Every water treatment quote names a unit, and the unit name tells you very little on its own. What decides whether the equipment works in your home is the sizing behind it: how much water your household uses, how hard or contaminated the supply is, and how many faucets and fixtures might run at the same time. If the sizing is wrong, a good unit can still run out early, let hard water through between service visits, or cost more than it needs to. Before you compare two bids, ask each company to show you the numbers it used.
Ask what inputs the sizing used
Start with the questions that feed the calculation. Ask which water test results they relied on, and whether those results came from a lab report, a field test kit, or the company's own spot check. Ask whether the household size and daily water use were measured, estimated from a questionnaire, or simply assumed. Ask how many bathrooms, water heaters, and outdoor or appliance connections they counted. A company that cannot name its inputs is guessing, and you should know that before you sign.
Get the sizing written on the quote
A sound quote lists the key sizing figures in plain terms: the capacity the unit is rated for, the flow rate it can deliver at the tap, and how often it is expected to regenerate or change media based on your inputs. Read those lines the way you would read any spec. If the quote gives only a model name and a total price, ask for the worksheet or the figures behind it. Keep a copy with your other paperwork so the numbers are on record if the equipment is later questioned.
Ask what changes the answer
Households grow, appliances get added, and water use shifts with the seasons. Ask the company what would make the sizing wrong: a new bathroom, more occupants, a pool fill, a change in the water source, or a new test result that shows higher hardness or a different contaminant. Ask whether they would revisit the sizing at no charge if your use turns out to be different from what they assumed. Their answer tells you whether the sizing is a starting estimate or a commitment.
Understand the two ways sizing goes wrong
An undersized unit runs out between regenerations or cartridge changes, so hard water or untreated water reaches your taps at the worst times, usually peak mornings and evenings. An oversized unit costs more up front and can use more space, salt, or media than your home needs. Neither outcome is automatic, but each one is worth asking about directly. Ask the company which risk they are guarding against with the size they recommend, and why they chose that side.
Compare bids by their sizing, not just their price
When you line up two or three quotes, put the sizing inputs side by side before you look at the totals. If one company used a higher daily water use figure, a different test result, or a different peak flow, the bids are not measuring the same job, and a lower price may just mean a smaller system for your actual home. If the inputs match and the outputs differ, ask why. You can also check whether each company's approach to sizing is described on its own pages and on the award winners list at Best Water Companies STL, which is a reasonable place to begin your shortlist before you ask each company for its worksheet.