Two different things that get the same complaint
Almost everyone who rings about "low pressure" is describing low flow. The shower has gone weak. The bath takes an age. Run the kitchen tap and the one upstairs dies. Those are flow symptoms, and the fix for a flow problem is usually not the fix for a pressure problem.
Pressure is force, measured in bar. It pushes water out of an opening, and it is what a shower valve and an unvented cylinder are designed around. It does not tell you how much water there is.
Flow rate is quantity over time, measured in litres per minute. It fills a bath, and it is what a combi has to heat as it passes through. It decides whether two showers can run at once.
You can have high pressure and poor flow: the London terrace with a good main in the street and a narrow, sixty-year-old supply pipe up the garden. One tap is fine. Two is not enough water to go round. You can also have good flow and low pressure, which is what a loft tank gives an upstairs bathroom — plenty of water, almost no force behind it.
Measure your flow rate: a jug, a phone and thirty seconds
This is the one measurement worth doing before you spend any money.
- Get a jug with markings on it. A 1-litre measuring jug is ideal. Do not use a bucket unless you know its capacity, because guessing the volume ruins the number.
- Pick the right tap. Start at the kitchen cold. On almost every house that one is fed straight off the incoming main, so it tells you what the property is given before the rest of the plumbing gets involved. Take off any aerator first, or you are measuring the aerator.
- Turn everything else off. No washing machine, no dishwasher, nobody flushing.
- Open the tap fully and time the fill. Stopwatch on as the jug goes under, off when the water reaches the 1-litre line.
- Do the arithmetic. For a 1-litre jug, litres per minute equals 60 divided by the seconds. One litre in 4 seconds is 15 litres per minute. In 6 seconds, 10. In 10 seconds, 6. In 20 seconds, 3.
- Repeat three times and take the middle result. The first pour is always the sloppiest.
Then repeat it in four places
The single reading is nearly useless on its own. The pattern across the house is what diagnoses the problem.
- Kitchen cold. Your baseline: roughly what is coming into the property.
- Bathroom cold. Much lower than the kitchen means the restriction is in the internal pipework, not the incoming supply.
- Bathroom hot. Cold good and hot poor puts the fault on the hot side only — the cylinder, its outlet valve, or a scaled heat exchanger. Not the supply pipe.
- Kitchen cold again, with the bathroom cold running. If the first tap collapses when the second opens, you have a flow ceiling. That is a supply pipe or a stopcock, and no pump will invent water that is not arriving.
Write the four numbers down. Handing them over saves the first half hour of any visit.
Pressure is harder to measure, and mostly you can infer it
Measuring pressure properly needs a gauge screwed onto an outside tap or a washing machine valve. They cost very little, and if you are minded to buy something it beats a pump.
The legal position sets the floor. Under section 65 of the Water Industry Act 1991 a water undertaker must supply water constantly and at a pressure that reaches the top of the top-most storey of every building in its area. CCW set out the practical standard: Ofwat requires a minimum of seven metres static head, which is 0.7 bar, in the communication pipe — the length that comes off the main and ends at your outside stop valve. Past that point, in your own supply pipe and inside your property, the company has no pressure obligation at all. Almost every pressure complaint we attend sits on the customer's side of that line.
Pressure that is genuinely too high is a different fault with a different fix, covered in my water pressure seems too high.
Gravity-fed systems fail as flow. Mains-fed systems fail as pressure.
Gravity-fed
A cold tank in the loft, a vented copper cylinder in the airing cupboard, and hot water that arrives because it is falling downhill. Pressure here is nothing but height: roughly 0.1 bar for every metre between the water level in the tank and the outlet. A shower head in a loft conversion two metres below the tank sees almost nothing, and no work on the cylinder will change that, because the geometry of the house is the pressure.
What goes wrong on these is flow, and it is nearly always a valve or a blockage rather than a component:
- A gate valve on the cylinder outlet seized part-open. They corrode internally and the handle still turns.
- A loft tank filling slowly, or sitting low because the float valve is scaled.
- Scale and debris at the cylinder outlet, or in old galvanised distribution pipe.
- An airlock, which makes the flow stutter rather than run steadily weak.
On a gravity system a shower pump is a legitimate answer, because it draws from a cistern rather than the main.
Mains-fed
A combi, or an unvented cylinder. Pressure comes from the street and is usually plentiful. Flow is limited by the narrowest thing between the main and the tap — in London, very often the private supply pipe from the boundary to the house, which may be old lead or half-inch iron with decades of deposit inside it.
What throttles flow on a mains-fed house, in the order worth checking:
- The internal stopcock, left three-quarters shut by whoever was last under the sink. Free to fix, and startlingly common.
- A pressure reducing valve set low, or failing.
- An old, narrow or partly collapsed supply pipe between boundary and house.
- Scale in a combi's hot water heat exchanger, which shows as hot flow much worse than cold.
On a mains-fed house a pump is usually the wrong purchase, and often not a legal one. The Water Supply (Water Fittings) Regulations 1999 require you to notify your water undertaker and get its consent before installing a pump or booster drawing more than 12 litres per minute connected, directly or indirectly, to a supply pipe. Pumping harder on a shared main takes water off your neighbours.
The wrong purchases this confusion causes
| What you buy | What was actually wrong |
|---|
| A shower pump on a combi | Nothing a pump can help. Combis are flow-limited by design, and pumping a mains-fed outlet is a consent issue. |
| A new cylinder | A gate valve on the old cylinder's outlet, seized half shut. |
| A pressure reducing valve | Low flow, which a PRV makes worse, not better. |
| A new combi, sized on price | A supply pipe that cannot deliver what the old boiler wanted, let alone the new one. |
| A new supply pipe | An internal stopcock that was never fully opened. |
Every row on that table is avoided by measuring four taps first.
When a drop in either one is a leak
Weak flow that has always been weak is a design or pipe-size issue. A sudden, unexplained change is a different animal, because water escaping before it reaches your taps takes pressure and flow with it.
- Every outlet drops at once, cold and hot, with no work done. That points upstream of the house — the supply pipe, or the main. Check with the water company first; if the street is fine, the loss is on your pipe and it is yours to fix.
- Flow falls and the meter keeps moving with everything closed. Close to conclusive. The method for confirming it is in water meter turning with everything off.
- Hot flow collapses and the boiler keeps losing pressure. Two symptoms of one heating leak, usually under a floor.
- Flow is fine but there is a wet patch, a warm spot underfoot or an unexplained rise in the bill. A leak big enough to destroy a floor is often far too small to register at the tap. Do not wait for the flow to tell you.
The underground supply pipe leak catches people out, because the water soaks into the ground and there is nothing to see. The only evidence is a meter that will not stop and flow that has quietly got worse.
What this is worth calling us for, and what is not
Not our job: a shower that has always been weak on a gravity system, a scaled combi heat exchanger, or low pressure your water company's own test puts in the street. Those belong to a plumber, a heating engineer and the water company, and we will say so rather than charge you to find out.
Our job: locating a loss you cannot see. We confirm the leak is real with an isolated pressure test, then trace it with acoustics on a pressurised supply, thermal imaging through screed and tile, or tracer gas on a drained run — so the floor or the garden is opened at one point rather than searched. £150 per hour, agreed before we attend, no call-out fee, and if we do not find the leak you do not pay for the looking time. The rate card is on the pricing page. The written report supports a trace-and-access claim; what your policy covers is the insurer's decision, not ours.
If the shower has gone cold rather than weak, or runs hot for a few seconds and then cold on a combi, that is covered in no hot water in the shower.
