When choosing a water filtration system, "pressure" is only half the story. To ensure your filters don't restrict your water supply or fail to clean the water properly, you need to know your Flow Rate.
In this guide, we’ll explain how pipe size and water velocity determine your flow, and how you can estimate yours at home.
1. The Relationship: Pressure vs. Flow
Think of water in a pipe like a crowd of people in a hallway.
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Pressure is how hard they are being pushed from behind.
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Pipe Diameter is the width of the hallway.
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Velocity is how fast they are walking.
If you have high pressure but a tiny 15mm pipe, you can only get so much water through before it becomes noisy and turbulent. For most domestic and light commercial systems, we aim for a "Goldilocks" velocity of 2.0 meters per second (m/s)—fast enough to be efficient, but slow enough to protect your plumbing.
Estimated Flow Rates by Pipe Size & Velocity
Use this chart to see what your pipe is "rated" for at common design velocities. 2.0 m/s is the industry recommended maximum for cold water.
| Pipe Diameter (mm) | 1.0 m/s (Quiet) | 1.5 m/s (Standard) | 2.0 m/s (Max Rec.) |
| 15 mm | 10.6 L/min | 15.9 L/min | 21.2 L/min |
| 22 mm | 22.8 L/min | 34.2 L/min | 45.6 L/min |
| 25 mm | 29.5 L/min | 44.2 L/min | 58.9 L/min |
| 28 mm | 36.9 L/min | 55.4 L/min | 73.9 L/min |
| 32 mm | 48.3 L/min | 72.4 L/min | 96.5 L/min |
| 50 mm | 117.8 L/min | 176.7 L/min | 235.6 L/min |
2. What is Water Hammer?
If you design a system where water travels faster than 2.0 m/s, you risk Water Hammer.
When you suddenly turn off a tap, that fast-moving water has a lot of momentum. It slams into the closed valve, creating a shockwave that travels back through the pipes. This sounds like a loud "thud" or "clash." Over time, water hammer can burst joints, damage expensive filtration membranes, and ruin solenoid valves.
3. Increasing or Decreasing Pipe Diameters
Selecting the correct pipe size is a true "Goldilocks" challenge in engineering—too small, and the system becomes constrained; too large, and you risk unnecessary costs while introducing new complications. Here’s what can go wrong when the balance isn’t right:
Decreasing Diameter Too Much (The "Choke")
When you squeeze a fluid through a pipe that is too narrow, you're essentially fighting physics.
- Significant Pressure Drop: The primary concern is a substantial loss in pressure.The Darcy-Weisbach equation shows that pressure loss due to friction is inversely proportional to the fifth power of the pipe's diameter (ΔP ∝ 1/D⁵). This means even a marginal reduction in pipe size results in a dramatic increase in pressure loss.
- Increased Velocity and Erosion: To maintain the same flow rate through a smaller pipe, the fluid velocity must increase. This high-speed flow can erode the pipe walls, particularly at elbows and bends, much like sandpaper wearing down a surface.
- Excessive Noise and Vibration: The rapid fluid movement can cause "singing" or whistling sounds in the pipes. This also leads to mechanical vibrations that can stress the system and loosen fittings over time.
- Greater Pump Strain: The pump must work harder to push fluid through the increased resistance of a smaller pipe. This not only increases energy consumption but also accelerates wear and tear, leading to premature pump failure.
- Risk of Cavitation: If high velocity causes the pressure to drop below the fluid's vapor point, bubbles can form. These bubbles then collapse violently—a phenomenon known as cavitation—which can pit metal surfaces and severely damage components like pump impellers.
Increasing Diameter Too Much (The "Slowdown")
While oversizing may seem like a safer option, it brings its own set of technical and financial challenges.
- Sedimentation: Oversized pipes cause fluid velocity to drop significantly. In systems carrying solids—such as wastewater or slurry—this reduced flow can prevent particles from staying suspended, leading to sediment buildup and eventual blockages.
- Stagnation and Bacterial Growth: In potable water systems, low flow creates "dead legs," or areas of stagnant water. This stagnation reduces chlorine residuals, encouraging biofilm formation and bacterial growth, including Legionella.
