Craft Quantities
Plan & Takeoffs

What to check, and in what order

The water pressure is low

Start with the scope: one outlet, all the hot outlets, or the whole house. One outlet is a local restriction. All the hot is the hot water system. Everything is the supply or the main stop valve. And most complaints about pressure are actually about FLOW — how much arrives per minute, which is a different quantity.
  • 6Candidate causes
  • Most commonMost likely
  • No — candidates onlyA diagnosis

Where this starts

Pressure and flow are not the same thing and the difference matters for the diagnosis. PRESSURE is the force pushing the water; FLOW is the volume arriving per minute. A tap can have perfectly good pressure and a poor flow because something narrow is in the way, and a shower can have plenty of flow and feel weak because the pressure is low. Most household complaints are flow complaints.

THE SCOPE NARROWS IT FASTER THAN ANYTHING ELSE. Go round the house and establish which outlets are affected: one tap, one room, all the hot outlets, or every outlet including the kitchen cold. Each answer points at a different part of the system, and it eliminates the others.

ONE OUTLET is a local restriction — an aerator full of scale, a blocked filter, a partly closed isolation valve under the basin, or a tap cartridge.

ALL THE HOT and none of the cold is the hot water system: a scaled heat exchanger, a partly closed valve on the cylinder, a failed shower valve, or in a vented system simply the height of the cold tank.

EVERYTHING is upstream: the main stop valve not fully open, a filter or meter restriction, or genuinely low supply pressure from the main. This is also where a shared supply and neighbours' demand show up, because it comes and goes with the time of day.

What it could be, and what tells them apart

These are candidates to check against your own building, in the order they most often turn out to be the answer. None of them is a diagnosis of your situation.

  1. A partly closed stop valve

    Most common

    Everything affected at once, often dating from recent work, with the internal stop valve found not fully open.

    How to check: Find the internal stop valve and open it fully. It is the most common single cause and the cheapest to eliminate.

  2. A scaled or blocked aerator or filter

    Most common

    One outlet only, developing gradually, often worst on the tap used most — with visible scale when the aerator is unscrewed.

    How to check: Unscrew the aerator from the affected tap and run it without. A dramatic improvement identifies it in seconds.

  3. An isolation valve under the fitting

    Common

    One outlet, often dating from when that fitting was installed or serviced, with the small screwdriver-slot valve on its supply partly turned.

    How to check: Look under the basin or behind the appliance for a small isolation valve and check its slot is in line with the pipe.

  4. Scale or restriction in the hot water system

    Common

    All hot outlets poor, cold fine, usually developing over years in a hard water area.

    How to check: Compare hot and cold at the same tap. A clear difference places it in the hot side rather than the supply.

  5. Low supply pressure at the main

    Common

    Everything affected, worse at peak times of day, and often shared with neighbours on the same main.

    How to check: Test at the outside tap or the kitchen cold — the outlet closest to the incoming main — at different times of day.

  6. A gravity-fed system doing what it does

    Less common

    In an older vented system, weak flow at upstairs outlets and much better downstairs, unchanged for years rather than newly developed.

    How to check: Whether the difference follows HEIGHT in the building. A gravity system's pressure is the height of the tank above the outlet, and always was.

What to check, in order

The order matters — each step is here because of what it rules out for the ones after it.

0 of 5 checked
  1. 1

    It is the single split that eliminates two thirds of the causes, and it costs a walk round the house with the taps.

  2. 2

    That is usually the outlet closest to the incoming main, so it tells you what the supply is actually delivering before the house's own pipework gets involved.

  3. 3

    It is the commonest cause of a whole-house complaint, it is free to fix, and it is very often left part-closed after work on the system.

  4. 4

    It separates a fitting problem from a supply problem immediately, and scale in an aerator is the commonest single-outlet cause.

  5. 5

    Identical performance points upstream of the hot water system; a clear difference places the problem inside it. Either way it halves the search.

The calculations behind this

Several of the causes above are a quantity that was specified or assumed. These compute them, with the formula and the standard behind each.

Domestic Water Pipe Sizing by Fixture Unit Calculator →Water Meter Sizing Calculator (Fixture Units) →Hydronic Circulator Pump Head Loss Calculator →

When to stop and get somebody in

  • Where the flow has dropped suddenly across the whole house with no valve found part-closed, which can indicate a problem on the incoming supply.
  • Where low flow is accompanied by discoloured water or air in the pipes, which points at work on the main or at corrosion rather than a restriction.
  • Anything involving an unvented cylinder, whose valves and controls are a competence-restricted area and which contains mains pressure.
  • Where a pump is being considered as the answer — a pump on an inadequate supply can breach water regulations and can starve neighbours on a shared main.

This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.