaning cycle, and the backwash setting reverses the water flow.
Multiport valves can have up to eight settings, including filter and backwash. Rinse is the setting you should choose after backwashing to loosen any debris remaining in the pool’s pipes. Recirculate allows you to safely operate the pump while bypassing the filter, and the closed setting stops water from entering the filter when you service it.
Does the Pump’s Size Matter?
Absolutely. Without the pump, the filter can’t function. If the filter is too small, the pump will eventually overheat, and you’ll be looking at a costly replacement. Therefore, it’s crucial to match the filter’s gallon-per-minute flow rate with the pump’s flow rate.
The tricky part of sizing a filter to a pump is factoring in the pump’s horsepower. Usually, the manufacturer lists the pump’s flow rate in the specs and the operating label, so you don’t have to figure it out on your own. However, if you need a rough guide, a 1 HP pump has a flow rate of about 50 GPM.
In-Ground Pool Filters: Recap
Diatomaceous earth is the most efficient. It lasts the longest compared to the other two filters, but DE is the hardest to clean. Cartridges have the shortest lifespan, yet they’re the easiest to clean. Sand requires infrequent cleaning; however, the downside is a less efficient filtration rate unless you opt for a sand alternative.
Valves are found on all three types of pool filters. Some are mounted to the side, but most are positioned on the top of the filter. Side valves are usually found on cartridge-style filters. They are necessary for side mounting a filter, and they take up less space on small equipment platforms.
Multiports are the most common type of pool filter valve. Unlike the two flow settings found on side valves, multiport valves give you a lot of control over the flow of water. They offer as many as eight settings, so you can clean, rinse and service your filter with less hassle.
Finding the correct size filter for your in-ground pool only requires a few math calculations, none of which are complicated. It’s perfectly okay to oversize the filter, but don’t get one that’s too small. Make sure the filter meets the pump’s flow rate, and you should be good to go.