Estimate whole-house filter demand from realistic simultaneous use, then compare it with both treatment-rated flow and pressure-loss data. A bathroom-count table is a rough marketing shortcut; it is not a substitute for the actual installation requirements.
Make three demand scenarios
List fixtures on the treated branch and use their documented or measured flow rates. Keep US gallons, Imperial gallons and liters distinct. For a simple volume-and-time measurement, see the bucket-test method .
| Illustrative scenario | Included draws | Total |
|---|---|---|
| Quiet use | One 1.5 GPM tap | 1.5 GPM |
| Normal busy period | Two 2 GPM showers plus a 1.5 GPM tap | 5.5 GPM |
| Planned overlap | The same draws plus a 3 GPM appliance fill | 8.5 GPM |
These are assumed inputs to demonstrate addition, not standard fixture ratings. Replace them with your own figures. An appliance fill can be intermittent; distinguish a brief overlap from sustained demand. Do not add outdoor irrigation if it is actually on an untreated branch.
Compare the full treatment path
At each scenario, ask whether the complete system can maintain its relevant treatment claim and leave enough pressure at the downstream fixtures. A maximum mechanical flow may exceed the claim-rated service flow. Several stages in series also add their losses at the same flow.
Record inlet pressure during demand, the complete-system loss and the downstream pressure requirement. If a seller provides only “up to 15 GPM,” request the conditions and a pressure-loss curve. A missing curve is missing evidence, not permission to assume no loss.
Include service demand
A backwashing unit can require a different flow from everyday household service. Confirm that the source can sustain that cycle and that the drain arrangement supports it. Increasing tank size can increase service requirements; it is not automatically a cure for weak supply.
An installer may need a formal fixture-unit or code-based design calculation for the project. This homeowner worksheet helps communicate real use; it does not replace that design or prescribe a universal 20% allowance.
Agree a measurable acceptance condition
Give the installer the chosen normal and peak scenarios. Ask for documented treatment performance, acceptable downstream delivery and a service plan under those conditions. If the existing house supply already fails the scenario, resolve that limitation before blaming or buying a filter.
Pentair’s housing data illustrate why flow is paired with pressure loss and exact model identity. The pressure-rating guide separates those hydraulic questions from the assembly’s maximum operating pressure.
Measure an individual outlet with the repeatable bucket-test example .