Quick Answer: The A.O. Smith AOW-S500 is a well-built, NSF/ANSI 44-certified cabinet softener with a better-than-average warranty (6 years on the valve, 12 on the tank and cabinet), a KDF prefilter and Wi-Fi. It's a good buy for city-water homes with little or no iron. Don't buy it for a well: its rated iron limit is under 0.3 ppm. Get the SoftPro Elite for Well Water instead. If you'd rather have a separate-tank layout with a prefilter included, look at the Aquasure Harmony 50k.
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You probably know A.O. Smith from the label on your water heater. That's a big part of why people look at its softeners: if scale is killing your heater, buying the softener from the heater company feels like a safe bet. It's a reasonable instinct, but the brand name tells you less than the spec sheet does, and the spec sheet has one limit that rules out a lot of homes.
In this guide
Which A.O. Smith softener are we talking about?
A.O. Smith sells two families of softeners:
- A DIY retail line with model numbers like AO-WH-SOFT-350C (35,000 grain) and AO-WH-SOFT-450T (45,000 grain). You'll mostly see these at Lowe's.
- The S-Series "Pro" line: the AOW-S400 (36,000 grain) and AOW-S500 (49,800 grain). A.O. Smith's product page offers a "Find a Local Pro" link and its documents are labeled Pro and Wholesale. When I last checked, the AOW-S500 wasn't available to order on Amazon, so plan on buying it through a pro or a wholesaler.
This review covers the AOW-S500, the larger S-Series model. Everything below comes from A.O. Smith's product page and its S400/S500 sell sheet. I haven't installed one.
The AOW-S500, line by line
- Capacity: 49,800 grains (the S400 is 36,000)
- Flow rate: 7 gpm
- Salt storage: 200 lb
- Plumbing: 1" MIP inlet/outlet with a twist-style bypass; installs before the water heater
- Operating range: 20–120 psi, 40–120°F
- Max iron: less than 0.3 ppm
- Certification: NSF/ANSI Standard 44
- Pre-treatment built in: a KDF prefilter that A.O. Smith says reduces chlorine taste and odor, plus a self-cleaning sediment prefilter
- Controls: 2.7" monochrome touchscreen, salt-level sensing, battery backup that keeps settings during an outage, and a controller that adapts regeneration to your usage. Wi-Fi is on the S500 only, not the S400.
- Warranty: 1 year complete unit, 3 years electronics, 6 years valve, 12 years media tank and cabinet
- Sized for: 3–5 bathrooms, per A.O. Smith's product page
The sell sheet doesn't give capacity at different salt doses, a grains-per-pound efficiency number, or a maximum hardness. Those are the three numbers I'd most want for comparing it with other units. Our guide on how to compare water softener spec sheets explains why they matter and what to ask the seller.
The iron limit is the dealbreaker for well owners
A max iron of under 0.3 ppm is very low for a salt softener. Plenty of cabinet softeners in this class publish clear-water iron limits several times higher. If you're on a private well, the odds are good you're above 0.3 ppm. Running iron through resin that isn't rated for it leads to fouling, orange staining in the brine tank, and capacity that drops year after year.
You have two options on a well:
- Put a dedicated iron filter ahead of the AOW-S500. That's more equipment, more cost and more floor space.
- Buy a softener that's actually sized and rated for iron. Our guide on how iron changes water softener sizing shows the math.
For most well households, option 2 is simpler. Hence the first alternative below.
7 gpm and "3–5 bathrooms" don't quite add up
A.O. Smith positions the S500 for 3–5 bathrooms, but its flow rate is 7 gpm. Every shower, faucet and appliance running at once draws its own couple of gallons per minute. Look at the gpm stamped on your showerheads and add up what realistically runs together on a busy morning. Two showers, a dishwasher and a washer filling can get close to 7. A five-bath home with teenagers can go past it.
What happens when you exceed rated flow? Pressure drops at the fixtures, and some hard water can slip through during the peak. For a 2–3 bath home, 7 gpm is fine. For a true 4–5 bath house, I'd want a softener with a published service flow comfortably above 10 gpm. Pentair's 1.5 cu ft rotary-valve system, for comparison, is rated at 11.9 gpm.
The 49,800-grain capacity is more than a typical family of four on city water needs. Bigger isn't automatically better, as we explain in is a higher grain rating always better? The adaptive controller should keep it from wasting salt, though A.O. Smith doesn't publish an efficiency figure.
S400 or S500?
The two S-Series models share a manual, a sell sheet, the 7 gpm flow rating, 200 lb of salt storage, the KDF and sediment prefilters, the 20–120 psi range and the warranty. A.O. Smith's literature lists only two differences:
- Capacity: 36,000 grains on the S400 and 49,800 on the S500.
- Wi-Fi: the S500 has it. The S400 doesn't.
Since flow is the same, the extra capacity doesn't help on busy mornings. It only means fewer regenerations. For a household using around 4,000–5,000 grains a day (say four people on 13–16 gpg water at about 75 gallons each), the S400 would regenerate roughly every week and the S500 roughly every ten days at full capacity. Both are fine. I'd pay for the S500 if you want the app alerts or your hardness is above about 20 gpg. Otherwise the S400 does the same job. Either way, you'll probably be buying through a pro or a wholesaler.
What it does well
- The warranty is better than most big-box cabinets. Six years on the valve and twelve on the tank and cabinet beat the common 1-year parts / 3-year board / 10-year tank pattern.
- The KDF prefilter is a nice extra on city water. It reduces chlorine taste and odor at the point of entry. If you were planning to add a carbon stage, this may be enough. If not, read using a carbon filter with a softener.
- Salt sensing plus Wi-Fi means you get a heads-up before you run out of salt.
- Settings survive outages thanks to the battery backup.
Better choices for some households
Well water with iron → SoftPro Elite HE for Well Water (64,000 grain). SoftPro builds this version specifically for well supplies and uses fine-mesh resin, the resin type you want when iron is in the picture. SoftPro claims up to 75% salt savings and 64% less wastewater versus conventional softeners, and backs it with 10 years on the valve and lifetime on the tank. SoftPro's own pages say to add an iron prefilter once iron goes above roughly 3–5 ppm, and they don't publish an exact ppm rating for the Elite. So test your water first, but it's built for the job the AOW-S500 isn't. The extra capacity also helps, since iron eats into a softener's usable grains. Check price on Amazon: SoftPro Elite for Well Water
Prefer separate tanks → Aquasure Harmony 50,000 grain. A two-tank system with a digital metered control head and fine-mesh resin that Aquasure rates for up to 10 ppm of iron, plus a triple-purpose prefilter for sediment, chlorine and taste. Aquasure pitches it for 3–4 bathroom homes. Choose it over the S500 if you'd rather have a separate-tank layout with more resin and more iron tolerance. Before you order, confirm the listing's rated service flow against your peak demand. Downsides: there's no app or salt-level sensing, the prefilter is one more cartridge to change, and it takes up more floor space than a cabinet. Check price on Amazon: Aquasure Harmony 50k
No salt at all → A. O. Smith AO-WH-DSCLR. If you mainly want to protect the water heater and can't run a drain, A.O. Smith's own salt-free descaler is rated for up to 6 years or 600,000 gallons on city or well water, with a 6-year warranty. It needs no salt, backwashing or electricity. Read the limits, though: A.O. Smith says it's for water under 15 gpg hardness and under 0.3 ppm iron, and it reduces scale without softening, so you'll still see spots and won't get the slick feel. Check price on Amazon: A. O. Smith Salt-Free Descaler
Who should buy the AOW-S500
Buy it if: you're on municipal water with a little chlorine taste, have 2–3 bathrooms, and want a compact cabinet with strong warranty coverage and an app that warns you about low salt.
Pass if: you're on a well (check the iron first; anything at or above 0.3 ppm is outside its rating), you have a large home with heavy simultaneous use, or you want detailed efficiency numbers before you buy.