Summary

Gas pressure testing is the final safety verification step in any gas installation, repair, or commissioning job. It establishes whether the gas supply pipework and fittings are leak-free before the system is energised. Every Gas Safe registered engineer must be competent in gas tightness testing — it is a mandatory requirement of the Gas Safety (Installation and Use) Regulations 1998.

In UK domestic practice, the procedures follow IGEM/UP/1B (Institution of Gas Engineers and Managers — Utilisation Procedure 1B), Tightness testing and direct purging of small LPG/Air, natural gas and LPG installations. This is the definitive specification for domestic-sized installations: pipework up to 35mm copper / 32mm steel with a maximum operating pressure not exceeding 2 bar at the ECV outlet. (IGEM/UP/1C is a different document — it covers strength testing, tightness testing and purging of meter installations, and IGEM/UP/1 and UP/1A cover industrial and commercial installations.)

Edition change: IGEM/UP/1B Edition 4 was published in March 2026 and becomes mandatory from 1 October 2026, superseding Edition 3. The headline change for natural gas is that permissible pressure drops are now determined by installation volume (IV) rather than meter size, and where a drop is within the whole-system limit you must now isolate the appliances and re-test the pipework alone, which must show no perceptible movement. Until 1 October 2026 both editions are technically valid.

A tightness test must be carried out:

Understanding the test procedures, pressure limits, pass/fail criteria, and what to do on failure is fundamental knowledge for any Gas Safe registered engineer.

Key Facts

Quick Reference Table

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Gas Type Standing Pressure Working Pressure Test Pressure (tightness)
Natural gas (mains) 21 mbar ≥20 mbar at the appliance 20–21 mbar
LPG — Propane 37 mbar (low pressure regulator) 35–37 mbar 36–37 mbar against a valve; 30–31 mbar through a regulator
LPG — Butane 28 mbar 27–28 mbar 20–21 mbar (28 mbar system); 36–37 mbar (37 mbar system)

Maximum permissible pressure drop over the 2-minute test — natural gas, existing installation with appliances connected (IGEM/UP/1B Edition 4, by installation volume):

Installation Volume (IV) Max permissible drop
≤ 0.005 m³ 8 mbar
> 0.005 – ≤ 0.010 m³ 4 mbar
> 0.010 – ≤ 0.015 m³ 2.5 mbar
> 0.015 – ≤ 0.035 m³ 1 mbar

New pipework, or any installation with no appliances connected: no perceptible movement. Under Edition 4, if the whole-system test shows a drop that is within the limit above, you must then isolate the appliances and re-test the pipework alone — that pipework-only test must show no perceptible movement. Under Edition 3 (valid until 1 October 2026) permissible drops were determined by meter type rather than installation volume — check which edition your training and paperwork reflect.

Test Type Purpose When Used
Tightness test (let-by) Verify pipework soundness; check ECV seats After any new installation or disturbance
Working pressure test Confirm adequate supply pressure under load Commissioning any new appliance
Visual inspection Identify visible defects before pressure test As part of any service or installation
Electronic leak detection Locate a detected leak in existing pipework When tightness test fails

Detailed Guidance

Equipment Required

Before any gas pressure test, ensure:

The Domestic Tightness Test (IGEM/UP/1B Procedure)

This procedure applies to natural gas domestic installations:

Step 1 — Visual inspection: Before applying any test pressure, carry out a visual inspection:

Step 2 — Connect manometer:

Step 3 — Purge test point nipple:

Step 4 — Let-by test (ECV check):

Step 5 — Tightness test of installation pipework:

Step 6 — On failure:

Step 7 — Pass — working pressure test:

Step 8 — Document results:

New Pipework Installation Testing

IGEM/UP/1B does not require a separate strength test for domestic installation pipework. New domestic pipework is proved by the tightness test at 20–21 mbar, with the stricter pass criterion: no perceptible gauge movement over the 2-minute test.

Strength testing at elevated pressure is a requirement of the other documents in the UP/1 family, and applies to different work:

So for a new extension, a complete re-pipe, or a routine appliance change in a domestic property, the IGEM/UP/1B tightness test alone is the correct procedure. If the job involves the meter installation itself, or a commercial property, check which document applies before testing — the procedures and pressures are not interchangeable.

Landlord Gas Safety Check (LGSC)

The annual gas safety check required under the Gas Safety (Installation and Use) Regulations 1998 for rented properties includes a tightness test of the installation pipework. The procedure is the same as above.

The Landlord Gas Safety Record (LGSR) must be completed for each appliance checked, and the tightness test result recorded. Copies must be:

Unsafe Situations and GIUSP Classifications

The Gas Industry Unsafe Situations Procedure (GIUSP) is set out in IGEM/G/11 Edition 2 (with amendments July 2022, June 2024 and July 2025), published for registered engineers as Gas Safe Technical Bulletin TB 001. It defines three classifications — note these are letter codes, not numbered ID codes:

Code Situation Action
ID — Immediately Dangerous Fault presents an immediate danger to life or property With the customer's permission, disconnect and make safe; label and warn — do not use
AR — At Risk Fault not immediately dangerous, but a risk exists if use continues With the customer's permission, turn off and label; warn and advise immediate repair
NCS — Not to Current Standards Does not meet the current standard but is not ID or AR Advisory only — record and recommend upgrade; the appliance may continue in use

IGEM/G/11 Table 1 gives worked examples of ID and AR situations, but it is not exhaustive — the engineer must exercise engineering judgement and be able to justify the classification.

A failed tightness test with a confirmed, unlocated gas escape is an ID situation. The engineer must:

  1. Turn off the gas at the ECV
  2. Issue the GIUSP Warning/Advice Notice to the customer (and to the landlord where the property is let)
  3. Label the installation with the classification and the engineer's details
  4. Record on the job sheet and, on a landlord check, the LGSR
  5. Report to the gas emergency service / transporter where required by the GIUSP

The engineer must not leave a property with a confirmed gas leak unless the gas is off at the meter and the customer has been warned.

Frequently Asked Questions

My gas meter has no test point. How do I carry out a tightness test?

Older meters may not have a test point nipple fitted. Alternatives: (a) fit a temporary test point adaptor to the outlet union of the meter; (b) use a test point on the first isolation valve downstream of the meter. The test must be carried out downstream of the ECV — testing upstream is testing the incoming supply, which is the gas transporter's responsibility, not yours.

Can I carry out a tightness test for a tenant?

Yes — if you are Gas Safe registered in the appropriate category (Domestic Natural Gas). You must complete the Landlord Gas Safety Record and provide copies to the landlord and tenant as described. If you are not Gas Safe registered, you cannot carry out any gas work, including testing.

A joint I've just made is bubbling on the leak detection test. What do I do?

Turn off gas at the meter. Remake the joint: for compression fittings, this means disassembling and replacing the olive if possible, or tightening further (one quarter-turn at a time). For threaded joints, remove and re-apply PTFE tape or jointing compound. For soldered joints, resolder (drain and dry the pipe first). Re-test the joint after any repair.

How do I test for gas leaks in concealed pipework?

An electronic gas detector is the most effective tool for locating leaks in inaccessible pipework. Run the probe along the suspected route of the pipe (along skirting boards, through inspection hatches). The detector will alarm as it approaches the source. Once the general area is identified, if access is needed, open up carefully — do not use power tools near a suspected gas leak (spark risk).

Regulations & Standards