Safety Notice
Only Gas Safe registered engineers are legally permitted to open, service, or reset the inside of a gas boiler.
This guide is strictly for qualified professionals working on-site who need a quick refresher on how to access Max Mode for testing and diagnostics.
For homeowners: never attempt to open or adjust your boiler. Always contact a Gas Safe engineer.
How to Reset a Viessmann Gas Boiler
To safely reset a Viessmann gas boiler and enter Service (Max) Mode:
1️⃣ Turn the control dial to “RESET” (position 6 on the dial).
2️⃣ Wait until the screen displays “SERV”.
3️⃣ Within 2 seconds, turn the dial back to the dot (home position).
This sequence activates Service Mode, allowing the boiler to run at full power for diagnostic testing.
Understanding Max Mode
Max Mode (also known as Service Mode) is used during professional maintenance to force the boiler to operate at maximum output.
It helps engineers:
- Test heating performance under full load
- Check radiator heat distribution after a power flush
- Verify burner and fan operation during servicing
- Record output readings for compliance documentation
Dial Settings Reference
| Dial Position | Function |
|---|---|
| 6 | Maximum Output |
| 1 | Minimum Output |
Using these settings allows accurate testing during performance checks or commissioning.
Important Safety Reminder
Never attempt to open or reset a gas boiler unless you are Gas Safe registered.
Unauthorized tampering is dangerous and illegal in the UK.
For homeowners in Surrey and London, always book a qualified Gas Safe engineer through Certificates4UK Ltd for:
- Boiler servicing and breakdown repairs
- CP12 Gas Safety Certificates
- EICR and EPC compliance checks
Contact Certificates4UK Ltd
Need a reliable Gas Safe engineer for boiler repairs, safety checks, or certification? We’re here to help.
We cover:
Molesey • Walton • Woking • Cobham• Weybridge • Sunbury-on-Thames • Surrey • London
Before replacing any parts or booking costly work, let one of our qualified engineers carry out a professional inspection to save you time and money.
Booking Options:
- 🌐 Book Online – Simple, quick, and available 24/7
- 📞 Call: 07596 076700 or 07869 520044
- 📧 Email: info@certificates4uk.co.uk
📍 Proudly serving Molesey, Surrey, and London, we specialise in:
Boiler repairs, Gas Safety Certificates (CP12), Installation, plumbing, EICR testing, EPC inspections, and full property compliance.
⭐ Established in 2014, C4UK has been providing professional heating and property compliance services across Surrey and London. 160+ five-star reviews, Certificates4UK Ltd is the trusted choice for safe, professional, and cost-effective heating services.
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