This system have its circulating water pressure at the highest point is near atmospheric pressure because feed and expansion tank open to atmosphere. An open central heating system must have a small water tank called feed and expansion tank at the highest point in the house. Usually, the F&E tank located in the loft or roof space by the side of or above large cold water storage tank. The Expansion vessel may or may not be directly connected to a water supply. It should be about 1/3 full so that it can accommodate the expanded water. There is always air dissolved in the water without it the fish can not survive! When the water is heated in the boiler the dissolved  air will separate and try to escape. This air will find its ways out through the F&E tank as it is open to the atmosphere. As air escapes water from the tank will fill that space. The water in F&E tank will be used to supply (feed) the required to fill for amount of air that escaped from the heating system. Hence there must be an arrangement to replenish the lost water. So that there will be always cold water available in the F&E tank. more 

Filling Point: Normally, the cold water supply is taken from the house cold mains supply via a no return valve. The filling point is connected to the cold water supply by a filling loop, while filling and must be removed after filling to the required pressure. Most Combination Boiler systems are fitted as closed systems.

By-pass Valve: When the main circuit is closed or restricted, the by pass or the bypass valve will ensure water still to circulate through the heat exchanger, preventing the boiler from over heating with water. If there is a by-pass-valve the pump's pressure will open the by-pass-valve and provide flow through the heat exchanger.

Expansion Vessel:  The vessel has a flexible neopren (synthetic rubber) diaphragm inside separating the water and Nitrogen. The nitrogen is pre charged at about 0.7 to 1 bar is on one side of the vessel. When the water in the system is heated it will expand and the diaphragm is forced out of shape into pressurized part of the vessel to accommodate the increased volume of water.

Pressure Relive Valve: When water temperature is above 100C, it expands very rapidly and by becoming steam it can exert very high pressure. In a closed system, it can cause pipe work to burst or leak at joints. The pressure relief valve prevents damage by releasing the hot water safely. This is a SAFETY device and should be properly fitted and serviced. It is spring loaded to be normally closed and opens only when the system pressure exceeds its preset value, about 3 bar. Domestic central heating boilers