The filter is the piece of equipment most responsible for whether your water looks good, and it is also the one most people maintain on a vague schedule or when something has already gone wrong. Two numbers make it straightforward: the pressure when the filter is clean, and the pressure now.
Learn your clean starting pressure
Right after a thorough cleaning, with the pump running normally, note the pressure gauge reading. That is your baseline, and it is specific to your pool. When the gauge climbs meaningfully above it, typically around 8 to 10 psi higher, the filter is loaded and it is time to clean.
Write that number on the filter housing with a marker. It saves a lot of guessing later, and it means anyone else working on the pool has the same reference.
One counterintuitive point: a slightly dirty filter actually filters finer than a perfectly clean one, because the trapped debris helps catch smaller particles. Cleaning too often is a real thing, and it is why chasing a spotless filter can leave water hazier than a sensible interval would.
Sand filters
Backwashing reverses flow to flush debris out to waste. Do it when pressure hits the threshold, not on a calendar. Run the backwash until the sight glass runs clear, then rinse per the valve instructions before returning to filter, or you will send trapped debris back into the pool.
Backwashing wastes water, so do not do it more than needed. And sand does not last forever: over years it becomes rounded and smooth and stops filtering effectively, and it can channel so water bypasses the media entirely. Replacement every several years is normal, and if the filter has never been changed in the life of an older pool, that is likely your problem.
Cartridge filters
These are common on newer pools and there is no backwashing, which means no water wasted. When pressure climbs, you shut down, remove the cartridges, and hose them down thoroughly, spreading the pleats to reach between them.
Twice a season, soak the cartridges in a filter cleaning solution to remove oils and mineral deposits a hose will not touch. Sunscreen, body oils and hard-water minerals build up inside the fabric and reduce flow in ways that rinsing does not fix.
Owning a spare set is the trick that makes cartridge filters easy: swap the clean set in, take the dirty set out to soak and dry properly, and the pool is down for ten minutes instead of a day. Cartridges have a service life of a few years depending on use and care, and a cartridge that will not come back to reasonable pressure after a proper soak is done.
DE filters
Diatomaceous earth filters give the finest filtration of the three and take the most attention. Backwash when pressure rises, then recharge with the correct amount of fresh DE through the skimmer per the manufacturer's instructions. Too little DE lets the grids clog with debris directly, which damages them.
Once or twice a year the grids should come out for a full cleaning and inspection. Torn grids let DE back into the pool, which shows up as a fine powder on the floor after backwashing.
Handle DE powder carefully and follow the product label, since the dust should not be inhaled. Some local jurisdictions have rules about where DE backwash discharge may go, which is worth confirming.
Run time matters as much as cleaning
A perfectly clean filter that only runs a few hours a day will not keep a pool clear in Piedmont summer heat. The pool needs enough daily runtime to turn the full volume of water over, and in peak season that is more hours than most people run.
A variable-speed pump changes this equation entirely, because running longer at lower speed costs dramatically less energy than running short at full speed and filters better while doing it.
Signs you have a bigger problem
- Pressure that climbs back to the threshold within days of cleaning
- Pressure that never comes down to baseline no matter what you do
- Very low pressure with weak return flow, which suggests a suction-side restriction or air leak rather than a dirty filter
- Sand or DE returning to the pool, meaning damaged internal components
- A filter that is simply undersized for the pool, which shows up as constant maintenance and water that is never quite right
The seasonal rhythm in the Triad
Expect to clean more often in the heat of July and August, after heavy storms that wash in clay silt and organic material, during spring pollen season, and after heavy bather load. Less often in the shoulder seasons when the pool is used lightly and the water is cooler.
Keep a simple log of cleaning dates and pressure readings for one season and you will know your pool's rhythm better than any general advice can tell you.
If your filter is fighting you, it may be undersized, worn out, or mismatched to the pump. Get in touch or call Oasis Pools at (336) 471-0103 and we will evaluate the equipment as a system.