- Thermal Loss: Larger pipes have increased surface area, resulting in greater heat loss when transporting hot water or steam. This inefficiency reduces the temperature of the fluid before it reaches its destination.
- Cost Inefficiency: Oversized pipes increase costs beyond the initial purchase. They require larger valves, more insulation, heavier supports, and additional space in trenches or buildings.
- Air Pockets: Low flow in oversized pipes can fail to clear air bubbles from high points in the system, leading to air locks that may block flow entirely. In short, bigger isn't always better—proper sizing is essential for efficiency and system functionality.
Summary Comparison Table
|
Feature |
Pipe Too Small |
Pipe Too Large |
|
Pressure |
High loss / Low delivery pressure |
Minimal loss |
|
Velocity |
Excessively high (noisy/erosive) |
Too low (sedimentation risk) |
|
Energy Use |
High (pump works harder) |
Low (but initial cost is high) |
|
Maintenance |
Frequent repairs from wear/leaks |
Issues with cleaning and bacteria |
4. The Impact of Distance and Bends (Pressure Drop)
Your "Incoming Pressure" is not what you get at the tap. Every meter of pipe and every elbow "steals" a bit of pressure through friction.
Maximum Pipe Length to Maintain 2.0 m/s Velocity
If your pipe run is too long, the friction will eventually "eat" your pressure, and your flow rate will drop.
| Pipe Diameter | 2 Bar | 3 Bar | 4 Bar | 5 Bar | 6 Bar |
| 15 mm | 78 m | 117 m | 156 m | 195 m | 234 m |
| 22 mm | 131 m | 197 m | 263 m | 329 m | 394 m |
| 28 mm | 179 m | 269 m | 358 m | 448 m | 538 m |
Pro Tip: Every 90° bend acts like an extra 0.5m to 1.5m of pipe. A dirty filter can add the equivalent of 10m to 20m of pipe friction! Always size your filtration equipment one step larger than you think you need to account for this "pressure tax."
5. The "Bucket Test": How to find your flow rate
The "bucket and stopwatch" test is a great baseline, but it typically measures the fixture’s limit, not the home's total incoming capacity.
Here’s why it can be misleading:
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The Bottleneck Effect: You might have a high-capacity main line, but a tiny aerator or a partially closed valve at the sink will "choke" the reading, giving you a false low.
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Friction Loss: Testing a tap far from the water meter accounts for the resistance of all the internal piping, which reduces the flow rate before it even reaches your bucket.
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Fixture vs. System: It tells you how fast that specific tap fills, but it doesn't account for "residual pressure"—how much the flow drops when a second person showers.
Pro-Tip for Accuracy:
To get the truest reading of your incoming water, perform the test at the hose bib (outdoor faucet) closest to your water meter and ensure no other appliances are running.
1. What You Need
- A 10-Litre Bucket: (Standard size at most UK DIY stores like B&Q or Wickes).
- A Stopwatch: (Use the timer on your phone).
- Access to the "First" Tap: Use the outdoor bib tap closest to the water meter or the kitchen cold tap (remove the plastic aerator/nozzle if possible).
2. The Step-by-Step
- Stop All Water: Ensure the washing machine, dishwasher, and any showers are OFF.
- Full Flow: Turn the tap on to its maximum flow.
- Time the Fill: Start the timer the moment the water hits the bucket and stop it exactly when the water reaches the 10L mark.
- Check the Results: Use the chart below to see how your home compares.
Flow Rate Cheat Sheet
Based on filling a 10-litre bucket:
|
Time to Fill 10 Litres |
Flow Rate (LPM) |
Performance Level |
|
30 Seconds |
20 LPM |
Excellent (Great for Rain Showers) |
|
40 Seconds |
15 LPM |
Good (Standard UK Performance) |
|
60 Seconds |
10 LPM |
Average/Minimum (Standard Requirement) |
|
Over 100 Seconds |
< 6 LPM |
Poor (Will struggle with multiple taps) |
The Calculation (For any bucket size)
If your client has a different-sized container (like a 12L bucket), they can use this simple formula:
