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.
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 → Well water, contaminant by contaminant