Guide
How often filters actually need changing
Manufacturer intervals are a starting point, not an answer. What actually determines the interval is your water and your household, and both push in the same direction.
Why the number on the box is a starting point
Manufacturer intervals are set against test water at a test flow rate. Your water is dirtier or cleaner than that, and your household uses more or less than the assumed volume. Both shift the real interval, usually shorter.
Sediment is the biggest variable. A prefilter on a well with fine silt can need changing monthly, while the same cartridge on clean municipal water lasts a year. Hardness shortens membrane life. Heavy chlorine loading shortens carbon life.
The practical approach is to use the stated interval for the first cycle, then observe. If pressure has dropped noticeably or taste has returned before the interval is up, that is your real interval and you should book against it rather than the label.
Typical intervals by equipment type
Sediment prefilters: three to twelve months, and the widest range on this list. Judge by pressure drop rather than by date.
Carbon block and under-sink carbon: six to twelve months. Carbon does not signal exhaustion in any obvious way, which is the problem with it. A cartridge left in past its capacity stops adsorbing and can release what it holds.
Reverse osmosis prefilters and postfilters: six to twelve months. The membrane itself lasts two to five years, and it lasts longest where the prefilters have been changed on time and the water is soft.
Ultraviolet lamps: annually, without exception, and this one is not negotiable. A UV lamp keeps glowing long after its output has fallen below the dose needed to inactivate anything. A lamp that looks fine and is doing nothing is the worst failure mode in this field because it is invisible.
Water softeners: no filter to change, but the brine tank needs salt kept topped up and the resin bed benefits from a cleaner periodically. Whole-house tank media such as carbon or a neutraliser is replaced every three to five years depending on throughput.
The failures that cost the most
Leaving a UV lamp beyond a year. Everything looks normal and nothing is being disinfected. On a private well with a bacterial history that is a genuine health failure rather than an inconvenience.
Ignoring a sediment prefilter until pressure drops. By that point the filter has been loading the downstream equipment for months, and on a reverse osmosis system that means a membrane costing several times the cartridge that should have protected it.
Letting a softener run out of salt for an extended period. The resin stops regenerating, hardness passes straight through, and scale begins forming in the appliances the softener was bought to protect.
Changing a cartridge without sanitising the housing. A housing that has held a spent filter holds biological growth too, and a new cartridge dropped into it is contaminated on day one.
Building a schedule you will actually follow
Write the date on the cartridge in marker when you fit it. This sounds trivial and it is the single most effective habit in home water treatment, because the question that defeats most people is not when to change it but when they last did.
Set a calendar reminder at the interval, not a mental note. Filters fail silently and gradually, so there is no event to prompt you.
Buy the next set of consumables when you fit the current one. The most common reason a change slips by two months is that the replacement was not in the cupboard.
If you would rather not track it, a maintenance plan exists for exactly that reason. The value is not the labour, it is that somebody else owns the calendar.
When to stop maintaining and replace the unit
There is a point where a system is not worth keeping. If the cost of a year of consumables approaches a meaningful share of a new unit, and the existing one is over a decade old, replacement is usually the better spend.
The same applies where parts are no longer made. Plenty of equipment in this region was fitted by installers who are no longer trading, and some of it uses proprietary cartridges that are now hard to source at sensible prices.
And a system fitted for a problem you no longer have is worth removing rather than maintaining. If the utility has since treated for the contaminant at the plant, your equipment is consuming cartridges to do work somebody else is already doing.
Common questions
How do I know when a carbon filter is exhausted?
You largely cannot, and that is the difficulty with carbon. It gives no reliable warning. Taste returning is a late signal rather than an early one, and by then the cartridge has been past its useful capacity for a while. Carbon is the case for changing on schedule rather than on symptom.
Can I change filters myself?
For most under-sink and whole-house cartridges, yes, and we will show you how at the installation rather than keeping it to ourselves. Shut off the supply, depressurise, swap the cartridge, sanitise the housing and flush. The jobs worth handing over are UV lamp changes where the quartz sleeve needs handling and membrane replacements where the system needs rebalancing afterwards.
Is a maintenance plan worth it?
It depends what you are buying. If you are buying labour you could do yourself, often not. If you are buying somebody else owning the calendar, frequently yes, because the common failure in home water treatment is not a bad filter change but a change that never happened. Judge it on whether you realistically track intervals.
Related reading
- water filter installation: maintenance plans
- water filters: uv disinfection
- water filters: reverse osmosis
Other guides
Next step