Gas Safe Registration (ACS) Core Gas Safety (CCN1) - Domestic Natural Gas Foundation Syllabus

What is the Gas Safe Register and what does it replace?

The Gas Safe Register is the official statutory body for gas registration in Great Britain, Isle of Man and Guernsey. It replaced CORGI in April 2009 as the registration scheme that legally must be held by anyone working on gas.

What is a Building Regulations notification and who notifies for registered gas engineers?

Notifiable work (e.g. installing a boiler or fire) must be notified to Building Control. Gas Safe Register operates a self-certification scheme: the registered installer notifies Gas Safe Register, who informs the local authority and issues a Building Regulations Compliance Certificate to the customer.

What is a Gas Safety Record (Landlord/CP12) and how often is it required?

A record of the safety check carried out on each gas appliance and flue in rented property. Landlords must have an annual gas safety check (every 12 months) by a registered engineer and provide the record to tenants.

What is a Benchmark commissioning checklist used for?

Benchmark is the industry standard installation, commissioning and service record (logbook) for domestic heating appliances; completing it validates the manufacturer's warranty and records that the appliance was installed and commissioned correctly.

List the factors that determine correct pipe sizing for a gas installation.

Gas flow rate/total load (kW or $\text{m}^{3}/\text{h}$), permissible pressure drop, length of pipe run (including equivalent lengths for fittings/bends), pipe material and bore, and gas relative density. The pipe must deliver the required volume within the allowed pressure drop.

What is the maximum permissible pressure drop across a domestic natural gas installation pipe, from meter outlet to appliance?

$1\ \text{mbar}$ ($1\ \text{mbar}$ maximum drop between the meter outlet and any appliance inlet) for natural gas, per BS 6891.

Name common gas pipework materials and one key requirement for buried or concealed pipework.

Copper, steel (mild/stainless), and polyethylene (PE, for underground service pipes). Concealed/buried pipe must be protected against corrosion (wrapped or sheathed) and, where in walls, suitably protected and not in cavities; PE must not be used inside buildings.

State the gas law relationship between volume, pressure and temperature (general gas equation).

$$\frac{P_{1}V_{1}}{T_{1}} = \frac{P_{2}V_{2}}{T_{2}}$$ where $P$ is absolute pressure, $V$ is volume and $T$ is absolute temperature in kelvin ($T(\text{K}) = ^{\circ}\text{C} + 273$).

What are the three stages of the IGEM/UP/1B tightness test procedure on a domestic installation?

1) Let-by test (check the emergency control valve is not letting gas by); 2) Tightness test (pressurise and check for pressure drop/leakage); 3) Purging. The system is tested at the appropriate test pressure with all appliances connected.

Describe the let-by test and what result indicates the ECV is sound.

With the ECV closed and the installation pressurised to test pressure via a gauge, leave for $1$ minute. The ECV is sound (no let-by) if there is no rise in pressure on the gauge during that minute; any rise indicates gas leaking past the valve.

For a new domestic natural gas installation, what test pressure and stabilisation/test times are used for the tightness test?

Raise to approximately $20\ \text{mbar}$, allow about $1$ minute stabilisation, then a $2$-minute test period. For a new installation (no appliances) the permitted pressure drop is zero; for installations with a meter/appliances a small permitted drop applies per IGEM/UP/1B tables.

What permitted pressure drop is allowed over the 2-minute test for an existing installation with a U6/G4 meter and appliances?

The maximum permitted pressure drop depends on installation volume; for a typical domestic U6 meter installation it is commonly up to $4\ \text{mbar}$ (and as low as $0\text{–}1\ \text{mbar}$ for new/no-appliance work). Always read the exact value from the IGEM/UP/1B tables for the installed volume.

What is purging and why is it essential after a tightness test?

Purging is displacing air with gas (or gas with air) to remove the air/gas mixture from the pipework. It is essential because a gas/air mixture in the flammable range is explosive; purging ensures only gas (or only air) remains, eliminating the dangerous mixture before commissioning.

What is the function of the Emergency Control Valve (ECV)?

The ECV is the main valve, fitted at the end of the service pipe (usually before/at the meter), that allows the consumer to shut off the entire gas supply to the premises in an emergency. It must be accessible and clearly marked with on/off positions and ideally an attached emergency notice.

What are the two main types of domestic gas meter and their typical maximum flow rating?

Diaphragm (positive displacement) meters such as the U6/G4 are most common in homes, with a typical maximum rating around $6\ \text{m}^{3}/\text{h}$. Ultrasonic/rotary types are also used. The meter measures volume of gas consumed for billing.

What is the standard nominal working (operating) gas pressure for a domestic natural gas appliance at the burner?

Around $20\ \text{mbar}$ at the appliance inlet/governor for natural gas (commonly $\pm$ tolerance per manufacturer). Distribution governor maintains the supply so that approximately $21\ \text{mbar}$ is available at the meter outlet.

Distinguish standing (static) pressure from working (dynamic) pressure.

Standing pressure is the pressure measured with no gas flowing (all appliances off). Working pressure is the pressure measured while gas is flowing (appliance operating). Working pressure is normally slightly lower than standing pressure due to flow losses.

What instrument measures gas pressure on site, and in what unit are domestic readings normally taken?

