Use this checklist to prepare a useful fault report before replacing a whole-house filter. Weak flow is an observation, not a diagnosis. A scored symptom quiz cannot reliably choose between a cartridge, valve, supply or well fault, especially when several problems coexist.
Capture the pattern without changing the system
Record affected outlets, hot/cold differences, when the problem began and the last maintenance or plumbing event. Compare outlets known to share the treated route. An outdoor tap on a bypassed branch is not a like-for-like comparison with an indoor treated tap.
| Observation | What it helps narrow | What remains uncertain |
|---|---|---|
| One outlet weak, comparable outlets normal | Local fixture or branch investigation | Exact component or cause |
| Only hot side affected | Hot-water route and equipment investigation | Whether an upstream issue also contributes |
| Weakness begins after cartridge work | Installed part, seating and valve-route check | Whether the new part is actually restrictive |
| Several outlets weaken during demand | Shared supply or treatment-path investigation | Location and number of restrictions |
| Well pressure behaves unusually | Well-system service assessment | Pump, controls, tank or source diagnosis |
Do not alter a well pressure switch, regulator or heater control just because the table points toward that part of the system.
Make paired readings comparable
Where suitable gauges are already fitted, record inlet and outlet pressure at the same time and under a repeatable flow condition. Note exactly which components lie between the gauges. Compare with the clean-system record if available.
An increased difference supports investigating that measured section. Both readings falling can indicate an upstream limitation, but does not prove there is no additional downstream restriction. Instrument error and a changed draw can also distort comparisons; see the gauge-accuracy example .
Treat bypass testing as a controlled service decision
A bypass comparison can change both treatment and hydraulic routing. It should only be performed when the route, operating procedure and permitted untreated-water uses are understood. A short test is not automatically safe. Do not use a health-protection bypass as a way to keep normal household consumption going while waiting for parts.
If bypass testing is appropriate, record which stages were excluded and whether the draw conditions were comparable. Improvement implicates the bypassed path; it does not identify one cartridge by itself. Return to the required treatment route and follow any specified restart procedure.
Send a repair brief
Give the installer model and cartridge codes, dated symptoms, affected outlets, recent changes, readings with locations and flow conditions, and any alarms or leaks. State the treatment duty that must remain protected. Request diagnosis before a larger housing, coarser cartridge or parallel arrangement is proposed.
For planned restriction reduction after the fault is identified, use the pressure-drop guide . For a changed well supply, EPA’s well guidance covers when testing needs attention. This article organizes evidence for repair; it does not establish a failed part remotely.
Take the recorded symptoms into the matched-flow pressure diagnostic guide .