Renovation

Check Your Water Pressure Before You Renovate — Not After the Tiling

Pressure and flow are different numbers and only one of them fills a bath. Measure both at the highest, furthest outlet before the design is fixed.

By Vladimir Castravet·

There is one test that costs almost nothing before you start and a fortune after. Measure the pressure and the flow at the tap — they are different numbers, and only one of them fills a bath. A mixer shower generally needs about 12 litres per minute to feel right, and your supply is held to a minimum of 0.7 bar at the communication pipe. Then design the bathrooms and the heating to those measured numbers rather than to a guess. The estimator prices the plumbing scope, and our renovation work covers the stage where this decision has to be made.

Two numbers, not one

Pressure is the force behind the water, measured in bar. Flow is how much water actually arrives, measured in litres per minute.

People say “the pressure is bad” when what they almost always mean is that the flow is bad. A house can have perfectly respectable pressure at the stop tap and still deliver a disappointing shower, because the pipework between the two is narrow, furred, kinked, or has been reduced somewhere along its length by a previous owner’s helpful plumber.

Flow is what fills a bath. Flow is what a rainfall head needs. Flow is what runs out when two people shower at once. It is the number that matters most and the one nobody measures.

The test, which takes fifteen minutes

Flow. Take a container of known volume — a bucket you have measured, or a two-litre jug. Open the cold tap fully. Time how long it takes to fill. Convert to litres per minute.

Pressure. A screw-on gauge costs very little and fits an outside tap or a washing machine connection. Read it with everything off (static) and with a tap running (dynamic). The gap between those two numbers tells you as much as either alone — a large drop means restriction in the pipework.

Three rules about where and when:

  1. Test at the highest and furthest outlet you plan to build, not at the kitchen sink. If the new shower is going in the loft, the ground floor tap is not the answer.
  2. Test at a busy time. Weekday evening, not midnight. Mains pressure in a London street varies with demand.
  3. Test more than once. One reading is an anecdote.

Write the numbers down. They belong in the brief, alongside the room sizes.

The figures

| The figure | What it means | Source | |---|---|---| | 7m static head (≈0.7 bar) | The minimum standard water undertakers are held to at the communication pipe. | Water Industry Act 1991, s.65 | | 10m head at 9 l/min | Ofwat's low-pressure reference level, measured at the boundary stop tap. | Ofwat | | 48°C | Hot water to a bath must be limited to a maximum of 48°C by an in-line blending valve or other temperature control with a maximum temperature stop. | AD G §3.65–3.66 | | 100°C | Precautions must prevent the temperature of stored water at any time exceeding 100°C. | Requirement G3(3) | | 60°C / 55°C | To control legionella, stored hot water generally at 60°C and distributed at not less than 55°C. | AD G §3 / HSG274 | | Unvented cylinders | Must have the required safety devices and a correctly terminated discharge pipe; installation is work for a competent person. | AD G §3.22–3.23 | | 125 l/person/day | Water efficiency cap for a new dwelling — 110 where planning imposes the optional requirement. | reg. 36 / AD G2 | | Commissioning notice | Not more than 5 days after commissioning where a building notice, full plans or initial notice applies; otherwise not more than 30 days. | AD G §3.78 | | Plumbing compliance | All plumbing must comply — backflow prevention, approved fittings, notification of certain installations to the water undertaker. | Water Supply (Water Fittings) Regulations 1999 |

Approved Documents are statutory guidance to the Building Regulations 2010 for England. Compliance is judged against the Requirements in Schedule 1; your building control body has the final word on your specific building.

The combi boiler trap

This is the single most common way a well-designed renovation disappoints.

A combi boiler makes hot water on demand, and its hot water output is limited by its own flow rate. Whatever that figure is, it is shared. Add a second bathroom and run both showers together and each one gets roughly half.

So the sequence that causes the problem is completely reasonable at every step: the family adds an ensuite, the plumber connects it to the existing boiler because the boiler is fine, and the first time two people shower simultaneously everybody is disappointed. Nothing was installed badly. The system was never sized for the house that now exists.

The same applies to a large rainfall head. Rainfall heads are specified by how they look and consume water by how big they are. A head that wants twenty-odd litres a minute on a supply delivering twelve will produce a gentle drizzle, beautifully.

Ask the question at design stage: what is the flow, what does the specified sanitaryware need, and what happens when two outlets run at once?

What to do if the number is too low

Re-pipe the restriction. Often the mains supply is fine and the incoming pipe inside the property is old, narrow or partially blocked. Upgrading the internal run is frequently the cheapest real fix.

Fit an accumulator. A vessel that stores mains-pressure water and releases it at the flow you need during peak demand. It does not add pressure beyond the mains, but it removes the flow bottleneck. It needs space.

Move to an unvented cylinder. Stored hot water at mains pressure, which handles multiple outlets far better than a combi. It needs space, safety devices and a correctly terminated discharge pipe, and it is work for a competent person.

Break tank and pump. Where the mains genuinely cannot deliver. You are generally not permitted to pump directly off the incoming supply, because that draws down pressure for your neighbours — so the compliant route is a tank, then a pump.

Or change the specification. A smaller shower head that is well-chosen can feel better than a large one that is starved. This is the cheapest answer of all and it is worth considering honestly before spending on plant.

Every one of these takes space, and space is decided at design stage. That is the entire point of testing early: the decision is cheap before the layout is fixed and expensive afterwards.

While you are testing, check two more things

The bath blending valve. Hot water to a bath must be limited to a maximum of 48°C by a blending valve or equivalent control with a maximum temperature stop. It is a small item that is genuinely a safety requirement, and it is regularly missing from older installations.

The stored water temperature strategy. Legionella control wants stored hot water generally at 60°C and distributed at not less than 55°C — which is why the 48°C limit at the bath is achieved by blending at the outlet rather than by simply storing water cooler.

The takeaway

Test the pressure and the flow before the design is fixed, at the highest and furthest outlet you plan to build. Then design the bathrooms and the heating to the measured number. Take the decision before — not after the tiling.

It is the same principle that governs testing a floor before choosing a finish: measure the thing you are building on, while measuring is still free.

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