A manometer (water column or electronic/digital manometer), connected to a test point. Domestic pressures are read in millibars ($\text{mbar}$); $1\ \text{mbar} \approx 10.2\ \text{mm}$ water gauge.

What is gas rating, and what is the formula to determine an appliance's gas rate from the meter for natural gas?

Gas rating measures the actual volume of gas an appliance burns over time to verify its heat input. Time the gas used over a known volume, then $$\text{Gas rate}\ (\text{m}^{3}/\text{h}) = \frac{3600}{t}\times V$$ where $t$ is seconds for volume $V\ (\text{m}^{3})$ to pass through the meter.

How do you convert a measured gas rate (m³/h) into appliance heat input in kW for natural gas?

$$\text{Heat input (kW)} = \frac{\text{Gas rate}\ (\text{m}^{3}/\text{h}) \times \text{CV}\ (\text{MJ/m}^{3})}{3.6}$$ Using a CV of about $38.7\ \text{MJ/m}^{3}$. (Dividing $\text{MJ/h}$ by $3.6$ gives kW.)

An appliance burns $0.05\ \text{m}^{3}$ in $120$ seconds. Calculate its gross heat input in kW (CV $= 38.7\ \text{MJ/m}^{3}$).

Gas rate $= \frac{3600}{120}\times 0.05 = 1.5\ \text{m}^{3}/\text{h}$. Heat input $= \frac{1.5\times 38.7}{3.6} \approx 16.1\ \text{kW}$.

What is the difference between a service valve and a control valve in a gas installation?

A service (isolation) valve is fitted close to an appliance to allow it to be isolated for maintenance without shutting off the whole supply. A control valve regulates or shuts off gas as part of the appliance's operation (e.g. a multifunctional gas valve, thermostat or solenoid).

Why is ventilation required for gas appliances, and what is the basic air requirement principle?

Ventilation supplies the combustion air needed for complete combustion and removes products, preventing CO build-up and oxygen depletion. Air supply must match the appliance heat input; open-flued appliances need permanent air vents sized to the appliance rating (per BS 5440-2).

Per BS 5440-2, what is the free-air ventilation requirement for an open-flued appliance above 7 kW net input?

An open-flued appliance requires permanent ventilation of $5\ \text{cm}^{2}$ of free air per kW of net heat input above the first $7\ \text{kW}$ (i.e. no vent needed for the first $7\ \text{kW}$, then $5\ \text{cm}^{2}/\text{kW}$ thereafter).

Define and contrast an open-flued appliance, a room-sealed appliance and a flueless appliance.

Open-flued: takes combustion air from the room and discharges products via a flue to outside. Room-sealed: takes air from and discharges products to outside through a sealed system, isolated from the room. Flueless: discharges products into the room (e.g. cooker), relying on room ventilation.

What is the difference between a natural-draught (Type B) and fanned-draught (fan-assisted) flue?

Natural-draught flues rely on the buoyancy of hot flue gases (the stack/chimney effect) to draw products up and out. Fanned-draught flues use a fan to mechanically push or pull combustion products through the flue, allowing horizontal/longer runs and smaller flue sizes.

Describe how to carry out a flue flow test and a spillage test, and what each confirms.

Flue flow test: with the flue cold, use a smoke pellet/match at the appliance position to confirm smoke is drawn up and clears the flue (no spillage), proving the flue is clear and pulls. Spillage test: run the appliance (after warm-up, with doors/windows shut and extract fans on), hold a smoke match at the draught diverter/canopy edge; smoke must be drawn in, confirming products are not spilling into the room.

In combustion analysis, what is the significance of the CO/CO₂ ratio and what is the maximum acceptable value for a correctly burning appliance?

The CO/CO$_2$ ratio indicates combustion quality; a low ratio means efficient, complete combustion. The maximum acceptable ratio for a safe domestic appliance is $0.004$ ($CO/CO_2 = 0.004$, i.e. $4:1000$). A higher ratio indicates incomplete combustion and a potential CO hazard.

What immediate actions define the gas emergency response if a gas escape is suspected indoors?

Do not operate electrical switches or naked flames; open doors and windows to ventilate; turn off the gas at the ECV; extinguish naked flames and do not smoke; evacuate if necessary; and call the National Gas Emergency Service on $0800\ 111\ 999$.

Why is carbon monoxide especially dangerous, and what are its properties and key early symptoms?

CO is a colourless, odourless, tasteless toxic gas. It binds to haemoglobin forming carboxyhaemoglobin roughly $200\text{–}250$ times more readily than oxygen, starving the body of oxygen. Early symptoms mimic flu: headaches, dizziness, nausea, breathlessness, tiredness and collapse — fatal at high exposure.

Under the Gas Industry Unsafe Situations Procedure (GIUSP), what are the two classifications of unsafe situation and what action does each require?

Immediately Dangerous (ID): presents an immediate danger to life or property — the appliance/installation should be turned off and, with permission, disconnected and labelled; if permission is refused, RIDDOR/Gas Emergency action follows. At Risk (AR): a fault that could cause danger but is not immediately dangerous — turn off with the customer's permission and advise; remedial action recommended. (A third category, Not to Current Standards, is recorded but is not unsafe.)