How often should a whole-house water filter be changed?

NSF, the group that tests and certifies home water treatment products, says the recommended change cycle "varies from product to product." Home filtration systems usually have established service cycles, and the cycle may be a set number of gallons or an estimate of how many months a cartridge lasts in the average home. To keep the filter reducing contaminants, NSF says to replace it according to the manufacturer's recommendations.

Here is what real numbers look like, from published product data.

• 3M Aqua-Pure whole-house cartridges (AP900 series). The product sheet lists a 100,000 gallon capacity, which it describes as approximately one year of cartridge life under most water quality conditions. The same sheet says the cartridge must be replaced at the required intervals stated on the label and manual.
• Culligan whole-home media systems. Culligan does not list one date. It says some media, such as its Cullar and Filtr-Cleer filters, can last several years, that its Iron-Cleer filter usually lasts several years, and that its pH neutralizing media is consumed and needs periodic replenishment. It adds that if you notice a drop in water pressure you may need a replacement filter, and that an arsenic reduction filter needs regular water tests to say when the media is due.

### Typical intervals by filter type

Published manufacturer schedules show how wide the range is. Your own cartridge's rating always wins over these examples.

Aquasana also says the main filter in a whole-house system can last anywhere from 1 to 10 years, and that some filters, such as pitcher filters, last only 1 to 2 months. So a two-month sediment cartridge and a ten-year tank are both normal, for different jobs.

Do the math on the rated gallons. A 100,000 gallon cartridge rated for one year assumes about 274 gallons a day on average, which is 100,000 divided by 365. A large household or one that irrigates from the house line burns through gallons faster than the "one year" label suggests. Then the gallons run out before the calendar does.

That leads to the practical rule. Go by whichever limit you reach first, gallons or months. Then treat the rated life as a ceiling, not a target.

Water quality and use change how long a filter lasts, and the filter itself will tell you when it is worn out if you know what to watch.

Pressure drops at every faucet. NSF explains why: as debris fills the pores of the filter, water can't move through effectively, and flow slows dramatically. Culligan says the same, that a drop in water pressure may mean the filter needs replacing. A filter on the main line affects the whole house, so low pressure at every tap, along with a filter that was fine last year, points at the filter first. Our page on low water pressure in the whole house walks through the other causes if a new cartridge does not fix it.

Taste or smell comes back. NSF makes an important point here. The clogged-pores problem is easy to spot, but it is not easy to tell when the surface of carbon or similar media has become full and can no longer adsorb contaminants. That kind of failure does not slow the flow at all. The water just stops getting treated. So if the chlorine taste or a musty smell returns after months of clean water, the media is likely spent, even if pressure is fine.

Sediment shows up downstream. If you see grit or a cloudy or brown tint that the filter used to catch, the cartridge may be full or bypassed. Our page on tap water that turns brown helps you tell a filter problem from a city line disturbance.

The gauge says so. Some systems have pressure gauges or indicator lights. NSF's home water treatment guide notes that some filters have indicator lights and some recommend replacement time ranges. If yours has either, use it along with the calendar, not instead of it.

There are three outcomes, and the worst is the quiet one.

1. 1Flow drops. A clogged sediment cartridge restricts water. This is the easy one to notice.
2. 2Treatment stops. NSF says the surface area of carbon media eventually fills and no more contaminants can be adsorbed. The filter still passes water, but it is not doing its job.
3. 3The claim no longer holds. The City of Phoenix warns in its water quality report that failing to follow the manufacturer's instructions on operation and maintenance for home treatment systems can result in potentially unsafe water. The EPA says the same about lead filters: using the cartridge after it has expired can make it less effective at removing lead.

Your city reports its test results every year, and a filter is optional treatment on top of that, often chosen for taste, odor and sediment. The risk from skipping maintenance is that the filter stops doing the job you bought it for and nothing tells you.

### Does it matter which replacement cartridge you buy?

Yes, and this catches many people. NSF says filter cartridges are not universal. A non-certified cartridge may look the same size and appear to fit the housing, yet even the smallest size difference could let water go around the cartridge instead of through it. Non-certified cartridges also may not match the manufacturer's quality, which can mean poor filtration or the introduction of materials that were never verified as acceptable for drinking water contact.

So buy the model the manufacturer names for your housing, and check the package for the NSF mark and the standard that applies. NSF/ANSI 42 covers aesthetic claims like chlorine, taste and odor. NSF/ANSI 53 covers contaminants with a health effect. NSF also says certification to a standard does not mean a product reduces every contaminant, so check the claim for the specific contaminant you care about.

Two more rules from the 3M sheet apply to most whole-house housings. The system is for cold water use only, and if your supply pressure is above 80 psi you must install a pressure reducing valve before the system.

Here is a clean way to set it up.

1. 1Find the rated life. Look in the owner's manual or on the cartridge for gallons, months or both.
2. 2Write the install date on the housing. A marker on the cap or a note on the wall near the filter takes ten seconds.
3. 3Watch the meter if you have one. If not, estimate. Multiply the people in your home by their daily use, and add outdoor use that runs through the filter.
4. 4Set a reminder for a bit before the rated date. Change early if pressure drops or taste returns.
5. 5Test your water once a year. Culligan advises regular water testing for filters that target specific contaminants such as arsenic.
6. 6Replace the whole set. If your setup has a sediment stage and a carbon stage, replace each on its own cycle.

Filters that handle your drinking water at one tap, like an under-sink reverse osmosis system, follow different cycles. Our page on reverse osmosis filter replacement and maintenance covers those, and Culligan lists separate timelines for RO prefilters, postfilters and membranes.

If you would rather not track cartridges, or you are not sure what your existing system needs, we can look at the setup and give you an upfront price for a filter change. If the real issue is scale rather than sediment, a softener is a different tool, and our guide on signs you need water treatment helps you decide.

Here are the short versions.

Is once a year right? For one common 3M cartridge, about one year is the rated life under most conditions. Other products differ, so use your own product's number.

Can I just change it when the water pressure drops? Pressure drop is a good sign, but NSF says a full carbon filter may not slow the flow at all. Use the pressure sign and the schedule together.

Can I clean and reuse a cartridge? Only if the manufacturer says the cartridge is reusable. Otherwise replace it on the rated cycle.

Does a whole-house filter make water safe to drink? Not by itself. A whole-house carbon filter mostly addresses taste, odor and sediment. Certification differs by product, so read the claim on the package.