The pump and filter are the only parts of a pool that never stop working, and they're the parts almost nobody understands. Homeowners obsess over the tile and the water feature and then let a contractor put an undersized filter on a pool that gets hammered by pine pollen every March. That's backwards. Circulation and filtration are the pool.
Here's the actual engineering, in plain language: what turnover means, why the plumbing matters more than the horsepower, and how the three filter types genuinely differ once you own one.
Turnover rate: what you're actually sizing for
Turnover is the time it takes to circulate a volume of water equal to the pool's volume through the filter. The old rule of thumb is one turnover in eight hours, though what actually matters is that the whole body of water gets moved, mixed, and sanitized regularly — because water doesn't queue up politely to be filtered.
Running longer at lower flow generally filters better than blasting water through at high flow, because slower water gives the filter media time to actually capture particles. A pool with well-aimed returns and one good turnover beats a pool with lazy circulation and two.
Head loss, and why pipe size beats horsepower
A pump doesn't move water through the plumbing so much as it fights the plumbing. Every foot of pipe, every 90-degree elbow, every valve, the filter itself, the heater, the salt cell — each one resists flow. Add it all up and you get total dynamic head. A pump's real output is whatever flow it can produce against that resistance, not the number on the box.
Here's the part that costs people money: resistance rises steeply with velocity. Push water through a pipe twice as fast and the friction losses go up dramatically. Which means undersized plumbing forces the pump to work far harder for the same flow, and you pay for it every hour of every day for the life of the pool.
The consequences of skimpy plumbing:
- A bigger, louder, more expensive pump that consumes more power to deliver the same flow.
- High water velocity, which is noisy and, over years, erodes fittings.
- Poor suction-side performance and a pump prone to losing prime.
- A hard ceiling on upgrades — you can't add a water feature or heater later without choking the system.
Generously sized pipe and sweeping turns instead of tight elbows cost very little at rough-in and pay back forever. It's one of the most consequential decisions in a custom pool build, and it's invisible once the deck is poured, which is exactly why cut-rate builders cut it.
Sand filters
Water is forced down through a bed of specially graded sand; particles wedge in the gaps between grains; clean water exits through laterals at the bottom. Simple, tough, forgiving.
- Filtration: the coarsest of the three, generally trapping down to roughly 20 to 40 microns. It'll keep water clear but it lets fine particulate through, so a sand pool often has a subtle haze a DE pool won't.
- Cleaning: backwash — reverse the flow and flush the trapped debris out the waste line. Easy, takes minutes, no disassembly.
- The catch: backwashing dumps a meaningful volume of water out of your pool, which you then have to refill and rebalance. Do it often and you're buying water and chemicals continuously.
- Media: the sand bed itself lasts a long time, generally several years, before the grains round off and lose their bite and need replacing.
Cartridge filters
Water passes through pleated fabric elements. More pleats, more surface area, and the surface area is the whole game.
- Filtration: meaningfully finer than sand, generally in the 10 to 20 micron neighborhood. Water looks noticeably crisper.
- Cleaning: no backwash, so no water wasted. That's the headline advantage. You open the tank, pull the cartridges, and hose them out.
- The catch: it's manual labor, and it's your labor. A few times a season you're hosing pleats, and once or twice a year you're doing a proper chemical soak to strip out oils and organics a hose can't touch.
- Media: cartridges wear out and get replaced on a cycle of a few years, sooner if you undersize them.
- Sizing rule: oversize the cartridge. A generously sized filter means lower pressure, less frequent cleaning, and longer element life. This is the cheapest upgrade available at build time and almost nobody takes it.
DE (diatomaceous earth) filters
Fabric-covered grids are coated with DE powder — the fossilized skeletons of diatoms, microscopically porous. It's the finest filtration in residential pools.
- Filtration: the best, generally down into the low single-digit microns. DE pools have a distinctive sparkle you can see.
- Cleaning: backwash, then recharge with fresh DE powder through the skimmer. Periodically you break the unit down and clean the grids by hand.
- The catch: the most maintenance of the three, it wastes water on backwash like a sand filter, DE powder is a consumable you keep buying, and it's a respiratory irritant you should handle carefully. Some municipalities also restrict DE discharge.
The pressure gauge tells you when to clean
Stop cleaning on a calendar. Clean on pressure. When a filter is clean, note the pressure reading — that's your baseline. As debris loads the media, resistance rises and the gauge climbs. The standard trigger is roughly 8 to 10 psi over baseline; check your manufacturer's spec for your unit.
Two things people get wrong. First, a slightly dirty filter actually filters better than a spotless one, because the trapped debris becomes a finer screen — so cleaning too early is counterproductive. Second, a rising gauge with weak returns means a clogged filter, but a falling gauge with weak returns means a suction-side problem: clogged skimmer or pump basket, a leak, or a plugged line. Learn that difference and you'll diagnose most problems from the pad in ten seconds.
Filter choice and Triad pollen season
This deserves its own paragraph because it's local and it's brutal. Every March and April, the Piedmont buries everything in a yellow film of pine pollen, and pollen grains are fine — small enough that a sand filter passes a good deal of it straight through and back into the pool. If you've ever wondered why your pool has a dull yellowish tint in spring no matter how much you shock it, that's the answer. Your filter isn't catching it.
Practical guidance for High Point and Guilford County:
- Cartridge or DE strongly outperforms sand in pollen season. If you're specifying a new pool here, that's a real consideration, not an upsell.
- If you're on sand, a clarifier during pollen weeks clumps fine particles into something the sand can actually catch.
- Skim aggressively and run more hours during a pollen event. Surface pollen is easy to remove; suspended pollen is a chore.
- Expect to clean the filter more often in spring. Watch the gauge, not the calendar.
If your filter is undersized, your plumbing is choked, or your pool goes cloudy every spring, those are solvable. We handle equipment-pad rework under pool upgrades. Request a design consultation or call (336) 471-0103.