Low boiler pressure is one of the most common issues experienced by UK homeowners and landlords, particularly during the colder months when heating systems work harder.
In this guide, you’ll learn why Viessmann boilers lose pressure, how to correctly and safely increase the pressure, and what specific causes you should investigate.
How to repressurise Viessmann boilers using the built-in filling loop (Standard Method)?
How to repressurise Viessmann boilers using the built-in filling loop is the most straightforward process for most boilers in the Vitodens range, especially the Vitodens 050-W, 100-W, and 111-W.
Step-by-step guide:
- Turn off the boiler
Switch off the boiler and allow it to cool slightly. - Locate the pressure gauge
On most Viessmann boilers, this will be on the display panel. Ideal pressure is 1.0–1.5 bar when cold. - Find the filling loop
Viessmann may use:- An internal filling loop lever system, or
- An external braided hose beneath the boiler.
- Open the taps or levers
Slowly turn or lift the levers so mains water begins flowing into the heating circuit. - Watch the pressure gauge rise
Stop when the gauge reaches 1.4 bar (never exceed 2 bar). - Close both levers/taps fully
This is vital—failure to do so is a common cause of overpressurisation. - Turn the boiler back on
Check for normal operation and reset any low-pressure fault code if displayed.
Why does my Viessmann boiler lose pressure?
Why your Viessmann boiler loses pressure is usually linked to water loss within the central heating loop. Pressure drops because water is either being released intentionally (bleeding radiators), escaping unintentionally (leaks), or being handled incorrectly (filling loop faults). Regular maintenance and timely troubleshooting can help prevent these pressure drops. If you encounter persistent viessmann boiler issues and solutions, it’s advisable to consult a professional technician to identify and rectify underlying problems. Keeping an eye on the pressure gauge and addressing any irregularities promptly can significantly extend the lifespan of your boiler. Additionally, understanding the various error codes displayed by your boiler can be crucial for effective troubleshooting. For instance, a ‘viessmann boiler f1 code explanation‘ often indicates an issue with water pressure that requires immediate attention. Familiarizing yourself with these codes can help you take proactive measures before more severe problems arise.
Causes of Low Pressure in Viessmann Boilers (With Fixes)
Below are the most common causes of pressure loss and how to fix each one. Each cause includes a dedicated “how to fix” section to keep the guide practical and logical.
What causes low pressure due to radiator bleeding?
What causes low pressure due to radiator bleeding is simply the release of trapped air from radiators, which allows water to escape the system. Every time you bleed a radiator, a small amount of water is lost, reducing overall system pressure.
How to fix low pressure caused by radiator bleeding
- Confirm pressure is below 1 bar.
- Follow the standard repressurising method above.
- Recheck radiators for cold spots to ensure no more air remains.
- Perform a final pressure top-up if needed.
What causes low pressure due to a leak in the central heating system?
What causes low pressure due to a leak in the central heating system is water escaping through pipework joints, radiator valves, towel rails, or even the boiler’s internal components such as the pump, heat exchanger, or pressure relief valve.
Common signs include:
- Visible drips under radiators
- Green/white crust on copper joints (sign of slow weeping)
- Pressure dropping daily or hourly
- Damp patches on walls or ceilings
- PRV discharge pipe dripping outside
How to fix leaks causing pressure loss
For visible leaks:
- Tighten loose radiator valve nuts using an adjustable spanner.
- Use PTFE tape on minor weeping compression joints.
- Replace worn TRV valve heads or lockshields.
For hidden leaks:
- Isolate sections using radiator valves to narrow down the leak location.
- Add a leak detection dye to the system (safe for most Viessmann models).
- Call a heating engineer for inaccessible leaks (behind floors, walls, or boilers).
Important: Never repeatedly repressurise a leaking system—it can damage the heat exchanger.
What causes low pressure due to a faulty expansion vessel?
What causes low pressure due to a faulty expansion vessel is the vessel’s inability to absorb system pressure changes. The expansion vessel is a diaphragm tank that holds air on one side and system water on the other. When the diaphragm fails or loses charge, pressure swings become extreme.
How to fix expansion vessel issues
- Check pressure behaviour:
- Pressure rises rapidly when heating is on → expansion vessel likely flat.
- A Gas Safe engineer should:
- Measure vessel charge with a Schrader valve tool.
- Recharge the vessel (usually to around 0.75–1.0 bar).
- Replace the vessel if the diaphragm has ruptured.
This is not a DIY fix.
What causes low pressure due to a stuck or faulty pressure relief valve (PRV)?
What causes low pressure due to a stuck or faulty PRV is the valve opening incorrectly and releasing water through the discharge pipe. Once a PRV has lifted once due to overpressure, it often fails to reseal perfectly.
How to fix PRV faults
- Check if the copper discharge pipe outside is dripping.
- The only remedy is PRV replacement by a Gas Safe engineer.
- PRV faults often occur because someone previously overfilled the boiler.
What causes low pressure due to incorrect filling loop installation?
What causes low pressure due to incorrect filling loop installation is either the loop being left slightly open, leaking from the loop, or wrongly connected hoses.
How to fix filling loop faults
- Ensure both levers/taps are firmly closed.
- Check washers in external hoses and replace if worn.
- Fit a new filling loop entirely if corrosion is visible.
How to Prevent Low Pressure in Viessmann Boilers
- Bleed radiators only when necessary and repressurise immediately afterward.
- Check system pressure monthly, especially in winter.
- Inspect radiator valves and connections annually for early signs of weeping.
- Get an annual service—expansion vessel checks are included.
- Avoid overfilling the boiler, as this stresses the PRV.
- Use inhibitor to protect against internal corrosion and pinhole leaks.
When You Absolutely MUST Call a Gas Safe Engineer
You must call a Gas Safe engineer immediately if:
- Pressure drops daily even after repressurising
- Water is escaping from inside the boiler casing
- The PRV discharge pipe is constantly dripping
- The boiler shows repeated F9, F22, or pressure-related codes
- You suspect a faulty expansion vessel
- You cannot safely locate the filling loop
Working on internal boiler components without certification is unsafe and illegal in the UK.
FAQs
Can I repressurise my Viessmann boiler while it is running?
No—always turn the boiler off before repressurising. Repressurising when the system is hot can give inaccurate gauge readings. Once the boiler has cooled down, you can safely proceed with the repressurising process to ensure optimal performance. If you encounter issues after repressurising, it’s helpful to know how to reset your viessmann boiler, which can often resolve minor errors and restore functionality. Regular maintenance and following these steps will help keep your system running efficiently.
What pressure should my Viessmann boiler be when the heating is on?
Typically between 1.5 and 2.0 bar, depending on system size. It will naturally rise when the heating is active.
Why does my pressure keep rising after I repressurise?
This usually points to an overfilled system or a flat expansion vessel, both of which require diagnosis.
Can cold weather cause boiler pressure loss?
Yes—pipe contraction, minor leaks widening, and frozen condensate runs can all contribute to pressure reduction.
Is it normal to repressurise a Viessmann boiler once or twice a year?
Yes. Occasional top-ups are normal. Frequent top-ups, however, indicate a deeper issue.
I’m Penny North, a home energy heating expert. My mission is to demystify new boilers and complex heating systems to help you achieve a warm, cosy home with lower energy bills and a smaller environmental footprint.