A boiler pressure gauge will usually sit between 1 and 2.5 bar when it is turned on. When the boiler is off the pressure should be between 1 and 1.5 bar. The boiler pressure level will rise when the boiler is working to heat a home or provide hot water.
Boiler pressure too high
Are you experiencing high boiler pressure? 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
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 is the only component other than the filling loop where the cold mains and the boiler water are close to each other. If the thin stainless steel of the plate heat exchanger corrodes through water from the higher pressure mains can 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, and 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 gets trapped in radiators they feel cold at the top but hot at the bottom. Bleeding the air from radiators causes the pressure in the heating system to go down.
If you bleed enough air from a radiator the boiler will show the low pressure fault code.
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 are on boilers to let out any excess pressure to avoid a dangerous situation.
They are designed to open at about 3 bar of pressure. Sometimes they open at a much lower pressure. This causes the pressure to drop.
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