# Water filter system

> A water filter system is a staged train of housings, not one cartridge. See the point-of-entry and point-of-use formats, sizing, and installed costs.

Canonical: https://watertreatmentpro.com/water-filter-system/

Last updated: 2026-11-09

A water filter system is a staged train of housings rather than a single cartridge. The stages run in a deliberate order, each protecting the next, and the housing size is set by the flow rate it must pass. Get the order right and the system is cheap to run; get it wrong and you replace cartridges monthly.

## Two Formats

**Point-of-entry.** A whole-house system installed on the main line after the meter or pressure tank. It treats every fixture: showers, laundry, dishwasher, water heater. Housings are large because they must pass 8 to 15 gallons per minute.

**Point-of-use.** A system at a single tap, usually the kitchen, or an under-sink unit feeding a dedicated faucet. Flow is 1 to 2 gpm, housings are small, and the treatment is more targeted.

Most homes benefit from both: point-of-entry carbon for chlorine and sediment, and a point-of-use RO for drinking water if dissolved contaminants are present.

| Format | Flow | Housings | Installed cost |
|---|---|---|---|
| Whole house, 2 stage | 8 - 15 gpm | 4.5 x 10 or 4.5 x 20 | $500 - $1,800 |
| Whole house, 3 stage | 8 - 15 gpm | Mixed | $900 - $2,500 |
| Under-sink carbon | 1 - 2 gpm | 2.5 x 10 | $150 - $500 |
| Under-sink RO | 50 - 75 gpd | 4 - 5 stages | $300 - $900 |

## Stage Order

The sequence follows one rule: remove what would damage or clog the next stage.

1. **Sediment.** 20 or 10 micron first, to take the coarse load. Protects the carbon and the UV sleeve if one is fitted.
2. **Carbon.** Removes chlorine, taste, and odour. Protects resin, membranes, and any downstream media from oxidants.
3. **Specialty media.** Iron, manganese, arsenic, or chloramine reduction, if the lab report calls for it.
4. **Polishing.** A final carbon block for taste, or a UV stage if bacteria are a concern.

Putting carbon before sediment is a common error. The carbon block loads with grit and blinds in weeks, while the sediment cartridge behind it stays clean and useless.

## Sizing by Flow

A filter system must pass the house's peak flow without excessive pressure drop. A 2.5 inch housing is rated for 4 to 6 gpm. A whole-house system needs 8 to 15 gpm. That is why whole-house systems use 4.5 inch housings.

| Housing | Cartridge | Practical flow | Pressure drop |
|---|---|---|---|
| 2.5 x 10 | Standard | 4 - 6 gpm | 2 - 5 psi |
| 4.5 x 10 | Big blue | 8 - 12 gpm | 1 - 3 psi |
| 4.5 x 20 | Jumbo | 12 - 20 gpm | 1 - 3 psi |

Undersizing shows up as weak showers and slow fixture fill, covered in [water pressure and filters](https://watertreatmentpro.com/water-pressure-and-filters/). The fix is a larger housing, not a coarser cartridge.

## What It Costs to Own

Two-stage whole-house system: $700 installed, sediment cartridges at $20 three times a year equals $60, carbon blocks at $40 twice a year equals $80. Annual total $140.

Over ten years: $700 + ($140 × 10) = $2,100, or $210 per year. That is the arithmetic for a mid-range system, and it is an estimate built on published ranges.

Under-sink carbon: $300 installed plus $80 per year in cartridges is $1,100 over ten years, or $110 per year, for one tap.

## What a Filter System Cannot Do

It cannot soften. Carbon and sediment filtration leave calcium and magnesium in the water, so scale continues to form in the heater, the dishwasher, and the fixtures. If hardness is above 7 GPG, a filter system is only part of the answer and a softener is the other part.

It cannot remove nitrate. That needs reverse osmosis or a dedicated nitrate-selective resin.

It cannot reliably remove bacteria. Carbon is not a microbiological barrier; UV is the tool for that.

There is also a category error to avoid. A filter system is sometimes sold as a "whole house water treatment system" when it is only filtration. If the water test shows hardness at 14 GPG, iron at 4 ppm, and a sulfur odour, a two-stage carbon system addresses none of the three main problems. The stages have to match the report, and the report has to come first.

| Goal | Filter system alone? | Needs |
|---|---|---|
| Chlorine taste | Yes | Carbon |
| Sediment | Yes | Sediment stage |
| Hardness | No | Softener |
| Nitrate | No | RO |
| Bacteria | No | UV |
| Iron above 3 ppm | No | Iron filter |

## Five Steps to Specify One

1. **Test the water** with a $40 to $150 lab panel; the stages follow the results.
2. **Measure peak flow** by counting simultaneous fixtures.
3. **Choose housing size** from the flow, not from the cartridge price.
4. **Set the stage order** as sediment, carbon, specialty, polish.
5. **Install a bypass** so cartridges can be changed without shutting the house down.

## Maintenance That Keeps It Working

Sediment every 3 to 6 months, carbon every 6 to 12 months. Mark the date on the housing with a marker; it is the only maintenance record most systems ever have.

If pressure drops before the interval is up, the cartridge is loading faster than expected, which means the upstream sediment stage is undersized or missing. Fix the cause rather than changing cartridges more often.

For the whole-house version in detail, see [whole house water filtration](https://watertreatmentpro.com/whole-house-water-filtration-filter/) and [how a whole house filter works](https://watertreatmentpro.com/how-does-a-whole-house-filter-work/). If the test shows hardness as well, [water softener systems](https://watertreatmentpro.com/water-softener-systems/) covers the equipment that addresses it.

A final note on buying. The housings, brackets, and wrenches are commodity parts; the cartridge is the product. Choose a housing that takes a standard size, then choose a certified cartridge from a manufacturer that publishes its certification listing. A system is only as good as the cartridge inside it, and the housing is just a pressure vessel that holds it.