How To Replace A Viessmann Vitodens 200 Expansion Vessel: Step By Step Guide

This guide explains everything you need to know about replacing an expansion vessel on a Viessmann Vitodens 200, including symptoms of failure, safety considerations, costs, and when boiler cover or servicing plans are the smarter option.

How To Replace A Viessmann Vitodens 200 Expansion Vessel Step By Step?

Replacing an expansion vessel on a Viessmann Vitodens 200 follows a precise process that must be carried out carefully.

Step 1: Isolate And Power Down The Boiler

Turn off the boiler at the control panel and isolate the electrical supply. Allow the system to cool fully before starting work.

Step 2: Drain The Heating System

Drain the boiler and central heating system using the drain-off valve until the pressure gauge reads zero. Open radiator bleed valves to speed up the process.

Step 3: Access The Expansion Vessel

Remove the boiler casing following manufacturer guidance. On the Vitodens 200, the expansion vessel is typically mounted internally at the rear or top of the unit.

Step 4: Remove The Faulty Expansion Vessel

Disconnect the vessel from the system connection and carefully remove it. Any remaining water should be collected safely.

Step 5: Fit The Replacement Expansion Vessel

Install the new Viessmann-approved expansion vessel, ensuring correct orientation and secure connections. The vessel must be pre-charged to the correct pressure, usually around 0.75–1.0 bar.

Step 6: Refill And Repressurise The System

Refill the heating system, bleed all radiators, and repressurise the boiler to the manufacturer’s recommended cold pressure.

Step 7: Test Boiler Operation

Restore power, fire the boiler, and monitor pressure stability while heating is running.

What Can Go Wrong If An Expansion Vessel Is Fitted Incorrectly?

Incorrect expansion vessel installation can cause more damage than the original fault.

If the vessel is undercharged, overcharged, or incorrectly connected, the boiler may continue to overpressurise. Poor sealing can lead to internal leaks, corrosion, and long-term component damage. In severe cases, the pressure relief valve may fail prematurely or the heat exchanger may be stressed.

This is why professional installation or boiler cover support is strongly recommended.

Is Replacing A Viessmann Vitodens 200 Expansion Vessel A DIY Job?

Replacing an expansion vessel is not considered a typical DIY task for homeowners.

Although the expansion vessel is not a gas-carrying component, accessing it requires partial disassembly of the boiler and working near sealed-system and electrical components. Incorrect installation can cause leaks, pressure instability, or internal damage.

For landlords, any work that affects boiler safety or reliability should be carried out by a qualified engineer to remain compliant with legal responsibilities and insurance requirements.

How Much Does It Cost To Replace A Viessmann Vitodens 200 Expansion Vessel?

Expansion vessel replacement costs vary depending on labour, accessibility, and whether the vessel is internal or external.

In the UK, replacing a Viessmann Vitodens 200 expansion vessel typically costs between £200 and £450, including parts and labour. Costs can rise if additional faults are discovered or system flushing is required.

Boiler cover plans often include this type of repair, making them a cost-effective option over time.

When Should You Consider Boiler Cover Or Servicing Instead Of DIY Repairs?

Boiler cover plans and boiler service plans should be considered before attempting complex internal repairs like expansion vessel replacement.

Replacing an expansion vessel involves working inside the boiler casing, depressurising the system, and handling pressurised components. For most homeowners and landlords, this carries risk, especially if the boiler is still under warranty or forms part of a rental property.

This is why many property owners choose to consider boiler cover plans from Boilers 2 Go, which provide access to qualified engineers, regular servicing, and protection against costly component failures like expansion vessels.

FAQs

Why Do Viessmann Vitodens 200 Expansion Vessels Fail?

Expansion vessel failure symptoms usually occur because the internal rubber diaphragm deteriorates over time.

The expansion vessel is designed to absorb pressure increases as heating water expands. When the diaphragm splits or loses its air charge, the vessel can no longer regulate pressure properly. As a result, boiler pressure rises rapidly when heating is on and drops sharply when the system cools.

On Viessmann Vitodens 200 boilers, this often shows up as repeated pressure loss, water discharging from the pressure relief pipe, or frequent boiler lockouts. Age, system corrosion, poor water quality, and lack of servicing all contribute to expansion vessel failure.

Can A Failed Expansion Vessel Damage Other Boiler Components?

Expansion vessel failure can directly impact other boiler parts if left unresolved.

Repeated overpressurisation stresses seals, valves, and the pressure relief system. Over time, this can shorten the lifespan of pumps, heat exchangers, and internal pipework. Addressing the issue early protects the entire heating system.

Can You Fit An External Expansion Vessel Instead?

Yes, external expansion vessels can sometimes be fitted if internal access is limited, but sizing and positioning must be correct.

Does Replacing The Expansion Vessel Affect Boiler Warranty?

Unauthorised or incorrect installation can affect warranty terms, which is why professional fitting is advised.

How Long Does Expansion Vessel Replacement Take?

Most replacements take between one and two hours, depending on access and system condition.

Can Low Boiler Pressure Be Caused By Other Faults?

Yes, leaks, faulty pressure relief valves, and filling loop issues can mimic expansion vessel symptoms.

Should Expansion Vessels Be Checked During Servicing?

Yes, checking vessel pressure is a key part of a proper annual boiler service.