Water Pressure Testing: Procedures and Pass/Fail Criteria

Why Pressure Testing Matters

A pressure test is the definitive way to prove a water system is leak-free before it goes into service or before you hand a job back to a customer. It's required under BS EN 806 (Specifications for installations inside buildings conveying water for human consumption) and referenced in the Water Supply (Water Fittings) Regulations 1999, which place a legal duty on installers to ensure pipework is watertight and fit for purpose.

Skipping or cutting corners on a pressure test isn't just a compliance risk — if a concealed pipe fails after the plasterboard goes up, you're looking at significant remedial costs and potential liability. A properly documented pressure test certificate protects both you and your customer.

Before You Start: System Preparation

A pressure test is only as reliable as the preparation that goes into it. Rushing this stage leads to false failures and wasted time.

• Isolate the system under test — close all stopcocks, service valves, and isolators. Remove or blank off any fittings that aren't rated for test pressure, such as flexible tap connectors, washing machine hoses, and pressure-sensitive appliances.
• Fit test ends — cap or plug all open ends with pressure-rated blanking caps. Never rely on compression olives alone to hold test pressure on open pipe ends.
• Check for temperature effects — allow the system to reach ambient temperature before testing, especially if it's been in a warm or cold environment. Temperature variation will cause pressure to drift even on a perfectly sound system.
• Pre-fill the system — for hydraulic (water) tests, fill the system slowly from the lowest point to purge air. Air pockets compress under pressure and will give misleading results. Vent from the highest points as you fill.

Test Pressures: What the Standards Say

The correct test pressure depends on the system type, the materials used, and the intended working pressure. The key reference is BS EN 806-4, which covers installation and testing, alongside guidance in the CIPHE Domestic Plumbing and Heating Installations guide and WRAS-approved installer guidance.

Cold Water (Copper and Plastic Pipework)

For domestic cold water systems, the standard hydraulic test pressure is 1.5 times the maximum working pressure, with a minimum test pressure of 1.5 bar. In practice, most domestic systems are tested at 1.5 bar to 3 bar, held for a minimum of 15 minutes. Many contractors use a standard of 1.5 bar for 1 hour on larger or more complex installations.

Hot Water Systems

Unvented hot water systems (cylinders, megaflo-type) have higher working pressures and must be tested accordingly. Refer to the cylinder manufacturer's instructions — test pressure is typically 1.5× working pressure, which on a system set to 3 bar working pressure means a test of 4.5 bar. Always isolate the cylinder itself and pressure test the pipework circuit separately unless the manufacturer's instructions explicitly permit otherwise.

Commercial and Industrial Systems

BS EN 806 and the associated Building Services Research and Information Association (BSRIA) BG 29 guide apply to commercial installations. Test pressure is again 1.5× maximum operating pressure, typically for a minimum of 1 hour, with a leak test at working pressure for a further period. Always check project specifications — commercial contracts frequently specify test duration and acceptable drop in writing.

Underfloor Heating (Water-Based)

Underfloor heating pipework is tested to 6 bar minimum, held for 24 hours before screed is poured, in line with manufacturer requirements and BS EN 1264. This is non-negotiable — screed covers the pipes, and any leak found after the pour means breaking the floor.

Conducting the Test: Step by Step

1. Connect your test pump — use a calibrated hydraulic test pump (hand pump or electric) with a test gauge. The gauge should be calibrated and have a resolution appropriate for the test pressure. A gauge reading to 10 bar is fine for a 3 bar test; avoid using a 0–100 bar gauge where you need to read small pressure changes accurately.
2. Bring the system up to test pressure slowly — increase pressure in stages (e.g., 1 bar, then 2 bar, then 3 bar). Check for visible leaks at each stage. If you see a leak, depressurise and fix it before continuing.
3. Start the clock — once you've reached test pressure and the system has stabilised (allow 5–10 minutes for pipe expansion on plastic pipework), record the start time and exact pressure reading.
4. Monitor at intervals — check and record pressure at 5- or 15-minute intervals throughout the test period.
5. Record end pressure and time — note the final pressure and elapsed time. Calculate the pressure drop.

