Generally once a year, technically upto 26,000 gallons will give you 100% effectiveness, after that it begins to degrade, most folks here use 2000-2500 gallons of water a month, the the one year mark works.
I've had few that for some odd reason pushed it to over 40,000 gallons. When I changed the filters they were really bad, way overdue.
When that homeowner saw them, they now are on a one year schedule.
We email a reminder out once a year to change them.
FYI, there are many home owners that have been told when buying other systems their carbon never (or 5years) needs changing because it is back washed.
Nothing in my years is more dangerous info, carbon can not be back washed, the contaminants it traps in it's microscopic pores are there forever.
Lastly, bacteria begins to grow after 18 months.
So they need to be replaced before that time.
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