Filters on a built-in: two schedules, and the one everybody mixes up

The air cartridge: a year, on the cabinets that have one

The air purification cartridge scrubs the air circulating inside the compartment it sits in, taking out odors and the ethylene that ripens produce faster than you want it ripened. It runs on time rather than on visible dirt: the icon flashes after about a year of the unit and the filter running, or earlier if the bulb inside the cartridge has failed. Two things about it catch owners out. The first is that a good many cabinets never had one, because Sub-Zero fits air purification only to the Classic cabinets, most of the Designer line and two of the PRO models, which leaves the 600 and 700 Series, undercounter units, freezers and wine storage without one. The second is that it has no part in refrigeration. It does not affect temperature, it cannot cause a warm compartment, and replacing it will not fix one. And because dual refrigeration keeps the two compartments completely separate, a cartridge in the fresh-food section is doing nothing whatsoever for the freezer.

• About a year: the icon flashes after roughly 365 days of the unit and the filter running
• An icon that flashes early usually means the bulb inside the cartridge has failed rather than the medium being spent
• There is no air cartridge at all in a 600 or 700 Series, an undercounter unit, a freezer or a wine cabinet
• It cannot make a compartment warm, and it cannot fix one that already is
• A cartridge, not a screen: it is not washable and not reusable
• It cleans the air in its own compartment only, which on a dual-refrigeration cabinet never reaches the freezer
• Nothing in the machine is harmed by running without one: what you lose is odor and ethylene control in that compartment

The water filter: gallons first, a year at the outside

The water filter is a carbon cartridge on the supply feeding the ice maker, and its schedule is not a calendar one. The control counts the gallons that pass through the cartridge until its rated life is used up, or a year from the last reset, whichever arrives first, so a household making ice every day gets there long before a weekend house does. Depending on the model it lives inside the fresh-food compartment near the top or behind the machine compartment grille, so look in both places before deciding your unit does not have one. Two symptoms are worth acting on without waiting for the indicator: a noticeable drop in water pressure, and cubes coming out hollow. Both mean the mold is taking less water than it was designed to take, and both are why an ice maker gets condemned when nothing mechanical has failed. Fit a new module to a starved supply and the next batch is exactly as hollow as the last one. Change the filter, then run water through and throw away the first two or three harvests, which will be cloudy with carbon fines and trapped air.

• Gallons or a year, whichever comes first: use sets the interval on this one, not the calendar
• Six months is the schedule for an undercounter ice maker, not for the filter in a built-in refrigerator
• Small, hollow or half-formed cubes: filter age first, then supply pressure measured at the inlet valve
• Cloudy ice for a day after a change is normal: dump the first two or three harvests and it clears
• It is a carbon cartridge, not a softener. It improves taste and catches sediment and does nothing about hardness
• Running without one is not a plan: some units need a bypass fitted to flow at all, and the rest send unfiltered sediment straight to an inlet valve that does not tolerate it
• Write the date on the housing with a marker, because that is more reliable than anyone's memory of last spring

The two indicators count different things, which is where the confusion starts

Neither light is a sensor looking at the cartridge, but they are not the same kind of counter either. The water filter indicator is fed by the control counting gallons through the cartridge, with a year from the last reset as the backstop, so a house of six making ice every day brings it on early while a house that barely draws water gets the full year. The air cartridge indicator is a run-time counter, roughly 365 days with the unit and the filter on, which is why a coastal weekend house opened a dozen times a year still gets told to change a cartridge that has barely worked. Reset whichever one you changed, following the procedure in the manual for that model, because a light that was never reset is reporting somebody else's schedule. And if you have just bought the house, assume nothing about either filter and start the clock yourself.

Hard water sits downstream of the filter, which is where the ice trouble really starts

Water in this county is hard, and a carbon filter does not soften water. It takes out sediment, chlorine taste and odor, and it passes calcium and magnesium straight through, because that is not what carbon is for. Everything downstream of the cartridge therefore keeps scaling: the inlet valve, the fill tube, the fill cup and the mold itself. The signature is a decline measured in months rather than a failure on a particular day, with cubes getting quietly smaller and a chalky white ring appearing where the water lands. Owners who have been diligent about the filter find this genuinely unfair, and it is the mix-up worth taking away from this article. A fresh filter and shrinking cubes together mean the problem moved past the filter some time ago.

• A new filter with still-shrinking cubes: look at the inlet valve and the fill tube, not the cartridge
• Chalky white deposit in the mold or around the fill cup is hardness, not a dirty filter
• Filters handle sediment and taste. Softening happens upstream of the kitchen or it does not happen
• An ice maker added during a remodel and fed from a saddle valve is a slow-flow story all of its own
• A valve that sat dry while the water was off in the house can give readings that are easy to misread afterward

The condenser is not a filter, and nothing protects it

No manufacturer fits a filter in front of a condenser, and the reason is obvious once you say it out loud: a filter would restrict exactly the airflow the coil exists to use. So the coil takes whatever the room gives it, unfiltered, for its whole life. Close to the water that is not ordinary dust. Onshore air carries a fine salt aerosol that holds moisture against the aluminum fins, and what settles binds into a mat rather than staying loose, packing down between the fins where a vacuum will not reach. This is the maintenance item that turns into money: a restricted coil raises head pressure, the compressor runs hotter and longer, and the unit starts catching up overnight instead of holding through the day. Cleaning the condenser every three to six months is the only general maintenance Sub-Zero asks for, and it is the item most owners have never done: the owner who has changed every filter on schedule and has never had the grille off is the common case, not the rare one.

• There is no filter over a condenser by design, because it would choke the airflow the coil needs
• Coastal loading binds into the fins and has to be combed out rather than blown off
• Pets, tracked-in sand and a door left open to the breeze all shorten the interval
• Three to six months is the general figure, and within a few blocks of the sand the coil sets the interval rather than the calendar

A schedule you will actually keep

Pick a fixed date for the yearly items and hang everything else off it. New Year, a birthday, the week the clocks change, it does not matter as long as it is a date you cannot forget. The air cartridge goes on that date, on the cabinets that carry one. The water filter goes in when the indicator asks for it and at a year if it has not asked, which in a house that makes a lot of ice comes round sooner than the date does. The condenser gets looked at every three to six months, and near the water it usually wants doing at every look. Buy the next cartridge the day you fit the current one, so the replacement is in the cupboard when the light appears rather than three days away. Write the date on the cartridge in marker. And if there is a Cove dishwasher in the same kitchen, its filter is a different animal again: it lifts out of the sump and rinses under the tap, so that one is a cleaning job rather than a purchase.

• Air purification cartridge: about a year, on a fixed date, on the cabinets that carry one
• Water filter: when the indicator asks and at a year at the latest, resetting it each time
• Condenser: every three to six months as the general figure, and as often as this climate demands within sight of the water
• Cove dishwasher filter: lifted out and rinsed, not replaced