The shower dribbles. The kitchen tap takes an age to fill the kettle. Upstairs, the basin barely trickles while someone runs a bath. Low water pressure is irritating, and the cause can be anything from a scaled-up shower head to a leaking pipe under the front garden. The trick is working out which before you start replacing things that weren't broken.
Start by being clear about what you're measuring.
Pressure and flow aren't the same thing
Pressure is how hard the water is being pushed, measured in bar. Flow is how much water actually comes out, measured in litres per minute. They're related, but they're not the same. A narrow, furred-up pipe can have perfectly good pressure behind it and still deliver a feeble flow. A tap on a gravity system can give a decent flow with very little pressure.
Two simple tests:
- Flow: time how long it takes to fill a bucket of known size, say 10 litres, from the kitchen cold tap fully open. Divide 10 by the number of seconds and multiply by 60 for litres per minute. Write it down.
- Pressure: a screw-on pressure gauge fitted to an outside tap or a washing machine valve reads static mains pressure when nothing else is running. They're cheap from plumbing suppliers. Read it with everything off, then with a tap running elsewhere in the house. A big drop as soon as one tap opens points to restriction in the pipes rather than a weak supply.
Do both tests at a quiet time and at a busy time, and keep the numbers. They make every later step easier, and they're the first thing a plumber or water supplier will ask for.
Narrow it down first
Before going through the causes, answer three questions.
Is it the whole house or one tap?
If one tap or shower is weak and everything else is fine, the problem is local: an aerator, a shower head, an isolation valve, a kinked hose or a tired mixer cartridge. If everything is weak, the problem is upstream: the stopcock, the supply pipe, a pressure-reducing valve or the mains itself.
Is it hot only, cold only, or both?
Weak hot with good cold points at the hot water source: the combi, the cylinder, or the pipework from it. Weak cold with good hot is less common on a combi, which is fed from the same cold main, and usually means a restriction on a cold-only branch such as a part-closed valve or a blocked filter.
Is it all the time, or only sometimes?
Pressure that drops at half seven in the morning and recovers by mid-morning suggests demand, not a fault. Pressure that has slowly got worse over months suggests scale or corrosion. Pressure that fell off a cliff suggests a valve, a leak, or work in the street.
The usual causes
The stopcock isn't fully open
Embarrassingly common, especially after plumbing work, a new kitchen or a house move. Someone turned it down and never turned it back up. Find the internal stopcock, usually under the kitchen sink, and turn it anticlockwise as far as it will go without force. Don't wrench a stiff one; old stopcocks seize and can snap. If there's an outside stop valve as well, find out whether anyone has touched it recently.
Aerators and shower heads full of limescale
In hard water areas, the little mesh aerator on the end of the tap and the holes in the shower head clog with scale and grit. Unscrew the aerator (some need the small key that came with the tap), soak it and the shower head in white vinegar or a citric acid solution for an hour or two, rinse and refit. Grit in the aerator straight after work on the mains is common too. If limescale is a running theme in your house, our guide to hard water and limescale covers prevention.
A failing or badly set pressure-reducing valve
Some properties have a pressure-reducing valve (PRV) on the incoming main to protect fittings from high street pressure. It's a brass valve, often near the stopcock or the meter, usually with an adjusting screw or knob on top and sometimes a small gauge. As they age they can stick or drift. If your gauge reads low with everything off and you have a PRV, it's the first suspect. Adjusting or replacing it is a plumber's job.
Old galvanised steel or narrow lead pipes
Galvanised steel pipe rusts from the inside, and the bore slowly closes up. Old lead supply pipes are often small in diameter to begin with. Both tend to give reasonable static pressure on the gauge and poor flow the moment a tap opens. The fix is replacing the pipe. With lead, that's worth doing for health reasons anyway.
A leak on the supply pipe
Water escaping between the street and your house reduces what's left for your taps, and it can go unnoticed for months. Clues include pressure that has fallen gradually, a damp patch or suspiciously green strip in the garden, and a meter that moves when nothing is in use. A water meter leak test will tell you whether you have one, and the leak detection hub covers what to do next.
Peak-time demand
In the morning and early evening the whole street showers, cooks and runs washing machines at once, and pressure at the end of the network sags. Homes at the top of a hill or at the far end of a long main feel it most. Check your pressure late at night and again at breakfast time. If it's fine at night and poor at peak, and your own pipework is in good order, the limit is the supply. Many water suppliers work to a minimum standard for pressure and flow at the boundary, but what that standard is, and how it's enforced, depends on the country and the supplier. Ask them. Some will come and measure it.
