2/2-way solenoid valves are used for switching, whereby in most cases the flow direction is kept constant or the inlet pressure at the valve is always greater than the outlet pressure. In special applications, this pressure difference may be reversed, thus changing the direction of flow. The response of a solenoid valve to a change in flow direction cannot be predicted across the board. In many applications, it is expected that a solenoid valve will not allow flow in the opposite direction and is therefore back pressure tight. Direct operated valves with a small nominal size are back pressure tight up to several bar. In the case of valves with a large nominal size, e.g. pilot-operated and positively operated valves from G1/2, sufficient back pressure tightness is no longer achieved in most cases and the valve becomes leaky or opens. There are several ways to prevent flow in the opposite direction:

1. Use of a pressure-compensated valve that relieves pressure both in and against the specified flow direction. Such valves are usually expensive and only available in smaller nominal sizes.
2. Use of a non-return valve in series with the solenoid valve to prevent backflow. The main disadvantage of this solution is that a certain minimum pressure is required in the flow direction to open the non-return valve, which can be a problem, especially in low-pressure applications.
3. Use of two solenoid valves that are connected antiserially and controlled simultaneously. With an antiserial circuit, one solenoid valve always ensures that backflow is effectively prevented when the system is switched off. The opening of both valves in the activated state also ensures that any pressure drop through the valve in the opposite direction is minimized. The following illustration shows such an interconnection using two antiserially switched, positively controlled valves.