The expansion vessel should be checked before restoring the system pressure. The expansion vessel has an air chamber separated by a rubber diaphragm. The air space is to allow space for the water to expand as it gets hotter, and the diaphragm keeps the air and water seperated. After a long time, air migrates through the diaphragm into the water. This means the expansion space gets smaller, and can disappear completely. Without the air space, the water pressure can increase until the pressure relief valve opens, providing it is not stuck.
To check the expansion vessel, you will need to find the pressure relief valve, usually found inside the boiler. The valve can be opened by twisting the top. Usually anti-clockwise, but some valves operate clockwise. Open the valve and allow the pressure gauge to fall to zero. Find the air valve on the expansion vessel. The valve looks like a car tyre valve. Momentarily press the centre pin on the valve, and check that air comes out. No air may come out, or water may come out. If water comes out, you will have to replace the expansion vessel.
Measure the air pressure using a tyre pressure gauge. The pressure can be set at 1 bar for most systems.
If the air pressure is low, you can pump it up using a car tyre pump. While pumping the air in, open the pressure relief valve so the water pressure stays at zero until the air in the vessel is at the correct pressure.
Then close the pressure relief valve and refill the water to the correct pressure, (around the same as the air pressure).
There is a chance the pressure relief valve will leak after this procedure, and will have to be replaced. This is unfortunate, but if the valve is not operated to check that it works, it could be stuck, and will not work when needed. If it does not work correctly, the expansion vessel could burst, leaving a lot of water in the wrong places.
To check the expansion vessel, you will need to find the pressure relief valve, usually found inside the boiler. The valve can be opened by twisting the top. Usually anti-clockwise, but some valves operate clockwise. Open the valve and allow the pressure gauge to fall to zero. Find the air valve on the expansion vessel. The valve looks like a car tyre valve. Momentarily press the centre pin on the valve, and check that air comes out. No air may come out, or water may come out. If water comes out, you will have to replace the expansion vessel.
Measure the air pressure using a tyre pressure gauge. The pressure can be set at 1 bar for most systems.
If the air pressure is low, you can pump it up using a car tyre pump. While pumping the air in, open the pressure relief valve so the water pressure stays at zero until the air in the vessel is at the correct pressure.
Then close the pressure relief valve and refill the water to the correct pressure, (around the same as the air pressure).
There is a chance the pressure relief valve will leak after this procedure, and will have to be replaced. This is unfortunate, but if the valve is not operated to check that it works, it could be stuck, and will not work when needed. If it does not work correctly, the expansion vessel could burst, leaving a lot of water in the wrong places.