A boiler pressure gauge will usually sit between 1 and 2.5 bar when the heating is turned on and 1 to 1.5 bar when cold.
It is normal for the pressure to rise when the boiler is heating the radiators.
Boiler pressure too high
Is the pressure too high? This could be a result of over-filling, but if it occurs suddenly, it may indicate a system fault. It’s recommended to seek the assistance of a Gas Safe engineer if there are rapid or frequent changes in boiler pressure, as they can accurately diagnose and resolve the issue.
The most common causes of the pressure rising too high are:
- Accidentally over filling of the system
- Filling loop not turning off when closed
- Expansion vessel (pressure vessel) problems
- Plate heat exchanger leaking
- Slow pressure gauge
Accidentally over filling of the system
The pressure should be set between 1 and 1.5 bar when the boiler and radiators are cold. If the pressure is set too high when the boiler is cold it will go much too high when it is hot.
Filling loop not turning off when closed
When the filling loop valves are closed they should stop the flow of water into the boiler. Sometimes they don’t close fully and allow a slow trickle of water to pass through. This can slowly raise the pressure to the point where it’s too high.
Expansion vessel (pressure vessel) problems
The expansion vessel (also known as the pressure vessel is designed to keep the pressure in the boiler steady. If it is faulty or needs to be serviced then the pressure can rise too high. This is especially obvious when the heating is used.
Plate heat exchanger leaking
The plate heat exchanger has the cold mains water and the boiler water close to each other.
The thin stainless steel of the plate heat exchanger can corrode and pin hole. This allows water from the higher pressure mains to get into the lower pressure boiler.
This shows itself as a constant pressure rise even when the filling loop is disconnected.
Slow pressure gauge
This is quite common on older boilers. As you top up the pressure the pressure gauge doesn’t respond fast enough so you put too much pressure in.
The only solution to this is to replace the pressure gauge. This is a job for a qualified heating engineer.
Boiler pressure low
Most modern boilers will switch off and display a fault code when the boiler pressure is too low. But why is the pressure too low? These are the most common faults:
Boiler leaking
The pressure shown on the pressure gauge includes all the water held in the whole heating system. This includes the boiler, pipework, and radiators. It also includes the pipework and controls to a hot water tank if you have one.
Pipework leaking
The most common reasons for pipework leaking are:
- joints that have been poorly fixed,
- mechanical damage to pipework/fittings,
- corrosion eating through pipes.
Radiator leaking
Radiators leak from three main areas. The panels of the radiator are normally made of mild steel so can corrode. The valves
Bleeding radiators
When air collects in radiators it has to
Expansion vessel (pressure vessel) problems
The expansion vessel (also known as the pressure vessel is designed to keep the pressure in the boiler steady. If it is faulty or needs to be serviced then the pressure can rise too high. This causes the pressure relief valve to open which drops the pressure too low.
Boiler pressure relief valve
Pressure relief valves (PRV) in boilers are designed to open when the system pressure gets above 3 bar. They are a safety device that saves the boiler and radiators from the pressure going too high.
Once a PRV opens it very rarely closes properly. It is only held shut with a spring so any dirt or wear on the seals stops it sealing up.
If you see water coming from the metal pipe near your boiler it will be related to a fault with your expansion vessel, your filling loop, or your plate heat exchanger.
Boiler pressure increasing
When the boiler heats up the radiators the water inside the system expands which causes the pressure to increase. The pressure rising up to 2.5 bar is normal and nothing to worry about.
If you find the pressure goes above 2.5 bar or into the red on your pressure gauge then there is a fault with the boiler or the pressure was set too high initially.
The boiler pressure gradually increasing even after the pressure is drained is a sign that the filling loop is passing or the plate heat exchanger is damaged.
Boiler pressure switch
Many boilers know the pressure is at the right level through the use of a pressure sensor.
If the pressure is too low the boiler will display a fault which can only be cleared by increasing the pressure.
Unfortunately, over time the pressure sensors can get clogged giving various faults such as E119 or F75.
The sensors can be unblocked but it is normally better to replace them as the dirt can damage the sensor.
Boiler pressure vessel
Also known as the expansion vessel this component keeps the pressure inside the system steady. When it is faulty or needs to be serviced the pressure can rise too high.
Boiler pressure when off
When the radaitors are cold the pressure should be set between 1 and 1.5 bar. If it is too high it needs to be dropped by bleeding some pressure from the system.
If the pressure is too low then the system needs to be refilled.
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