Many homeowners buy a filter or softener for a problem that comes from a faucet, a supply line, a water heater or a shut-off valve. The water itself may be fine. Equipment only fixes a water problem, so the first job is deciding which kind of problem you have.
Start with where the symptom shows up
Write down exactly where you see the problem. The pattern is the first clue.
If it shows up at every tap, in hot and cold water, the source of the water is the likely cause. If it shows up only at one fixture, the cause is probably local, such as a clogged aerator, a worn cartridge or a dirty supply line to that fixture. If it shows up only on the hot side, look at the water heater and the hot line before you look at the water supply. If it shows up only outdoors, check the hose bib and any line that runs to it.
Run four checks before you call anyone
1. Compare hot and cold at the same tap. Run each for a minute and note the difference. A problem on one side only points away from the source water.
2. Test several taps. Include one near the point where the water enters the house and one at the far end. A problem that is strongest at one end may come from a line or a fixture rather than the supply.
3. Inspect the fixture. Unscrew the aerator at the tip of the faucet and look for grit, flakes or a colored film. Check the screen and any cartridge. Debris that comes out of the fixture is a plumbing or fixture problem, and a filter upstream will not fix it.
4. Note what changed. Write down when the problem started and what happened around that time, such as work on the street, a utility notice, a new water heater, a new appliance or a change in your softener's schedule. A clear timeline is often the fastest way to find the source.
Signs that point to the water itself
A problem in the water usually appears at all taps, returns after you clean the fixtures, and leaves deposits on glass, dishes or sinks. Cloudy water deserves a simple glass test. Fill a clear glass and set it on the counter. If the cloudiness settles to the bottom, it is likely fine particles. If the water clears from the top down or the bubbles rise, it is probably air in the line, which is common after a supply disruption and usually clears on its own.
Signs that point to plumbing or appliances
A problem tied to one fixture, one side of the house, a single appliance, or a sudden change after work on the lines usually has a mechanical cause. Rust-colored hot water with cold water clear points toward the water heater. Debris that appears only after a fixture or appliance is installed points toward that install. Noise, weak flow or pulsing at one tap points toward a restriction or a valve, not the water quality.
Gather evidence before you talk to a provider
If you want a professional opinion, bring the facts. Note which taps are affected, hot or cold, and when the problem appears. Take clear photos of stains, deposits or debris. If you are on a public water system, ask the utility whether there was work, a notice or a disruption in your area, and request its most recent water quality report. If you have a private well, a state-certified laboratory can run a test panel on a sample you collect with its instructions. Label every sample with the tap, the date and the time.
Questions to ask before a test or quote
Ask the provider which taps they will test, and whether the test covers the specific thing you see. Ask them to show you the test result rather than only describing it. Ask how they will separate a fixture or plumbing problem from a water quality problem, and get that answer in writing. Ask whether a diagnostic visit is charged separately and whether it is credited toward equipment if you buy. Ask who is responsible for any plumbing work the diagnosis requires. When comparing firms, the award methodology page explains how winners are chosen, which can help you judge what the listing does and does not tell you.
When to stop and get help right away
If you smell gas, leave the house and call your gas utility. If water is leaking into a wall, ceiling or electrical area, shut off the supply and call a plumber. If anyone in the household has symptoms after drinking or using the water, contact your doctor, and if the water comes from a public system, notify the utility. This guide cannot tell you whether the water is safe for a health condition, a pregnancy or an infant's formula, so those decisions belong with your doctor and the local health department.
Decide the source before you buy the fix
Once you know whether the problem is in the water, at a fixture or in an appliance, the next decision is much easier. Equipment should follow a confirmed diagnosis. A filter that treats the water will not repair a clogged valve, and a repaired fixture will not remove dissolved minerals. Match the fix to the source you have found.