I have a oil fired unvented heating system in my dwelling. when the central heating is switched on some rads are very noisy as if there is air in them, but when they are 'bleed' the do not relaese air. after a duration of time, some rads have a loud hissing/presure noise. the pressure gauge read 1.5 bar when cold and the pump is set at 3.
Well yes and no. Obviously the higher the pump setting, the higher the pressure of water in the feed pipe. If the lockshield valves on the rads are screwed down too far, the pressure increases, and the water makes a hissing/rushing sound as goes through the valve.
Try opening up a valve on one of the rads that is making a noise and see if it stops. Just be aware that it may unbalance the system, and some other rad may be starved of hot water.
The bit I don't get is when air in an unvented system is gone, it should stay gone.
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