What should be kept open when the boiler is warmed up?

Keeping the boiler vent open when the boiler is warmed up is essential for allowing safe and efficient operation. The boiler vent is designed to release excess steam or pressure that may build up as the water heats and converts to steam. This venting function helps to prevent pressure from reaching dangerous levels, which could potentially lead to equipment failure or hazardous situations.

Moreover, an open boiler vent ensures that any gases produced during operation can safely escape, maintaining a safe working environment. This also aids in the proper functioning of the boiler by allowing for adequate air supply, which is critical for combustion processes in certain boiler designs.

The other choices typically serve specific functions that might not necessarily pertain to the boiler's venting needs during operation. Keeping the flue damper open is crucial only during certain phases of operation to ensure proper exhaust of combustion gases. The condensate return valve is important for managing the return of condensate but does not affect the venting process. The safety relief valve should only be opened automatically under certain conditions if the pressure exceeds safe limits, rather than being manually kept open as part of regular operation.