Ask ten well owners how often they change their sediment filter and you'll get "every three months," said with varying confidence. Ask them why three months and you'll get silence. The truth: the right interval for your well might be three weeks or nine months, and the manufacturer printed "3 months" because they had to print something.
Your system will tell you exactly when the filter is done — if you give it a way to talk. That's a $15 pressure gauge.
What the Filter Actually Does
A whole-house sediment filter — usually a 10-inch cartridge in a housing after the pressure tank — catches sand, silt, rust flakes, and scale before they reach your fixtures, appliances, and any downstream treatment. It protects everything behind it: the UV system that can't disinfect cloudy water, the softener bed, the water heater, and every faucet aerator in the house.
As it loads up with sediment, it doesn't fail dramatically — it just chokes. Flow drops, pressure at the taps sags, and eventually a really packed cartridge can start unloading captured fines back into the stream.
The Real Answer: The 10–15 PSI Rule
Install a pressure gauge on each side of the filter housing (many housings have ports; gauge kits are cheap, or put one gauge downstream and compare against your pressure switch gauge upstream).
A fresh cartridge drops 1–3 psi. Change the cartridge when the pressure drop across it reaches 10–15 psi. That's it. That's the whole rule the "every 3 months" advice is imitating.
No gauges yet? Two fallback signals:
- The shower test: noticeable pressure loss at fixtures that recovers completely with a new cartridge = you waited too long.
- The eyeball test: a clear filter housing lets you see the cartridge browning. Visual isn't as good as pressure (a cartridge can look filthy and flow fine, or look OK and be blinded with fine silt) but it beats guessing.
Change it regardless at ~6 months even with low pressure drop — a wet cellulose cartridge sitting for months can grow biofilm.
Picking the Right Cartridge (Micron Ratings Decoded)
Micron rating = the particle size the filter catches. Smaller number = finer filtration = clogs faster.
| Rating | Catches | Use when |
|---|---|---|
| 50 micron | Sand, visible grit | First stage for sandy wells |
| 20–30 micron | Fine sand, silt | General first stage |
| 5 micron | Fine silt, most rust | The default single-cartridge choice; required ahead of UV |
| 1 micron | Very fine silt, some cysts | Only with a coarser stage ahead of it |
Two mistakes cost people the most:
- Going too fine, alone. A single 1-micron cartridge on a silty well clogs in days. If you need fine filtration, run two stages: coarse (20–50µ) then fine (5µ or 1µ). Each stage lasts far longer, and the pair costs less per month than burning fine cartridges weekly.
- Wrong style for the sediment. Lots of sand? Add a spin-down pre-filter ahead of everything — a reusable screen you purge with a valve twist, no cartridge to buy. Fine cloudy silt? Pleated (washable, more surface area) for larger particles; melt-blown depth cartridges for the fine stuff.
Buy cartridges in multipacks — per-cartridge cost drops by half, and having spares on the shelf is the difference between a 10-minute swap and a week of procrastination at 14 psi of drop.
Swap procedure, for completeness: bypass or kill the pump, relieve pressure at a faucet, unscrew the housing (strap wrench), swap cartridge, check the O-ring (a dab of silicone grease yearly — never petroleum jelly), hand-tighten, repressurize, check for drips.
Your Action Plan
- This week: add gauges (or at least note your baseline pressure at a reference faucet with a fresh cartridge).
- Monthly, 30 seconds: glance at the pressure drop. Under 10 psi, walk away.
- At 10–15 psi drop: swap the cartridge. Note the date.
- After three swaps: you now know your well's real interval — and whether a two-stage setup or spin-down would pay for itself.
Step 4 is where the money is, and it only works if the dates exist. Log each swap in the WellKept app — date and pressure drop — and the app shows your actual replacement rhythm: "cartridges last 11 weeks in summer, 5 weeks after spring runoff." Set a reminder at your real interval, keep a multipack on the shelf, and the filter stops being something you think about.
Frequently Asked Questions
How often should a whole house sediment filter be changed on well water? When the pressure drop across it reaches 10–15 psi — which might be 3 weeks on a silty well or 9 months on a clean one. Absent gauges, every 3 months is a defensible default, but install gauges: they turn guesswork into a 30-second monthly glance.
What happens if you don't change a sediment filter? Water pressure throughout the house sags as the cartridge chokes, the pump works harder against the restriction, and a badly overloaded cartridge can shed captured sediment downstream or grow bacteria in the trapped organic material. It quietly becomes the most expensive $8 part you didn't replace.
What micron sediment filter is best for well water? Five micron is the standard single-cartridge choice, and it's the minimum protection required ahead of a UV disinfection system. If your well carries heavy sand or silt, run a 20–50 micron stage (or a reusable spin-down filter) first so the fine cartridge doesn't clog in days.
Should the sediment filter go before or after the pressure tank? After. Placing the filter before the pressure tank forces the pump to push through a clogging restriction and can cause erratic pressure-switch behavior as the filter loads. The standard order is: well, pressure tank, sediment filter, then any treatment (softener, UV), then the house.
Why does my sediment filter turn brown so fast? Iron. Orange-brown slime or staining on cartridges usually means dissolved iron oxidizing in the housing, not just dirt — and cartridge filters are a terrible iron treatment. If cartridges brown within weeks, test your iron level; over ~0.3 mg/L you'll save money with a proper iron filter.