Water pressure & flow, explained
You can buy the best shower on the market and still be let down by a dribble — because how a bathroom actually feels to use is decided by your water supply, not the fittings hung on the wall. Pressure and flow are the two numbers behind every strong shower, fast-filling bath and tap that runs without a fight. This guide explains what they are, why they matter, how we measure them before quoting a full bathroom installation, and how we design a bathroom that performs around the supply you actually have in your Plymouth home.
Pressure and flow are not the same thing
People use the words interchangeably, but they describe two different things — and a good bathroom needs both. Pressure is the force pushing the water, measured in bar. Flow is how much water actually arrives, measured in litres per minute. You can have decent pressure but poor flow if a pipe somewhere is narrow, furred up or too long — the force is there, but not enough water can get through to make a shower feel generous.
The easiest way to picture it: pressure is how hard you can push, flow is how much gets delivered. A pinhole can hold high pressure but pass almost no water. A wide, short pipe run at modest pressure can deliver a lovely, soft, drenching flow. A shower that feels weak is usually a flow problem, not a pressure one — and that distinction changes everything about how we spec your bathroom. It is the single most common thing homeowners get wrong when they blame the shower head for a supply that was never going to run it.
Because the two behave differently, we measure both at survey. A quick pressure gauge tells us the force; timing how long an outlet takes to fill a container tells us the flow. Once we know both figures, we can tell you honestly what your supply will and won’t run — before you fall in love with a rainfall head it can never feed. That honesty at the quoting stage is exactly what stops a bathroom disappointing you the day it is finished.
The three plumbing systems in Plymouth homes
Which system your home has decides how much pressure and flow you start with, and it is the first thing we identify on a survey. Most Plymouth properties fall into one of three types, and each behaves very differently at the shower head.
Gravity-fed (tank in the loft)
Common in older Plymouth stock — the 1960s–90s homes in Peverell, Mannamead and Plympton. A cold tank in the loft feeds a hot cylinder in the airing cupboard. Pressure depends on how high the tank sits above the outlet, so it is often low, especially on the top floor. Great candidate for a pump if you want a strong shower.
Combi boiler (mains-fed)
Very common in newer builds and updated homes. Heats water on demand straight off the mains, so there is no tank or cylinder. Pressure is usually good, but flow is limited by how much the boiler can heat at once — which is why some combis struggle to run a bath and a shower together.
Unvented (pressurised cylinder)
An insulated cylinder fed directly from the mains at high pressure — the best of both worlds for most bathrooms. Strong, balanced hot and cold, easily enough to run a big shower and fill a bath quickly. Increasingly what we fit or recommend when a home’s system is due for renewal anyway.
Knowing your system matters because it sets a ceiling. A gravity-fed home on the top floor of a Plymstock semi will never run a mains-pressure rainfall head unaided — but a pump, or a switch to an unvented cylinder, transforms it. We work all of this out at the bathroom installation survey so the spec matches reality.
Why pressure and flow decide how the bathroom feels
Every outlet in the room has a minimum it wants to perform properly, and if the supply falls short the fittings simply cannot make up the difference. Here is where it shows up most.
The shower
This is where poor supply is felt hardest. A skimpy flow turns a drench into a drizzle, and low pressure makes a thermostatic valve swing hot and cold when another tap is opened. Big rainfall heads are especially thirsty — they need real flow to feel like anything at all. If a strong shower matters to you, and it does to most people, this is the outlet we design the whole supply around. It is closely tied to your choice of walk-in shower and valve.
The bath and basins
Low flow means a bath that takes an age to fill — irritating every single evening. Basins are more forgiving, but you still notice a tap that barely trickles. Where two outlets run at once, poor flow means opening the kitchen tap steals pressure from the shower upstairs. Balancing all of this is part of getting the first-fix plumbing right, with feeds sized to the demand rather than left to chance.
None of this is visible in a finished-bathroom photo, which is exactly why it gets overlooked until it is too late to change cheaply. We would far rather have the awkward conversation at the quote — “your supply won’t run that head unless we pump it” — than hand over a beautiful room that underwhelms every morning.
How we test and design around your supply
Fixing pressure and flow is nearly always cheap and easy to design in before the walls close up, and expensive or impossible to bolt on afterwards. Here is how we approach it on every job.
1. Measure at survey
We gauge the pressure and time the flow at the outlets that matter, and identify which of the three systems you have. Ten minutes of testing saves a world of disappointment later.
2. Match the spec to the supply
We steer you towards fittings your supply can actually run — the right shower valve and head, and a bath that fills at a sensible rate — so nothing you choose over-promises.
3. Design in a fix if needed
If your heart is set on a big shower and the supply won’t feed it, we plan the fix: a shower pump on a gravity system, upsized feed pipes, or a move to an unvented cylinder.
4. Upsize the pipework
At first fix we run feeds sized to the demand and keep them as short and direct as sensible, so flow isn’t choked by narrow or over-long runs buried in the wall.
