Boiler Pressure Too High or Too Low? A London Homeowner's Guide
Incorrect boiler pressure is one of the most common heating complaints in London. This guide explains what the correct pressure is, how to adjust it and when to call an engineer.
What Is Boiler Pressure
Your central heating system is a sealed pressurised circuit. The pressure gauge on the boiler (or on a nearby pipe) shows the pressure of the water in that circuit in bar. Most modern combi boilers operate correctly between 1.0 and 1.5 bar when cold. Under 0.8 bar is low; over 2.5 bar is high.
Low Pressure: Causes and Fix
Low pressure is the more common issue and is usually caused by a small leak in the system (often undetected) or radiator bleeding. Symptoms include boilers that show error codes like F1, E119 or A1, or simply lock out and display a pressure warning.
How to repressurise a combi boiler:
- Make sure the boiler is switched off and cool
- Locate the filling loop — usually a braided flexible hose with two valves beneath the boiler
- Open both valves (quarter turn each) and watch the pressure gauge
- When the pressure reaches 1.2–1.5 bar, close both valves immediately
- Switch the boiler back on — it should reset
If you need to repressurise more than once a month, there is likely a slow leak. Book an engineer to investigate.
High Pressure: Causes and Fix
High pressure (above 2.5 bar) is usually caused by overfilling the system or a faulty expansion vessel. The boiler safety valve may discharge water when pressure gets too high — you may notice a small drip from a copper pipe outside the property.
To reduce pressure, bleed a radiator. Open the bleed valve until water (not just air) starts to emerge, then close it. Check the pressure gauge. Do not continue if pressure does not reduce — call an engineer to check the expansion vessel and pressure relief valve.
When to Call a Gas Safe Engineer
- Pressure drops within days or hours of repressurising
- Pressure will not stabilise in the normal range
- You see water dripping from the boiler safety valve pipe (external)
- The boiler error code returns after repressurising
Frequently asked questions
What is the ideal boiler pressure for a combi boiler?
Between 1.0 and 1.5 bar when the heating is cold. When the system is hot and running, pressure may rise slightly to 2.0–2.5 bar — this is normal.
Why does my boiler lose pressure overnight?
A boiler that loses pressure regularly has a leak somewhere in the system — this could be at a radiator valve, a pipework joint or within the boiler itself. A Gas Safe engineer can pressure test the system to find it.
Can I repressurise my own boiler?
Yes — repressurising via the filling loop is a standard homeowner task that does not require a Gas Safe engineer. Only if you need to open the boiler casing or work on the gas supply does the work require registration.
What does it mean when the boiler pressure relief valve is dripping?
The pressure relief valve discharges when the system pressure exceeds its safe limit (usually 3 bar). This means either the pressure is too high or the valve itself is faulty. Call a Gas Safe engineer.