Gas Safety Certificate: 5 Checks Landlords Can’t Miss (CP12)

What are the main checks included in a Gas Safety Certificate and why should landlords in London pay attention?

A Gas Safety Certificate, often called a CP12, confirms that gas appliances, pipework, flues and ventilation systems in rented properties are safe for use. Landlords across the UK are legally required to renew this certificate every 12 months. Without a valid CP12, serving a Section 21 notice is not legally possible and noncompliance with the Gas Safety (Installation and Use) Regulations 1998 may result in fines, loss of insurance cover or blocked evictions. For landlords managing homes across Harrow, West and South West London, staying on top of this requirement protects tenants and provides legal security.

If you’re wondering what is actually checked during a gas safety inspection, these five key components of the CP12 certificate provide the full picture.

The 5 Key Components of a Gas Safety Certificate

Gas Appliance Inspection: What exactly is checked?

Each gas appliance in the property must be tested for operational safety and performance. This includes the boiler, gas cooker and any gas fire. The engineer will examine:

• Flame appearance, colour and stability

• Pilot light behaviour and ignition response

• Wear, corrosion or discolouration on burners

• Casing damage, loose panels or heat stress

• Proper function of safety cut-off mechanisms

Any appliance that does not meet safety requirements will be marked as unsafe. A CP12 cannot be issued unless all appliances listed pass the inspection. In older homes across West and NW London, ageing boilers and neglected cookers often require repair or isolation.

The engineer will also inspect appliance flues for blockage, corrosion or signs of poor ventilation. If combustion gases are not dispersing correctly, servicing may be required.

For more details, visit our article on gas appliance testing and CP12 requirements.

Why do landlords need a CP12 certificate and when should it be renewed?

By law, any property with a gas supply must be checked annually by a registered engineer listed on the Gas Safe Register. The legal obligation falls under the Gas Safety (Installation and Use) Regulations 1998 and is enforced by the Health and Safety Executive (HSE).

This annual inspection is not optional. If your certificate expires or isn’t shared with the tenant, you cannot legally issue a Section 21 notice to regain possession of your property. This applies across all boroughs, including Harrow and SW London, where local councils are increasingly proactive in enforcement.

The certificate must be issued before a tenancy begins, and tenants must receive a copy within 28 days. Managing this renewal window helps maintain compliance and prevents issues with tenancy agreements.

Gas Pipework Tightness and System Integrity

Gas pipework must be structurally sound and leak-free. The engineer will carry out a tightness test using a manometer or calibrated pressure gauge. A gas drop during testing suggests leaks or poor sealing.

During the visual check, the engineer will look for:

• Surface rust or corrosion near joints

• Loose connections or insecure pipe clamps

• Disconnected sections or poorly supported pipework

These risks are common in older properties across NW London and must be corrected before the CP12 is considered valid. Gas pressure loss, corroded joints or exposed flexible connectors can compromise safety and lead to enforcement action.

For technical details, see this article on gas pipework integrity checks.

How are flues and ventilation checked during a CP12 inspection?

This part of the inspection confirms that harmful gases like carbon monoxide are safely expelled. The engineer will perform a flue flow and draft test, assess vent spacing and check for blockages.

They will inspect:

• Flue exit points for obstructions or buildup

• CO detector placement and condition

• Internal vents and air bricks for proper airflow

• Chimneys or external ducting for clearance

A blocked or damaged flue increases the risk of CO exposure, which is flagged immediately. This is especially relevant in compact flats across SW London or where older fireplaces have been retrofitted for gas.

Appliance Operating Pressure and Gas Rate

Gas appliances must operate within specified pressure and gas rate parameters. The engineer uses a pressure gauge and flow meter to measure stability.

Issues include:

• Low mbar levels causing ignition delays

• Regulator faults affecting flame stability

• Appliances drawing inconsistent or excessive gas volumes

Incorrect operating pressure can lead to performance failure or safety shutdowns. The engineer records all readings on the CP12, and any variance outside manufacturer standards will prevent certificate issue.

What should be included in a valid CP12 report?

To meet compliance and legal requirements, your CP12 certificate must include the following:

• Gas Safe engineer’s full name, ID and registration number

• Full address of the rental property

• Appliance test results, pass or fail outcome

• Notes on any corrective actions required

• Inspection date and expiry date for the next check

• Engineer’s signature and company details

• Confirmation that a tenant copy was issued

Digital certificates are accepted, but landlords must ensure all fields are complete. Incomplete CP12s are often deemed legally invalid during Section 21 proceedings.

For guidance on documentation, refer to our CP12 record and delivery checklist.

What mistakes make a Gas Safety Certificate invalid?

Several common mistakes can lead to certificate rejection and legal disputes:

• CP12 was issued after a new tenancy began

• Engineer did not sign the form

• Tenant never received their copy

• Expired certificate at time of eviction notice

In one recent Harrow case, a landlord lost possession rights after issuing a Section 21 with an expired CP12. The court ruled the eviction invalid.

Set calendar reminders or use property management software to track renewal dates. Doing so keeps your legal options intact and protects tenant safety.