ACS Tightness Testing & Purging 3

Question 1

What is the purpose of a let-by test on a gas installation?

Accepted Answer

To check for leakage past the closed Emergency Control Valve (ECV)

Answer

A let-by test checks whether the Emergency Control Valve (ECV) is sealing correctly or allowing gas to pass when in the closed position.

Question 2

How is a let-by test correctly performed on a domestic gas installation?

Accepted Answer

Close the ECV and check for a pressure rise on the downstream side using a manometer

Answer

With the ECV closed, a manometer connected downstream will show a pressure rise if gas is passing through (letting by), indicating a faulty valve.

Question 3

During a let-by test, what does a pressure rise on the downstream side of the closed ECV indicate?

Accepted Answer

The ECV is allowing gas to pass through when in the closed position

Answer

A pressure rise downstream of the closed ECV indicates the valve is not sealing properly and is allowing gas to pass through, known as letting by.

Question 4

At what stage of checking a gas installation should the let-by test be performed?

Accepted Answer

As the first check, before the tightness test

Answer

The let-by test is performed first to confirm the ECV seals correctly, which is the essential foundation for all subsequent tightness testing.

Question 5

If a let-by test reveals that the Emergency Control Valve (ECV) is faulty, what is the correct action?

Accepted Answer

Notify the gas transporter, as the ECV is their responsibility to repair

Answer

The ECV is the property of the gas transporter (network operator), so any defect must be reported to them for repair — it is not the installer's responsibility.

Gas Safe ACS Practice Test

Gas Safe ACS Practice Test

ACS Tightness Testing & Purging 3