Pressure and flow are not the same thing
Pressure is the force behind the water, measured in bar. Flow is how much water actually arrives, measured in litres per minute.
You can have good pressure and poor flow if a pipe is undersized or partly blocked. You can have adequate flow and low pressure on a gravity system fed from a loft tank.
Showers need both. A thermostatic valve needs enough pressure to hold temperature, and a rain head needs enough flow to feel like anything.
Test flow yourself: run a tap fully open into a bucket for exactly thirty seconds, then double the litres collected. That gives litres per minute.
Under 6 litres per minute at the bathroom hot tap is poor. Ten to twelve is comfortable. Fifteen or more is excellent.
Common causes, cheapest first
Blocked aerator or shower head. Limescale in hard water areas restricts outlets dramatically and takes ten minutes to clear.
Partly closed isolation or stopcock valve, often left half shut after previous work.
Undersized or old pipework, particularly 10 mm runs to an upstairs bathroom or original galvanised pipe in older properties.
Gravity-fed system with insufficient head height, which is common in bungalows and loft conversions where the tank is barely above the outlet.
Combi boiler at its flow limit, especially when two outlets run at once.
Mains supply that is genuinely weak, which some streets simply have.
The fixes and what they cost
Ranges below are supply and fit, including VAT, for typical installations in our area.
| Fix | Typical cost | Suits | Realistic improvement |
|---|---|---|---|
| Descale head and aerators | £0-£60 | Hard water areas | Often dramatic |
| Open or replace isolation valves | £80-£180 | Any system | Full flow restored |
| Fit a shower pump (twin impeller) | £500-£1,100 | Gravity-fed only | Roughly doubles flow |
| Upgrade 10 mm pipe runs to 15 mm | £300-£900 | Any system | Significant on long runs |
| Fit an electric shower | £280-£650 | Any system | Consistent but modest flow |
| Mains booster / accumulator | £900-£2,200 | Weak mains supply | Large and sustained |
| Convert to unvented cylinder | £2,200-£4,000 | Gravity-fed homes | Transformational |
| Replace combi with higher output | £2,500-£4,500 | Combi at its limit | Better hot flow only |
Things that make it worse
Fitting a huge rain head to a system that cannot feed it. The larger the head, the more flow it needs to feel adequate.
Adding a pump to a mains-pressure system, which is not permitted and can damage a combi boiler.
Restricting a shower valve with excessive flow limiters left in from the factory. These can often be removed where the system supports it.
Long pipe runs with many bends. Every elbow costs a little flow, and a route taken for convenience during a previous job can cost a lot in total.
What we check at survey
Static pressure at an outside tap or the kitchen cold tap, which tells us the incoming mains condition.
Measured flow rate at the bathroom hot and cold outlets, using the bucket test rather than guessing.
System type, cylinder condition, tank height and pipe sizes to the bathroom.
Simultaneous demand: what happens to the shower when someone runs the kitchen tap.
We do all of this free of charge as part of the no-obligation survey, and we will tell you if the answer is a £40 descale rather than a job for us.
Frequently asked questions
How do I test my water flow rate?
Run the tap fully open into a bucket for thirty seconds and double the litres collected. Under 6 litres per minute is poor, 10 to 12 is comfortable.
Can I fit a pump to boost my shower?
Only on a gravity-fed system with a cold tank. Pumping a combi or mains-pressure supply is not permitted and can cause damage.
Why does my shower go cold when a tap runs?
Usually a non-thermostatic valve on a system at its flow limit. A thermostatic valve holds temperature, though it cannot create flow that is not there.
Is an unvented cylinder worth the cost?
In a gravity-fed house where several people shower, it is often the single biggest improvement available. It gives mains pressure at every outlet.
Will you tell me if I do not need work doing?
Yes. A fair number of pressure callouts are solved by descaling or opening a valve, and we would rather say so than sell an unnecessary job.