A whole house water filter goes on the main line after the meter or pressure tank and before the water heater, so every fixture in the building is fed from it. The job is mostly plumbing: isolate the line, cut in a bypass loop with shutoffs, mount the housing so the cartridge can be pulled out, then pressure test and flush before you trust it.
Choose the mounting point before you buy anything
The filter belongs on the incoming main, downstream of the meter on city water or downstream of the pressure tank on a well, and upstream of the water heater so hot and cold are both protected. Look for a straight run of pipe against a wall or joist that can carry the weight of a full housing, with clearance underneath to drop the sump and pull the cartridge out. Measure that clearance first: it is the single most common reason a filter ends up impossible to service. Outdoor or unheated locations need freeze protection, and a floor drain or a pan nearby saves a mess on every change.
Fit a bypass loop and shutoffs
Put a ball valve on the inlet side, a second on the outlet side, and a third across a bypass tee so the house can run with the filter isolated. That loop is what lets you change a cartridge without shutting off the whole building, and it is what saves you when a housing cracks on a weekend. Add a pressure gauge before and after the filter if the unit does not include ports for them: the difference between the two readings is how you know the cartridge is loading up rather than guessing from the calendar. Unions or push-fit connectors at the housing make future replacement of the whole unit a ten minute job.
Making the connection
Shut the main off and open a low fixture to drain the line, then cut the pipe cleanly and deburr it. Copper can be soldered or joined with press or push-fit fittings, PEX needs the right crimp or expansion tool, and threaded joints want PTFE tape or pipe dope on the male threads only. Mind the flow arrow on the filter head, because a housing fitted backwards will still pass water while filtering almost nothing. Support the housing on its bracket and lag it into solid framing rather than hanging it off the pipe, since the weight of a full sump plus water pressure will otherwise stress the joints.
Pressure test and flush before you walk away
Lubricate the sump O ring with silicone grease, seat it properly and hand tighten with the supplied wrench rather than overtightening. Open the outlet valve first, then crack the inlet slowly so the housing fills without a pressure spike, and watch the seal while the line comes up to pressure. Open the vent button to purge trapped air. Run a nearby cold tap until the water clears: carbon cartridges shed fine black dust when new and must be flushed until it disappears. Leave the area dry, come back in an hour, and check the housing and every new joint again.
Sizing and what to install with it
The filter has to pass the peak flow of the house without choking it, which is why manufacturers publish a flow rating and a capacity for each model. Undersize it and you will feel the pressure drop every time two showers run at once. Choose the cartridge for the actual problem: sediment for grit and cloudiness, carbon for chlorine, taste and odor, and a dedicated oxidizing filter if the real issue is iron. On well water, put the sediment stage first so it protects everything after it, and read our whole house water filter comparison before deciding between a cartridge unit and a backwashing tank.
Questions people ask about how to install a whole house water filter
Do I need a permit to install a whole house water filter?
Requirements are local rather than federal, so check with your city or county building department. Many areas treat a simple in-line filter as maintenance, while a backwashing unit with a drain connection often needs a permit.
Should the filter go before or after the water heater?
Before. Fitting it on the cold main upstream of the heater protects the tank from sediment and gives you filtered water at both hot and cold fixtures.
Can I install a whole house filter myself?
If you can cut and join the main line confidently and fit isolation valves, yes. Call a plumber where the main is buried, undersized, galvanized or too close to electrical service.
Why is the water pressure lower after installing the filter?
Either the cartridge is too fine or the unit is too small for the household flow. Compare the model's published flow rating against your peak demand, and check the inlet gauge against the outlet gauge.