Saunier Duval Boiler Repairs London

Saunier Duval Boiler Repair Help Centre

Common Saunier Duval faults, fault codes, safe checks, and when to call a Gas Safe engineer in London.

Jump to:
Fault Codes ·
Ignition Faults ·
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 (see guide below). More detail in our complete pressure guide.
2. Note the fault code: Saunier Duval boilers display F-codes (F28, F22, F75, etc.) similar to Vaillant. Write it down or take a photo 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 clock changes or dead batteries.
5. Freezing weather? The condensate pipe may be frozen (see winter section below).
6. Reset once only: Press the reset button, wait 60 seconds. If it locks out again, stop and call an engineer.

Saunier Duval Boilers in London

Saunier Duval is a French boiler manufacturer, now part of the Vaillant Group. While more common in continental Europe, Saunier Duval boilers are found in some London properties, particularly in homes with European connections or where installers sourced from European supply chains.

The good news: because Saunier Duval is part of the Vaillant Group, many components are shared or compatible with Vaillant parts, and technical support is available through the Vaillant network.

Saunier Duval Boiler Fault Codes

Note: Many Saunier Duval models use F-codes similar to Vaillant. If your code format looks different, tell us your model when you call.

F28: Ignition Failure

What is happening: The boiler is trying to ignite but cannot establish a stable flame after multiple attempts. This is the most common Saunier Duval fault code.

Common causes: Frozen condensate pipe (winter), gas supply interruption, faulty ignition electrode or lead, blocked flue, gas valve fault.

What to try: Check if your gas hob or other gas appliances work. In freezing weather, check the condensate pipe for ice. Reset once. If F28 returns, call an engineer.

F29: Flame Loss During Operation

What is happening: The boiler ignited successfully but the flame was lost during operation.

Common causes: Gas pressure fluctuation, dirty or worn flame sensor, flue blockage, wind affecting the flue terminal, intermittent gas valve issue.

What to try: Reset once and monitor. If F29 appears repeatedly, call an engineer for diagnosis.

F22: Low Water Pressure

What is happening: System pressure has dropped below the minimum operating level.

Common causes: Small leak in system, recently bled radiators, pressure relief valve discharge, expansion vessel needs recharging.

What to try: Top up pressure using the filling loop (see below). If pressure drops again within days, there may be a leak. Read our pressure guide for step-by-step instructions.

F75: Pump / Pressure Sensor Fault

What is happening: The boiler cannot detect a pressure change when the pump should be running, suggesting either the pump is not working or the sensor has failed.

Common causes: Pump stuck or failed, pressure sensor fault, air in the system, low system pressure.

What to try: Check the pressure gauge first and top up if needed. If pressure is fine and F75 persists, call an engineer. This often requires pump or sensor diagnosis.

How to Top Up Pressure (Saunier Duval)

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

Note: Most Saunier Duval boilers have a filling loop underneath or near the boiler. Some models have an integrated filling device.

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

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

If you need to top up frequently (more than once a month), there is likely a leak. Call us for a diagnosis.

Frozen Condensate Pipe (Winter)

In freezing weather, a frozen condensate pipe is one of the most common causes behind F28 faults on Saunier Duval boilers. This is often safe to resolve yourself.

Signs: Boiler locked out with F28, freezing temperatures outside, gurgling sound when boiler tries to start.

What to do:

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

Do not use a naked flame or heat gun on plastic pipework.

Full instructions: Frozen condensate pipe guide.

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

Why These Faults Are Common in London Homes

Hard water, older pipework, and London’s variable weather contribute to certain faults:

• F22 (low pressure): Older pipework and radiator valves can develop small leaks.
• F28 (ignition failures): Condensate pipes routed externally freeze during cold snaps.
• F75 (pump/sensor): Sludge and debris can affect pump operation and sensor readings.

Saunier Duval Boiler Models We Repair

We repair Saunier Duval gas boilers including:

• Thema / Themaplus: Common combi range
• Isofast / Isomax: Combi and system models
• Semia: Compact combi range
• Thelia: Older range
• Other Saunier Duval models: Including discontinued ranges

Saunier Duval: The Vaillant Connection

Saunier Duval is part of the Vaillant Group, which means:

• Parts availability: Many components are shared with or compatible with Vaillant parts
• Technical support: Access to Vaillant Group engineering knowledge
• Familiar fault codes: The F-code system is similar to Vaillant boilers

Engineers familiar with Vaillant boilers can usually diagnose and repair Saunier Duval units without difficulty.

Common Saunier Duval Boiler Problems

F28 ignition failures: The most common repair. Often caused by worn electrodes, ignition leads, or frozen condensate in winter.

Low pressure (F22): Common on all sealed system boilers. Usually a small leak or expansion vessel issue.

F75 pump/sensor faults: Can indicate pump wear, airlocks, or sensor failure.

Diverter valve problems: Hot water works but heating does not (or vice versa). Common on combi boilers as they age.

PCB issues: Less common but can cause random lockouts. Parts usually available through Vaillant network.

Repair or Replace?

Saunier Duval boilers can generally be repaired thanks to parts availability through the Vaillant network. The decision depends on age, condition, and repair cost.

Signs it may be time to replace:

• Multiple faults in the past 12 months
• Boiler over 15 years old
• Major component failure on an aging unit
• Efficiency noticeably declining (higher gas bills)

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.

Saunier Duval Boiler Servicing

Annual servicing keeps your Saunier Duval running efficiently and helps catch developing faults early.

We service all Saunier Duval models and provide a full report after each visit. Book your boiler service.

Renting? Landlords: Check This

Tenants: Your landlord is usually responsible for boiler repairs. Contact them first and keep written records.

Landlords: Stay compliant with our Gas Safety Certificate (CP12) service. See our London landlord checklist.

London Wide Coverage

We repair Saunier Duval boilers across all London boroughs including North, South, East, West, and Central London.

For coverage and service areas, see our London coverage page.

We Also Repair These Brands

What We Need From You (To Diagnose Faster)

• Boiler model: For example Themaplus, Isofast, Semia
• Fault code: The exact code shown on the display (F22, F28, F75, etc.)
• Pressure reading: (if visible) and whether it drops frequently
• Postcode: So we can route the closest engineer
• Gas supply check: Whether other gas appliances work (hob/cooker)

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