Checking Mains Water Pressure With a Gauge

What kind of gauge you need

The gauge for mains water is not the one on the front of your boiler. The boiler gauge shows the pressure in the sealed heating circuit, which should sit around 1 to 1.5 bar when cold and has nothing to do with the pressure at your taps. If that is the number you are worried about, see our combi boiler pressure guide instead.

A mains test gauge is a dial, usually reading 0 to 10 or 0 to 11 bar, on a short fitting with a three-quarter-inch BSP female thread. That thread fits the outlet of a standard outside tap and a washing machine or dishwasher valve. Many kits also include a rubber push-on adaptor for a kitchen tap spout, which works for a quick reading but tends to leak at higher pressures. Better gauges have a second, red drag needle that stays at the highest pressure reached, useful for catching pressure spikes overnight. Typical prices are around £10 to £30, with the branded gauges at the big merchants at the top of that range.

Most gauges read in bar and psi. One bar is roughly 14.5 psi and roughly 10 metres of water height, so a reading of 3 bar is the same as about 30 metres head.

How to take a reading, step by step

Choose a fitting as close to the incoming main as possible and before any pressure reducing valve, water softener or filter, so you measure what the pipe delivers rather than what a fitting lets through. In a house, the outside tap or the washing machine valve is usually best. In a flat, use the washing machine valve.

• Make sure no other taps, toilets, showers or appliances are running anywhere in the property, including a dishwasher part way through a cycle.
• Turn the tap or valve off, screw the gauge on hand tight, then open the tap fully.
• Read the needle once it settles. This is your static pressure.
• Leave the gauge on and open the cold kitchen tap fully. Watch the needle drop and read it again once it steadies. This is your dynamic, or running, pressure.
• Close the kitchen tap and check the needle returns to the static figure. If it creeps up slowly, something in the supply is restricting flow.
• Repeat at a busy time, around 7 to 9 in the morning, and late at night. Mains pressure varies across the day.

What your reading means

Ofwat’s reference level for adequate supply is 10 metres head, about 1 bar, with a flow of 9 litres a minute, measured at the boundary stop tap; the legal guaranteed minimum is lower, at 7 metres (about 0.7 bar). In practice most London homes see more than that, but pressure depends heavily on height. You lose roughly 0.1 bar for every metre you go up, so a top-floor flat can read noticeably lower than the ground floor of the same building.

The pattern of your two readings matters more than either number on its own.

• Static and dynamic both reasonable, with a modest drop when the tap runs: your supply is fine. If a particular tap or shower is weak, the problem is at that outlet.
• Static reasonable but dynamic collapses to near zero: pressure is there but the pipe cannot carry the flow. Common causes are a half-closed or scaled stopcock, an old narrow lead or iron supply pipe, or a sticking pressure reducing valve.
• Both low all day: the supply is weak before it reaches you. Check your water company’s postcode map for work in the area and ask a neighbour to compare.
• Both fine at night but low in the morning: demand in the street is dragging pressure down. It is a network issue if it is well below the minimum standard.
• Very high static pressure, or the drag needle showing spikes much higher than normal: the supply may need a pressure reducing valve to protect appliances and fittings.

Why pressure alone does not tell you about flow

Pressure is the push. Flow is how much water actually arrives each minute, and it is flow that fills a bath or runs two showers. A narrow pipe can show a healthy static pressure yet deliver a feeble flow the moment you open a tap, which is why the running reading matters.

Measure flow with a bucket at the cold kitchen tap. Open it fully, time how long it takes to fill a bucket of known size, and work out litres per minute. Nine litres a minute or more meets the minimum standard at the boundary. A combi boiler or unvented cylinder usually needs more, and the installation manual gives the figure. If your flow is poor after a new boiler was fitted, see our guide to low water pressure after a new combi.

If you have an unvented hot water cylinder, it will have its own pressure reducing valve on the cold inlet, commonly set at around 3 bar. The label on the cylinder states the setting. A gauge on the mains before that valve will read higher than the pressure inside the cylinder, which is normal.

When the reading means you need a plumber

A gauge is a diagnostic tool, not a fix. Once you have a pattern, you know which way to go. A weak supply across the street is for your water company to investigate. A restriction inside the property is plumbing work.

On a call-out for low pressure, an engineer confirms your readings, then works back from the tap to the stopcock, checks for a stuck or badly set pressure reducing valve, and looks at the supply pipe material and size. Where the fault is a stopcock, the fix is usually quick; see stopcock repair. Where the supply pipe is too small or corroded, the answer is a new one, and that part from the boundary to the house is the owner’s responsibility.

Visits are the £60 call-out, and the repair price is agreed with you on site before any work starts. Our low water pressure repair page explains what an engineer tests and replaces, and our guide on how to increase water pressure covers the options once you know the cause.