The boiler will not start when the pressure in the system is not high enough. For example: There are 6 radiant circuits and 5 of them have thermostatic valves switched off, a large resistance in the system will occur that may prevent the activation of the boiler. In this situation, the bypass valve will provide sufficient flow through the boiler and correct operation of the system. The valve is pre-set in order to work with minimal resistance. By adjusting the valve, you can lift the pressure barrier but we recommend that it is left at the factory settings.
How does the pressure differential bypass valve work?ebizuniverse