An external refrigerator filter sits in the small tube feeding the appliance rather than in a bay inside it, and for a number of households it is the better arrangement. It is bought for three distinct reasons, and it is worth knowing which applies to you before deciding what happens to the internal cartridge.
The three reasons to fit one
The appliance has no internal filter position at all, which is common on simpler and older models with a water and ice supply. The manufacturer's cartridges are expensive or electronically authenticated, so an external filter lets a standard cartridge do the work instead. Or a longer service interval and a bigger element are wanted, since an external housing can hold several times the media of a bay sized cartridge and therefore runs far longer between changes.
What to do about the internal one
Two options with quite different consequences. Keeping it as a second stage puts two cartridges in series, which slows the dispenser noticeably and doubles what you buy for very little gain. Removing it and fitting the manufacturer's bypass plug is usually better, and on models that authenticate the cartridge electronically it may not be permitted, in which case an external filter is either redundant or you are paying for two. Check the appliance's behavior before ordering anything.
Fitting it into the feed
The appliance is fed by a small diameter tube from a shut off valve, usually under a nearby sink or at a cold line behind the fridge. Close the valve, relieve the pressure, cut the tube square with a proper tube cutter, and fit the filter with the flow arrow pointing towards the refrigerator. Leave enough slack that the appliance can still be pulled out fully, and mount the housing to the wall or cabinet rather than leaving it hanging on the tubing.
Commissioning and remembering it
Open the valve slowly, check every joint by hand, then again an hour later, since push fittings often weep only after pressure settles. Run several litres through the dispenser and discard the first two or three batches of ice, because carbon fines produce grey water and cloudy ice at first. Then write the date on the housing, because an external filter has no indicator light and lives out of sight behind the appliance, which is exactly how these end up years overdue. Our external fridge filter guide covers cartridge matching.
Questions people ask about external refrigerator water filter
When is an external filter better than the fridge's own?
On fridges with no filter bay, on models with expensive or authenticated cartridges, and where a longer interval is wanted.
Should I keep the internal cartridge too?
Usually not. Two in series slow the dispenser and double the cost. Fit the manufacturer's bypass plug instead where permitted.
Which way round does it go?
Flow arrow pointing towards the refrigerator, mounted to a wall or cabinet rather than hanging on the small tubing.
How will I remember to change it?
Write the date on the housing. There is no indicator light and it sits out of sight, which is how these end up years overdue.