Sime Boiler Repairs London

Sime Boiler Repair Help Centre

Common Sime faults, fault codes, safe checks, and when to call a Gas Safe engineer.

We specialise in Sime boiler repairs across London.

Gas Safe registered engineers for safe diagnosis, repair and testing.

24/7 availability with London wide call handling and booking.

Jump to: Fault Codes · Ignition Lockout · Pressure · Frozen Condensate · Repair or Replace

Quick Checks Before You Call Anyone

These take 60 seconds and solve a surprising number of lockouts:

1. Check the pressure gauge: If below 1 bar, top up to 1.0 to 1.5 bar. More detail in our complete pressure guide.
2. Note the fault code: Sime boilers may show numeric or letter codes depending on model. Write it down before calling.
3. Check your fuse box: Boilers need electricity, a tripped switch is an easy fix.
4. Check thermostat and timer: Settings can change after power cuts or dead batteries.
5. Freezing weather? The condensate pipe may be frozen (see winter section below).
6. Reset once only: Use the reset button on the front panel once, then wait 60 seconds. If it locks out again, stop and call an engineer.

Sime Fault Codes

Note: Sime fault codes vary by model and age. If your code is not listed, call us with the exact code and model and we will advise the safest next step.

Ignition Lockout

What is happening: The boiler is trying to light but cannot establish a stable flame, so it locks out for safety.

Common causes: Gas supply interruption, frozen condensate pipe (winter), ignition components, airflow or flue issues, gas valve or PCB related faults.

What to try: Check if your gas hob or other gas appliances work. In freezing weather, check the condensate pipe for ice. Reset once only. If it locks out again, call an engineer.

Low Pressure

What is happening: System pressure has dropped below the minimum operating level and the boiler may stop to protect itself.

Common causes: Recently bled radiators, a small leak, pressure relief discharge, or the expansion vessel needing attention.

What to try: Top up using the filling loop and aim for 1.0 to 1.5 bar. If you need to top up frequently, there is likely a leak or a component issue. Use our pressure guide.

How to Top Up Pressure (Sime)

If you are not comfortable using the filling loop, stop and call a Gas Safe engineer.

Do not remove the boiler casing. If you cannot locate the filling loop easily, call an engineer.

1. Locate the filling loop (often underneath the boiler, a braided silver hose with one or two valves).
2. Ensure the boiler is off and cool.
3. Open the valves slowly, a quarter turn at a time.
4. Watch the pressure rise to between 1.0 and 1.5 bar.
5. Close both valves fully.
6. Switch the boiler back on.

Do not exceed 2.0 bar. If you overfill, stop and call an engineer.

Frozen Condensate Pipe (Winter)

In freezing weather, a frozen condensate pipe can cause ignition related lockouts on condensing boilers. This is often safe to resolve with the steps below. If you are unsure, or the boiler will not restart, call an engineer.

Signs: Lockout during freezing temperatures, gurgling when the boiler tries to start, repeated ignition failures.

What to do:

1. Locate the condensate pipe (white or grey plastic pipe running to an outside drain).
2. Pour warm water (not boiling) over the frozen section.
3. Reset the boiler once thawed.

Full instructions: Frozen condensate pipe guide.

If it keeps happening, we can reroute the pipe internally or add insulation to reduce future freezing.

Why These Faults Are Common in London Homes

Hard water, older pipework, and winter weather make certain faults more likely in London properties:

• Ignition lockouts: Winter condensate freezing can trigger repeated ignition failures.
• Circulation and overheating: Sludge and limescale can restrict flow, contributing to overheat protection faults.
• Pressure drops: Older systems can lose pressure after radiator bleeding or due to small leaks.
• Noise and kettling: Scale can cause kettling and overheating. See our guide to boiler noises.

Sime Models We Repair

We repair Sime boiler models including:

• Format: Widely installed domestic combi range
• Brava: Compact combi models
• Planet: System and regular boilers in older properties
• Dewy: Condensing models seen in some London homes
• Friendly: Older installations still in use

If you are unsure of your model, tell us what it says on the front of the boiler and we will identify it for you.

Common Sime Problems

Ignition faults: Repeated lockouts linked to gas supply symptoms, condensate freezing, ignition components, or airflow issues that need safe testing.

Low pressure: Common after bleeding radiators or due to a small system leak.

Poor circulation: Sludge, a tired pump, airlocks, or restricted pipework can trigger overheating and shut downs.

Fan and airflow faults: Airflow proving faults that need safe checks by an engineer.

PCB and sensor faults: Intermittent lockouts or inconsistent readings that need safe testing.

Repair or Replace?

Sime boilers typically last 10 to 15 years with proper maintenance. If yours is approaching that age and needs a major repair (PCB, fan assembly, heat exchanger), replacement may be more cost effective.

We give honest advice either way. Read more: Repair or replace? An engineer’s guide.

If you decide on a new boiler: New boiler installation.

Sime Servicing

Annual servicing helps keep your Sime running safely and efficiently. We service Sime models and provide a clear report after each visit.

We recommend servicing in late summer or early autumn, before the heating season. Book your boiler service.

Renting? Landlords: Check This

Tenants: Your landlord is usually responsible for boiler repairs. Contact them first and keep written records, especially if you have no heating or hot water.

Landlords: Stay compliant with our Gas Safety Certificate (CP12) service. See our full London landlord checklist covering gas safety, EPCs, and boiler maintenance.

London Wide Coverage

We repair Sime boilers across all London boroughs. See our London coverage area or find your local page:

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