This happens because the water in the heating system expands when hot and contracts when cold so on a sealed system or combi an expansion vessel is fitted the pipework in the boiler or outside the boiler. The expansion vessel is made in two halves and joined together in the middle with a rubber diaphragm. One side of the diaphragm has water in it and the other side has air. The air is given a pressure of ¾ to 1 bar or approx 14lbs/insq as the water expands the air is compressed and the gauge shows a small increase in pressure. If the air pressure is too low or no pressure because the tyre valve has leaked the air out, then when the water expands there is nothing to absorb the expansion since the diaphragm just gets pushed to the back of vessel, then the pressure climbs to 3 bar and the pressure relief valve (PRV) opens and lets the water out of boiler until pressure drops below 3 bar. Then system cools down and the pressure drops too low and boiler won?t fire until pressure is increased with filling loop; this makes the cycle start all over again. The first thing to do is remove the plastic cap covering the valve on the top of the expansion vessel and press in the pin to release some air the same as you would with a car tyre, if only air comes out stop releasing it and check the pressure of the air, if pressure is ½-1bar the its ok and it may be that the tube connecting the vessel to the boiler is blocked. The rubber diaphragm may have failed or split or the air it needs re pressurising back to 3/4-1 bar this is done with a foot pump when no pressure is in the system, his means bleeding a radiator of water until the water stops coming out and the gauge reads zero this MUST happen first, before using the foot pump. The tube that connects the boiler to the expansion vessel may be blocked with sludge. And cause the same symptom.