Combi boiler flow-rate limits
A combi heats water as it passes through, so there's a ceiling on how much hot water it can deliver at a useful temperature. That ceiling depends on the boiler's output and how cold the incoming water is. In winter, with colder mains water, the flow you get at shower temperature drops. Run a shower and the kitchen hot tap together and both suffer. That's the nature of the boiler type, not a fault.
Real faults do happen. A blocked filter on the cold inlet, a scaled-up plate heat exchanger or a sticking diverter valve can all cut the hot flow. These are jobs for a qualified heating engineer, and on gas boilers many countries require a registered professional by law.
Tank-fed systems and head height
Plenty of older homes have a cold water tank in the loft feeding a hot water cylinder, and sometimes the upstairs cold taps too. Here the pressure comes from gravity: the height of the water in the tank above the outlet. Roughly, each metre of height gives about 0.1 bar. A ground-floor tap several metres below the tank does fine. A shower in a loft conversion sitting barely below the tank gets almost nothing.
Raising the tank, where there's room, helps a bit. A pump helps more.
Pumps, and when they're allowed
On a gravity system, a shower pump or a whole-house pump drawing from stored water is the standard fix. Fitting one involves plumbing and electrical work, and the electrical side should be done by a qualified electrician under your local rules.
Pumping straight from the mains is a different matter. In many countries and supply areas you're not allowed to connect a pump directly to the incoming main, or only within set limits or with the supplier's consent, because it can rob neighbours of pressure and risks drawing water back into the network. The usual compliant routes involve a break tank or an accumulator. Check your local water rules before buying anything.
Boiler pressure isn't water pressure
This one trips up a lot of people. The gauge on the front of a combi or system boiler shows the pressure in the sealed heating circuit, the water that goes round your radiators. That typically sits around 1 to 1.5 bar when the system is cold. It has nothing to do with how hard water comes out of your taps.
Your taps run on mains pressure or, on older systems, on gravity from a tank. A boiler gauge reading 0.5 bar won't make the shower weak, and topping the boiler up won't make the shower stronger. If the boiler gauge keeps falling, that's a separate problem with its own causes, covered in our guide to a combi boiler losing pressure.
Diagnostic table
| Symptom | Likely cause | Test | Fix |
|---|---|---|---|
| One tap weak, rest fine | Clogged aerator or part-closed isolation valve | Unscrew the aerator and run the tap without it | Descale or replace the aerator; open or replace the valve |
| Shower weak, taps fine | Scaled shower head or kinked hose | Run the shower into a bucket with the head removed | Descale the head; replace the hose |
| Whole house weak since recent work | Stopcock or outside valve left part-closed | Check the stopcock turns no further anticlockwise | Open fully, without forcing |
| Whole house weak, gauge low with all taps off | Low supply or a faulty PRV | Gauge readings at night and at peak; look for a PRV | Ask the supplier; have the PRV adjusted or replaced |
| Good static pressure, poor flow when running | Narrow, corroded or lead pipework | Bucket test compared with gauge drop | Replace the pipework |
| Gradual decline, meter moves with everything off | Supply pipe leak | Meter test with the stopcock closed | Locate and repair the leak |
| Weak only at breakfast and early evening | Peak demand on the network | Compare night and peak readings | Contact the supplier; consider stored water |
| Hot weak, cold fine (combi) | Flow-rate limit, blocked inlet filter or scaled heat exchanger | Run one hot outlet at a time; compare summer and winter | Engineer service; accept the limit or change the system |
| Hot weak, cold fine (cylinder) | Low head height from the loft tank | Measure the height from tank to outlet | Raise the tank or fit a permitted pump |
| Boiler gauge low, taps fine | Heating circuit pressure loss, not water pressure | Read the boiler gauge when cold | Separate problem: find the cause before repressurising |
Who deals with what
- Aerators, shower heads and stopcock position: you.
- PRVs, pipe replacement and pumps: a plumber, plus an electrician for any wiring.
- Anything inside a gas boiler: a qualified heating engineer, registered where your country requires it.
- Pressure at the boundary, street mains and supply interruptions: your water supplier.
Get the bucket and gauge numbers first. Ten minutes with a stopwatch usually tells you which of those four you need.
Owen Pritchard
Owen handles everyday repairs – dripping taps, weak showers, blocked loos – and has firm views on coat hangers, caustic soda and anything sold as "flushable".
General guidance only. Gas, electrical and structural work must be done by a suitably qualified person, and rules differ from country to country.
