An excellent choice
I really did my homework on these filters. I have a 2 stage whole house filter system so I have to think carefully about what I use. For my first stage I have a sediment filter with added polyphosphate to deal with scaling (quality filter made by 3M). This A.O. Smith is my second filter. The things I have to worry most about in my water are chlorine and chlorine byproducts as well as atrazine. Note the following review is entirely in the context of needing a whole house filter with a very high flow rate. Carbon block filters can be NSF rated at 42 or 53. For the 42 standard, manufacturers can talk about reducing the problematic taste or appearance of contaminants. For the 53 standard, manufacturers can talk about the health effect of removed contaminants. The bad news for people with 10 x 2.5 housings is that, I don’t think, there is a single carbon filter on the market in this size, with a decent flow rate, that claims the NSF 53 standard compliance. I believe this is because the filter is simply too small to meet the standard. So we’re stuck with the NSF 42 standard. There is more bad news here. I don’t think there is a single filter in this size, with this flow rate, that is fully certified by NSF to meet the 42 standard. There appear to be three levels that NSF certifies. One is the materials used in the filter are to spec, the next is that the filter housing is to spec, and finally there is certification that the filter produces water that meets the standard. What you get with the A.O. filter here is certification from the NSF that the filter media meets spec. That appears to me to be as good as it gets after spending 1/2 of an entire Sunday researching this. On the upside this is, as far as I can tell, one of, if not the, highest flow rate filters in this size with an NSF certification. At 4 gpm it’s WAY higher than most filters in this class. The amount of water this filter will treat is not stated explicitly anywhere, but since they’re saying it might last 6 months they seem to be estimating it will last over 10,000 gallons which is amazing for this type of filter (I sincerely doubt it will last 10,000 gallons). However it does appear to be a true carbon block filter which could account for its longevity. So back to water quality. It’s easy enough to know if a filter like this is removing chlorine because of the taste and smell. However, at least for me, it’s much harder to know if it’s removing trihalomethane or atrazine. There is reason to believe it IS removing those contaminants because the mechanism of action is similar to how chlorine taste and smell are removed. However I can't be sure without doing expensive lab tests, that's what the NSF 53 standard is for. And again, I don't think there is a single filter in this form factor certified to the NSF 53 standard. You may spot something like the Matrikx and think "oh he's wrong" but that filter has a flow rate of .75 gpm, it's just not practical for whole house applications. Overall this filter is a very good choice. For the price and the size it may be the best filter choice if you need a high flow rate for whole house applications. 4 gpm will be able to work for 1 or two people but will struggle beyond that. On the negative side there is no mention of whether the carbon block is coconut shell so it probably isn't (coconut shell is preferred). Also, on first use I had about 5 gallons of water that came out pretty dark with carbon fines. This isn't necessarily bad but something to keep in mind if you use this as a pre-filter because those fines will not be good for the filter that come after this one. You'll want to flush from a faucet that doesn't not have a screen because I got two fairly large chunks of carbon in addition to fines in that first 5 gallons. Read more