When 2 or 3 boilers are connected to a common supply and return header, what type of stop valve should be used?

The appropriate choice for use when 2 or 3 boilers are connected to a common supply and return header is the non-pressure return valve. This type of valve is designed to facilitate the return of condensate from the heating system back to the boilers without allowing backflow that could disrupt the system. It ensures that the condensate can return effectively while maintaining correct flow dynamics between multiple boilers.

In systems where several boilers share a common supply and return, it's crucial to avoid the presence of unrestricted reverse flow. A non-pressure return valve serves this purpose by allowing the return flow while preventing any backflow into the system. This helps in maintaining operational efficiency and reduces the risk of boiler thermal shock or damage that can arise if water circulation is improperly regulated.

Other types of valves, such as check valves, could potentially be used to prevent backflow but may not provide the same operational assurance in terms of pressure management within a multiple-boiler setup. Similarly, pressure-regulating and globe valves serve different functions that might not align as effectively with the requirements posed by a shared header system. Therefore, the non-pressure return valve stands out as the most fitting choice for this configuration.