Before You Buy Any Water Filter
Please read this first. All water filters have specific requirements that manufacturers don't always make clear. Buying the wrong system for your home can mean wasted money and poor performance.
This applies whether you're buying from Amazon, a UK retailer, or anywhere else.
#1: Check Your Water Pressure First
⚠️ Critical for RO Systems
Reverse Osmosis (RO) systems require a minimum of 40 PSI (2.8 bar) water pressure, but operate best between 60-80 PSI (4.1-5.5 bar).
If your water pressure is below this threshold, the RO membrane cannot push water through properly. You'll get:
- ✗Very slow water flow (a trickle instead of a stream)
- ✗Poor contaminant removal (the membrane needs pressure to filter effectively)
- ✗Excessive water waste (more goes down the drain)
- ✗Premature membrane failure
💡 Solution: If your pressure is low, you'll need to install a booster pumpjust before the RO system. A good-quality RO booster pump costs between £60-120.
How to Check Your Water Pressure
Buy a pressure gauge (£10-30)
Look for one that reads 0-11 bar (0-160 PSI) and screws onto a standard garden tap or washing machine connection.
Connect to your cold water supply
Attach the gauge to an outside tap or the cold water supply under your kitchen sink. Make sure no other taps are running.
Read the pressure
Turn on the tap fully. The gauge will show your static pressure in PSI or bar.
| Pressure | PSI | Bar | Suitable For |
|---|---|---|---|
| Low | Below 40 | Below 2.8 | Carbon filters only. RO needs booster pump. |
| Marginal | 40-50 | 2.8-3.5 | RO will work but may be slow. Consider pump. |
| Good | 50-80 | 3.5-5.5 | All filter types work well. |
| High | Above 80 | Above 5.5 | May need pressure reducer to protect filters. |
UK Average: Most UK mains water is between 1-4 bar (14-58 PSI). Flats and top-floor properties often have lower pressure. Properties with gravity-fed systems (tank in loft) typically have very low pressure.
#2: Understand RO Waste Water (Purge)
All RO systems produce waste water. This is not a fault — it's how the technology works. The RO membrane filters water by pushing it through microscopic pores. Contaminants that can't pass through are flushed away with waste water (also called "purge" or "reject" water).
How It Works:
The RO membrane has a flow-through rate — the speed at which it can filter water. This rate is always slower than your incoming water pressure. The unfiltered water waiting to be processed goes down the drain. This is the only way the system can work effectively.
Traditional RO Systems
Waste ratio: 3:1 to 5:1
3-5 litres wasted per 1 litre filtered
High-Efficiency RO
Waste ratio: 1:1 to 2:1
1-2 litres wasted per 1 litre filtered
✅ Good News: Modern Systems Are Efficient
The high-efficiency systems we recommend (like the Waterdrop G3P600) have a1:1 waste ratio — meaning they waste the same amount as they filter. This is a huge improvement over older systems. The annual cost of waste water is typically only £15-20.
🔧 Already Have an RO System?
If you have an older RO system with high waste, you can retrofit a permeate pumpto reduce waste by up to 80%. No electricity required — it uses hydraulic pressure.
💧 Use the Waste Water
The waste water isn't dirty — it just has concentrated minerals. Collect it for watering plants, cleaning, or flushing toilets. Some people plumb it into their garden irrigation system.
#3: Installation Requirements
Under-Sink Space
RO systems need space under your sink — typically 35-45cm wide and 40-50cm tall. Measure your cupboard before buying. Some compact/tankless models are smaller.
Drain Connection
RO systems need a drain connection for waste water. Most kits include a saddle valve that clamps onto your existing drain pipe. This is a simple DIY job.
Separate Tap
Most RO systems come with a dedicated tap for filtered water. You'll need to drill a hole in your sink or worktop (usually 12mm). Some premium systems can connect to your existing mixer tap.
Electricity (Some Models)
Tankless RO systems and those with booster pumps need a power outlet under the sink. Traditional tank-based systems don't need electricity.
Pre-Purchase Checklist
Don't Fall for Vague Marketing Claims
"50% better filtration!" "Removes impurities!" "Good for hard water!" — these claims sound impressive but often mean nothing. Before you buy any filter, learn what questions to ask.
Read: How to Spot Vague Water Filter Marketing →Tip: Print our Red Flags Checklist and take it to the store when you shop.
Why Switch from Bottled Water?
The UK buys 7 billion plastic bottles every year. A home filter saves you money AND helps the planet.Read the full story →
Ready to find the right filter for your home?