Why criteria, not products
Most guides answer “which system” by describing systems. That only works if you already know which category you are in.
These seven questions decide the category. Each one is answerable before you spend anything, and each one rules something out as well as in. A criterion that only ever points at “buy something” is not a criterion.
1. Well or municipal?
How to answer: you already know.
Rules in: on a private well, you own the whole treatment train and the testing. EPA’s enforceable limits — nitrate at 10 mg/L, arsenic at 0.010 mg/L — apply to public water systems, not to your well.
Rules out: on municipal water, whole-house iron and bacteria treatment is usually unnecessary. Your utility already treats for both.
2. What do your test numbers say?
How to answer: a six-parameter panel — total coliform, nitrate, pH, hardness, iron, manganese. Add arsenic once.
Rules in: each number maps to one stage of the treatment train.
Rules out: everything, until you have them. Symptoms overlap. Orange staining is iron; black staining is manganese. They need different media.
3. Is your iron dissolved or oxidised?
How to answer: draw a glass and let it stand an hour. Clear then orange-brown means dissolved. Particles immediately means oxidised.
Rules in: dissolved iron below 5 mg/L can go through a softener, provided pH is above 6.7.
Rules out: a softener, if the iron is already oxidised. Penn State Extension is direct — oxidised iron and manganese foul the resin. Oxidation and filtration go upstream instead.
4. Does only the hot tap smell?
How to answer: run cold, smell it. Run hot, smell that.
Rules in: if both smell, hydrogen sulfide is in the groundwater and needs treatment sized by concentration.
Rules out: all water treatment, if only the hot tap smells. The cause is the water heater’s magnesium anode rod. An aluminium replacement is a part, not a system.
5. What is your peak flow demand?
How to answer: count fixtures that can run at once. A shower, a dishwasher and a washing machine is a different number from one bathroom.
Rules in: flow rate sizes the vessel and the UV lamp.
Rules out: any unit rated below your peak demand. An undersized system starves fixtures at the exact moment you notice.
6. Can you discharge brine?
How to answer: are you on sewer or septic? Is there a drain near the installation point?
Rules in: salt-regenerated softening, if discharge is straightforward.
Rules out: a salt-regenerated softener where brine discharge is restricted. That points at a conditioner — but note what changes. NSF/ANSI 44 covers cation exchange regenerated with sodium or potassium chloride. A saltless device sits outside that standard, and your hardness number will not fall.
7. Is anyone in the house sodium-restricted?
How to answer: ask.
Rules in: potassium chloride as the regenerant. Minnesota Department of Health suggests exactly that substitution.
Rules out: softening every tap. Minnesota Health recommends keeping an un-softened tap for cooking and drinking regardless.
The decision path: your situation, and what to weight
Answer the seven questions, then find your row. The output is not a product. It is the short list of criteria that decide your purchase, in priority order.
| Your situation | Weight these first | Where that lands you |
|---|---|---|
| City water, scale on fixtures and glassware | Hardness in gpg, brine discharge access | Softener, or a conditioner if you cannot discharge brine |
| Well, orange staining | Iron form, pH, combined iron and manganese | Oxidation and filtration, ahead of everything else |
| Well, black staining | Manganese, combined iron and manganese | Greensand between 3 and 10 mg/L; oxidation above it |
| Well, rotten-egg smell at the cold tap | Hydrogen sulfide concentration in mg/L | Method banded by concentration, sharing the iron vessel |
| Rotten-egg smell at the hot tap only | Nothing. The anode rod is the cause | No treatment purchase at all |
| Well, coliform positive once | Repeat test, well cap and casing, turbidity | Fix the well first. UV only if it recurs on clear water |
| Well, nitrate above 10 mg/L | Certified nitrate reduction, influent concentration | Point-of-use, drinking water only |
| Well, arsenic detected | Arsenic species first, then removal for that species | Point-of-use, with pre-oxidation if arsenic III |
| Blue-green staining in sinks | pH, then hardness measured after neutralising | Neutraliser first, then size the softener on the new number |
Two rows end in no purchase. That is the path working correctly.
Mistakes that cost the most money
Buying the softener first. It is the product people have heard of, so it gets bought before the water is understood. Penn State Extension: oxidised iron and manganese foul the resin. A softener-first install on well water with oxidised iron is a resin bed degrading from day one.
Sizing from a regional hardness map. USGS says of the widely-reproduced national map that its 1975 data “are not designed to be used to make local decisions or decisions on the scale of individual homeowner property.”
Accepting a removal percentage with no species named. Reverse osmosis removes over 98 percent of arsenic V and typically 60 to 65 percent of arsenic III. A supplier quoting one number is quoting the easier form.
Accepting a standard number without a claim. Under NSF/ANSI 58, TDS reduction is the only required claim. Lead, nitrate and arsenic are optional.
Reading a stain as a health risk. Iron at 0.3 mg/L and manganese at 0.05 mg/L are Secondary Maximum Contaminant Levels, which EPA does not enforce. Nitrate at 10 mg/L is an enforceable health limit. Selling the first with the urgency of the second is how filtration gets sold on fear.
Installing UV ahead of particulate removal. Bacteria sit in the shadow of suspended solids. The lamp still draws power and still shows a green light.
Three situations, and how the path changes
On city water. Your utility already treats for bacteria and usually for iron. Question 1 rules out most of the treatment train immediately. Hardness and taste are what remain, so questions 2, 6 and 7 carry the weight.
On a well with no treatment yet. Every question applies, and the order in the decision path is the order to buy in. Test first, then work down the table. Nothing here is worth buying out of sequence.
On a well with a softener already failing. Question 3 is the one to run first. A softener losing capacity early on well water usually means iron is reaching the resin. That is an upstream problem, and replacing the softener without fixing it buys the same failure again.
The objections worth answering
“Can I buy one system that handles everything?” Whole-house units exist, but the sequence is physical. pH correction has to precede the softener because it changes the hardness the softener sees. UV has to follow particulate removal because it cannot irradiate through turbidity. A single vessel cannot reorder that.
“Can I skip the test?” Every criterion above is a number. Orange staining and black staining look similar and need different media. Guessing produces an expensive system aimed at the wrong problem.
“My neighbour has one — can I get the same?” New Hampshire DES notes that wells within about 50 feet of each other “can present very different water quality.” Their result is not your result.
Who this is for
Anyone with a water test report and a purchase to make, on a private well or a municipal supply, sizing residential equipment for a single household.
When to buy nothing at all
Three cases where the correct answer is no purchase:
- Only the hot tap smells. Replace the anode rod.
- Hardness under 7 grains per gallon. Below the level Minnesota Health identifies, most households do not recover the running cost.
- You have not tested. Every criterion above is a number. Guessing at them produces an expensive system aimed at the wrong problem.
What certification does and does not tell you
Once you reach product comparison, one fact does most of the work: under NSF/ANSI 42 and 53, certification claims vary by product.
A system “certified to NSF/ANSI 53” is tested against the specific claims listed for that model, not against all fifty-plus contaminants in the standard. Ask which claim, not whether there is a badge.
→ What each standard covers → The sizing method and the treatment sequence → Point-of-use or whole-house? The exposure route decides → Well water, contaminant by contaminant → Reverse osmosis on city water: what NSF/ANSI 58 actually certifies → Pitcher, faucet mount, under-sink or fridge filter: what each can be certified for → What EPA says removes what