Sizing a Whole-House Filter: Flow Rate, Housing Size and Pressure Loss

An undersized whole-house filter is one of the most common and most avoidable installation mistakes. The symptom is familiar: water pressure was fine before the filter went in, and now the shower dribbles. The cause is almost always a mismatch between what the filter can pass and what the household actually demands. Getting this right means thinking about three things together — flow rate, housing size and pressure loss — before you buy anything.

Why flow rate is the starting point

Flow rate is the volume of water that passes through the filter per unit of time, usually expressed in litres per minute (L/min). Your household has a peak demand — the maximum flow it ever needs at one moment, such as two showers running while someone fills the kettle. That peak is what the filter must handle without choking.

A rough way to estimate peak household demand is to count the fixtures that could run simultaneously and apply a typical flow per fixture. A standard shower head runs at roughly 8–12 L/min; a kitchen tap at 6–9 L/min; a toilet cistern refill at 6–10 L/min. Add the two or three fixtures that realistically overlap in your home and you have a working peak figure. For most South African family homes this lands somewhere between 20 and 40 L/min, though larger properties with multiple bathrooms can exceed that — confirm with a licensed plumber before specifying.

The filter you choose must be rated to meet or exceed that peak figure at the inlet pressure available on your property. Manufacturers publish flow-rate curves, not just single numbers, so check the rating at your actual supply pressure, not at the optimistic maximum the packaging may show.

Housing size: what the numbers actually mean

Filter housings are sold by cartridge size, and cartridge size directly controls how much filter media water can pass through at any moment. The two common domestic sizes are:

• Standard (10-inch): suits lower-demand applications — a small flat, a single bathroom, or a point-of-entry filter feeding only part of the house.
• Big Blue (20-inch, large diameter): the correct starting point for whole-house use in a typical family home. The larger body holds more media, offers greater surface area, and produces less pressure drop at equivalent flow rates.

A 10-inch standard housing rated at 15 L/min will not become a 30 L/min filter by fitting a finer cartridge. If anything, a finer cartridge in an undersized housing restricts flow further. Housing size and cartridge rating must be matched to the demand calculation above.

For a useful breakdown of what different filter configurations cost once the sizing is sorted, the whole-house filtration installation cost guide covers the typical price ranges a South African homeowner should budget for.

Pressure loss: the number most people ignore

Every filter creates pressure drop — the difference between inlet and outlet pressure. A clean, correctly sized cartridge in good condition produces a small, acceptable drop. The problem arises in three situations:

1. Wrong housing size for the flow being asked of it.
2. Too fine a micron rating for the water quality being fed in (a 1-micron cartridge in silty municipal water blocks fast).
3. A cartridge that needs replacing — pressure drop increases as a filter loads up with captured particles.

Most installers recommend fitting a pressure gauge on both sides of the housing. When the differential rises by roughly 50–70 kPa above the clean baseline, the cartridge is due for replacement. Without gauges you are guessing, and the symptom — low pressure — arrives before most homeowners act.

Pipe diameter matters here too. A Big Blue housing fitted with 20 mm (¾-inch) pipe connections when your mains runs at 25 mm (1-inch) creates a restriction at the fittings regardless of how good the cartridge is. Connections should be matched to the existing pipe diameter, or you trade away pressure unnecessarily at the fittings themselves.

Understanding sediment filter micron ratings helps you choose a cartridge that does the job without loading up faster than necessary for your actual water quality.

If you want a properly sized system installed without the guesswork, compare 3 free quotes from vetted installers — no obligation.

Whole-house versus point-of-use: sizing implications

Sizing decisions change depending on what you are actually filtering. A whole-house filter treats every litre entering the building, so it must handle the full peak demand. A kitchen-only or under-sink filter serves one tap, so flow rate requirements are far lower and a small housing is adequate.

This distinction matters when budgeting and planning. Many homeowners find a layered approach works well: a correctly sized whole-house sediment or carbon filter at the mains entry, then a dedicated drinking-water filter at the kitchen. The whole-house unit does not need to achieve drinking-water purity; it protects appliances and plumbing, and takes the bulk load off the finer kitchen unit. The comparison between whole-house and kitchen filters explains the trade-offs in more detail, which is worth reading before committing to a single filtration approach.

For a broader overview of your filtration options and how different technologies fit together, the water filtration guide is a good place to orient yourself before getting specific about sizing.

What South African water conditions add to the equation

South African municipal water quality varies considerably by region, season and — frankly — by how much stress the local network is under. Sediment loads tend to spike after heavy rain or when ageing reticulation pipes disturb settled material. In areas served by older infrastructure, a 5-micron sediment pre-filter may block noticeably faster than manufacturer replacement schedules suggest, and sizing should account for more frequent cartridge changes or a larger housing that buys more time between services.

Drinking-water quality in South Africa is governed by SANS 241 (2024 edition). If your concern is compliance with that standard for drinking water, a whole-house sediment filter alone will not achieve it — you need appropriate treatment matched to the specific contaminants present in your supply, confirmed by a water test rather than assumptions.

Household size	Typical peak demand	Recommended minimum housing
1–2 people, 1 bathroom	15–20 L/min	Standard 10-inch
3–4 people, 2 bathrooms	25–35 L/min	Big Blue 20-inch
5+ people or 3+ bathrooms	35–50+ L/min	Twin Big Blue or consult installer

These are rough guides only. Have a licensed plumber confirm peak demand and inlet pressure before specifying.

Sizing depends on your specific supply pressure and pipe layout — a local installer can measure both in minutes. Get 3 free quotes and find the right fit for your home.

The installation factors that affect performance

Even a correctly sized filter can underperform if the installation is careless. Key points to confirm with your installer:

• Filter location: as close to the mains entry as practical, before any hot-water cylinder or irrigation feed, so the whole house benefits.
• Bypass valve: a bypass allows cartridge changes without shutting down the house — non-negotiable for a practical installation.
• Orientation: most housings are vertical with the cartridge hanging downward; horizontal installation may void cartridge ratings and traps air.
• Pressure testing: the installer should test inlet and outlet pressure after commissioning, not just check for leaks.

Getting these details right at installation is far cheaper than correcting them later, particularly if pipe modifications are required after walls or cabinets have been closed up.