Pass/Fail Criteria: Acceptable Pressure Drops

This is where many certificates fall down — installers either don't record the data clearly or apply the wrong criteria. Here's what the standards say:

BS EN 806 Hydraulic Test

• Test pressure: 1.5× maximum working pressure, minimum 15 minutes.
• Pass: No visible leakage and no pressure drop during the test period (or an insignificant drop attributable to temperature stabilisation).
• Fail: Any visible leak, or a sustained pressure drop that cannot be accounted for by temperature change.

Plastic Pipework (PEX, PB, MDPE)

Plastic pipe expands under pressure, which means you'll often see a pressure drop in the first 30 minutes that has nothing to do with leaks. BS 6700 (since superseded by BS 8558, but its test method is still widely used) and manufacturer guidance acknowledge this. The standard approach is:

• Pressurise to test pressure, then re-pressurise after 30 minutes to correct for expansion.
• Run the monitored test from this point — pressure should remain stable.
• Some specifications allow a drop of no more than 0.6 bar over 30 minutes for plastic systems, provided there is no visible leak and the rate of drop is reducing over time (indicating settling rather than leakage).

Fail Indicators

• Visible water at any joint, fitting, or pipe surface
• A continuous pressure drop that doesn't stabilise
• Pressure drop exceeding the specification for the pipe material and test duration
• Any drop greater than 0.1 bar per hour on metal pipework (after stabilisation)

What to Record on the Certificate

A pressure test certificate is only legally useful if it captures the right information. Under the Water Fittings Regulations, records should be retained and available for inspection. At minimum, your certificate should include:

• Date and time of test start and end
• Site address and system location (e.g., "first floor bathroom circuit", "main cold water ring main")
• System type (cold water, hot water, UFH, etc.) and pipe material
• Test medium (water/hydraulic, or air/pneumatic — see note below)
• Test pressure applied (in bar)
• Duration of test
• Pressure at start and end of monitored period
• Pressure drop (calculated)
• Pass or Fail result, clearly stated
• Gauge calibration reference (calibration date or serial number)
• Installer name, signature, and company details
• Any defects found and remedial action taken, with a re-test record if applicable

Air vs Hydraulic Testing

Hydraulic (water) testing is the standard and preferred method for water systems. Pneumatic (air or nitrogen) testing carries significantly higher risk — a pressurised gas system stores far more energy than a water-filled one, and a fitting failure under air pressure can be explosive. Air testing should only be used where water testing is genuinely impractical (e.g., in freezing conditions or where the system cannot be easily drained) and must be carried out at lower pressures (typically no more than 0.5 bar for air pre-tests) with appropriate safety precautions. Never air test at hydraulic test pressures.

Regulatory and Standard References

• BS EN 806-4:2010 — Installation and testing of water supply pipework inside buildings
• BS 6700:2006+A1:2009 — Design, installation, testing and maintenance of water supply services
• Water Supply (Water Fittings) Regulations 1999 — legal framework for water fittings in the UK
• WRAS (Water Regulations Advisory Scheme) guidance notes — approved fittings and installation standards
• BS EN 1264 — water-based surface embedded heating and cooling systems (for UFH pressure testing)
• BSRIA BG 29 — commissioning water systems in buildings (commercial applications)

Completing the Certificate in CertBox

When recording a water pressure test in CertBox, use the dedicated pressure test certificate type for your system. Fill in all mandatory fields — the form is structured to match the data requirements above. If you need to record a re-test after a failed first attempt, use the additional test fields to log both results on a single certificate rather than creating a separate document. This keeps the compliance trail clean and shows the remedial action taken between tests. Once signed and submitted, the certificate is time-stamped and stored securely for your records and your customer's.

Published 2026-07-06. This article is for general guidance only and does not constitute legal or professional advice. Always refer to the relevant standards and consult qualified professionals for definitive requirements.