5. Balance the outlets
We set things up so running one tap doesn’t rob another, and so hot and cold arrive in balance at the shower valve — the secret to a temperature that stays put.
6. Price it into the fixed quote
Whatever your supply needs is costed into your fixed written quote up front. You see the pump or cylinder in the price, not as a mid-job surprise. See bathroom costs in Plymouth.
The fixes, and what they cost you in convenience
When a supply won’t run the bathroom you want, there is almost always a route to fixing it. The right one depends on your system, your budget and how much disruption you’ll accept. These are the options we most often design in for Plymouth homes.
A shower pump
The classic fix for a gravity-fed home with a loft tank and hot cylinder. A pump boosts pressure and flow to the shower — sometimes the whole bathroom — turning a weak trickle into a proper drench. Needs a little space near the cylinder and a power supply, both easy to plan for at first fix.
Upsized feed pipes
Where flow is throttled by old, narrow or furred pipework, simply running new, correctly sized feeds while the walls are open can transform delivery — often at little extra cost because we’re in there anyway. Cheapest win of the lot when it applies.
An unvented cylinder
Swapping a gravity system for a mains-pressure unvented cylinder gives strong, balanced water throughout — not just the bathroom. A bigger job that touches the whole system, best done when your hot water is due for renewal anyway. We’ll tell you honestly if it’s overkill for your needs.
Sensible spec choices
Sometimes the smartest fix is no fix at all — choosing a shower and head that flatter your existing supply. A well-chosen valve on modest pressure can feel excellent. We’d rather match your ambitions to reality than sell you a pump you don’t need.
Whichever route suits your home, the point is the same: we work it out before anything is tiled in, so the finished bathroom performs the way you pictured it — and so you’re never left wishing you’d known at the quote stage. That planning is part of doing a job we’re happy to put our name on.
What this means for older Plymouth homes
A lot of the bathrooms we’re called out to sit in the city’s 1960s–90s housing stock — the solid semis and terraces of Peverell, Mannamead, Plympton and Plymstock. These homes were very often built with gravity-fed systems: a cold tank in the loft and a hot cylinder in the airing cupboard. It’s a perfectly good system, but two things tend to catch people out when they upgrade the bathroom.
The first is height. On a gravity system, pressure comes purely from how far the loft tank sits above the outlet, so a bathroom on the top floor of a two-storey home often has very little push to work with. That’s fine for a bath, but it’s why so many showers in these homes feel weak — there simply isn’t the head of water above them. The second is age: decades-old feed pipes are frequently narrow by modern standards, and can be partly furred up inside, both of which throttle flow before the water even reaches the room.
The good news is that a full bathroom installation is the perfect moment to put both right. Because we’re already inside the walls and floor, running new correctly-sized feeds costs little extra, and adding a shower pump or planning a switch to a mains-pressure unvented cylinder is far cheaper designed in now than bolted on later. So while an older home may start with a modest supply, it very rarely has to stay that way. We simply show you the options and the cost, and you choose. It’s the same principle we apply to a bathroom renovation or a full rework: fix the invisible things properly while the room is open, and the finish looks after itself.
Water pressure & flow FAQs
How do I know if my water pressure is too low?
The tell-tale signs are a weak shower, a bath that takes forever to fill, and taps that slow to a trickle when another outlet is running. It’s often worse upstairs, and worst of all on the top floor of a gravity-fed home. The only way to be sure is to measure it — which we do free at survey, gauging pressure and timing flow so we can tell you exactly where you stand and what, if anything, needs doing.
Will a new shower head fix a weak shower?
Rarely on its own. A weak shower is usually a flow problem in the supply, not the head — and fitting a big, thirsty rainfall head to a starved supply actually makes it feel worse. The real fix is to improve delivery with a pump, upsized pipes or a system change, then choose a head your supply can run. We work out which you need before quoting, so you don’t spend money on a head that can’t perform.
Can you improve pressure during a bathroom installation?
Yes — and a bathroom installation is by far the best time to do it, because the walls are open and the plumbing is exposed. Depending on your system we can add a shower pump, run upsized feeds, or plan a move to a mains-pressure unvented cylinder. It’s all designed in before boarding and tiling and costed into your fixed quote, so there are no surprises. Bolting the same fixes on to a finished bathroom later costs far more.
Does a combi boiler give good pressure for a bathroom?
Usually good pressure, yes, because it runs off the mains. The limit with a combi is flow — how much hot water it can heat at once — which is why some struggle to run a bath and a shower at the same time. For most single bathrooms a decent combi is fine; for a busy family bathroom or two showers running together, we’ll check the boiler’s output and tell you honestly whether it copes or whether an unvented system suits you better.
Proud of every bathroom we fit
Let’s make sure your bathroom performs
A beautiful bathroom that dribbles is a daily frustration. We measure your pressure and flow, tell you honestly what your Plymouth supply will run, and design in any fix before a single tile goes on. Tell us about your room and we’ll give you one clear, fixed written quote.