Every treatment unit lists a flow rating, and it is easy to treat that number as the answer to whether the unit will keep up with your house. It is only part of the answer. Water pressure drops as it passes through a tank, valves, and media, and the amount of drop depends on how fast the water is moving at that moment. A unit that looks generous on paper can leave a shower feeling weak when two fixtures run at once.
What the flow rating does and does not tell you
The flow rating is usually given at a single test condition. It tells you how much water the unit can move under that condition, not how much pressure you will have left at the tap. Ask the seller what pressure the rating assumes and what the unit's pressure loss is at the flow you expect at peak use. Ask for the answer in writing, with the model name beside it.
Work out your peak, not your average
Before you ask the question, write down the most water your house uses at one time. Count the fixtures that might run together on a busy morning: a shower, a washer filling, a dishwasher, an outdoor hose, a kitchen tap. Use the rated flow for each if you know it, or a realistic estimate from your own experience. The number that matters is the peak, not the daily total.
Questions to ask before you commit
Put these questions to each provider and compare the answers side by side. What is the pressure drop at my peak flow, not only at the rated flow? Does the figure include the bypass valve and the connections, or only the tank? Will the inlet or outlet pipe size need to change to reach that flow? Is the pressure figure from the manufacturer's data sheet, and can I see that sheet?
If a provider cannot answer the first question, treat that as information. A careful installer should be able to show you the numbers and explain how they reached the recommendation.
Check the numbers on the quotes
When you compare quotes, look for the same peak flow, the same pressure figure, and the same model family. If one quote is sized for a higher flow than another, ask why. A larger unit can cost more to install and may not be needed for your house. A smaller unit can starve the fixtures you use most. Neither should be chosen on price alone.
Where to go next
If you need a starting list of providers, our directory of water treatment providers in Missouri shows the firms we cover, and our methodology page explains how each provider is reviewed. Use them to build a short list, then put the same questions to every firm on it.