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My refrigerator has a carbon block water filter. I'm wondering when the proper time to replace it is, and don't want to replace it too soon.

Is using the pressure drop/flow rate a good method of knowing when to replace it, or should I go by the "months/gallons" rating, even though my city water might be cleaner than the water they used for the lifetime testing, so it wouldn't need replacement yet? Or will bacteria grow in the old filter, so I really should replace it every six months regardless of use?

What's the failure mechanism of the filters? Will they start to pass what they are supposed to block, or does their pressure drop increase while their filtering ability remains nearly constant?

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I find with mine that I notice slower flow and reduced pressure which is usually when I change it. Typically this is in the 6-8 month range, but as you mentioned it really depends on your water quality.

I don't believe bacteria growth is an issue but overtime the filtering ability will diminish along with the pressure and flow.

I think ultimately it is just easiest to replace on a fixed schedule. In the grand scheme of costs, if you change it 1 month earlier then you should, over the lifespan of your fridge you are probably only changing it a handful of extra times.

If your fridge has a built-in ice maker then the reduced pressure might be more of an issue - if you notice small ice cubes then this could be a sign its time to replace the filter.

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I'm curious about if there is a published study showing that bacteria don't grow in the filters. My plan is to measure the flow from the water dispenser every month, and replace the filter when the flow is reduced by 10%. So far, I've noticed no drop between months 5 and 7..... This plan could be flawed if there is a flow restriction device for the water dispenser itself.... I'll also keep an eye on the ice cube shape. My fridge does has a filter reminder, but we often accidentally reset it (bad design), so it isn't helpful (and I think that it's just a six month timer, anyway). –  Pigrew Jul 9 '13 at 19:34

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