My daughter lives in a house which has a central heating system that is not a combi but does appear to be a pressure system.
The small jpeg image
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shows what appears to be some kind of gas canister which gives a BAR reading on the dial.
The red knob on the top seems to be able to be turned in each direction and I assume turns the pressure on and off though there is no obvious reaction.
Does the complete system fail if the pressure falls or does the pressure just improve performance.
That is the expansion vessel. Inside there is a rubber diaphragm with air behind it. As the water in the system expands with heating, it pushes the diaphragm and compresses the air.
The knob is the pressure relief valve. It is designed to open if the pressure exceeds a preset safety value - normally 3 bar. Turning this will let water out and below a certain pressure the boiler will probably not light. In general, a system should be pressurised to about 1.5 bar or thereabouts. This is done using the filling loop which is the braided hose snaking down the wall. After filling, the hose should be disconnected. Another reason for not turning the red knob is that there is a fair chance that you will cause it do develop a slow dripping leak, for example if a small particle etc. gets lodged in it or a scratch on the valve seat. Often, this is impossible to fix and necessitates replacement of the valve. In short, you don't want to turn the knob.
Check the system pressure, and if you are unsure whether inhibitor is in the system, then add some. A convenient way to do that is with Fernox Superconcentrate which comes in a cartridge for a mastic gun. You can remove the filling hose, attach an adaptor that comes with the material and squirt it in. Then add the hose back and adjust pressure up to the correct value.
If turning the knob on top causes no reaction then I would guess that the system is not pressurised? If there is no pressure then chances are air will be drawn into the system. Mine used to run at about 1 bar but I found the higher the pressure the quieter the system ran. As others might have said, fill it with that braided loop then disconnect it. You *should* periodically turn that valve to make sure that it releases the pressure although as it's not quite a critical as if it were also a pressurised HW cylinder.
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