Understanding your boiler’s pressure readings and expansion tank behaviour can save you both hassle and money. Many homeowners in Bordon and nearby areas like Alton, Whitehill, and Farnham notice their pressure gauge dropping but are unsure what it really means. This guide walks you through the essentials so you can carry out safe checks before deciding whether you need a Gas Safe engineer.
Quick checks you can do now
Before diving deeper, there are a few simple things you can look at straight away. These help rule out easy fixes and give you confidence about what’s happening in the system.
- Check the pressure gauge on the boiler display or control panel.
- Inspect visible radiator valves for leaks.
- Listen for gurgling or air movement in radiators.
- Feel pipework for unusually cold spots that suggest air pockets.
If any of these appear off, it’s worth continuing with the steps below to find out what’s going on.
What it means when boiler pressure keeps dropping
Your heating system relies on the right amount of water pressure to circulate heat effectively. Most boilers operate best at around 1.2–1.5 bar when cold and up to about 2.0 bar when hot. If the pressure keeps falling below 1.0 bar, the boiler may lock out or refuse to fire.
Pressure loss doesn’t always mean a major fault. In homes across GU35, GU34, and GU9, the causes are often simple: trapped air, small leaks, or an underperforming expansion tank.
How to read and interpret your boiler pressure gauge
Most modern boilers have a clear analogue dial or digital readout. Cold system readings below 1 bar usually indicate insufficient pressure, while readings consistently above 2.5 bar suggest over-pressurisation or expansion issues.
- Green zone: safe operating pressure
- Red low zone: system under-filled
- Red high zone: pressure too high (possible expansion tank problem)
Assess the reading both when the system is cold and once it has been running for 20–30 minutes.
How to repressurise your boiler safely
If the pressure is low, most sealed systems allow you to top up the water using a filling loop. Here’s a safe method to follow.
- Turn off the heating and let the system cool.
- Locate the filling loop, usually a silver braided hose beneath the boiler.
- Open the valves slowly until the gauge rises to around 1.2–1.5 bar.
- Close both valves firmly and restart the boiler.
Common mistake: overfilling. If the pressure accidentally rises above 2.5 bar, you may need to bleed a radiator to bring it down again.
Expansion tank basics and signs of failure
The expansion tank works as a cushion for the expanding water when your system heats up. If it isn’t functioning properly, the pressure will fluctuate dramatically.
- Pressure rises too high when hot then drops too low when cold.
- Boiler frequently needs topping up.
- Water escapes from the pressure relief pipe outside.
Expansion tank issues are common in older systems around Bordon, Liphook, and Alton, often due to a failed internal diaphragm or a loss of air charge.
How to check your expansion tank
You can carry out a basic assessment without tools.
- Turn the boiler off and wait for it to cool completely.
- Locate the expansion vessel (usually a red or grey tank).
- Press the Schrader valve briefly. Air means it’s likely OK; water suggests failure.
If water comes out of the valve, the diaphragm has almost certainly failed, and the vessel will need professional attention.
When to call a professional heating engineer
Some issues are safely manageable for homeowners, but others require a Gas Safe engineer, particularly if you’ve identified leaks, frequent lockouts, or you suspect expansion tank failure. Homes in Bordon and surrounding areas like Whitehill and Farnham often have older systems where professional assessment is the safest option.
- Repressurising does not hold for more than a day or two.
- Pressure climbs rapidly into the red zone when heating is on.
- Water is dripping from the pressure relief pipe.
- You suspect the expansion tank diaphragm has failed.
Next steps
If you need help diagnosing or repairing pressure or expansion tank problems, contact us at (01420) 558993.