How can a fitter control the rate of steam or heat output of a radiator?

A fitter can effectively control the rate of steam or heat output of a radiator by installing adjustable capacity air vents. These vents allow for the regulation of the amount of air that can escape from the radiator. By adjusting the vent, the fitter can control the rate at which steam condenses in the radiator, thereby influencing the heating output.

In systems where heat output needs to be adapted based on varying needs, having the ability to adjust the air vent capacity is crucial. This feature enables fine-tuning of the heating system to achieve optimal comfort levels and efficiency, as it allows for higher or lower steam flow based on demand.

Other options presented do not provide the same level of control. Manual shut-off valves can stop the flow of steam entirely but do not allow for any form of modulation; they can only be fully opened or closed. Fixed capacity air vents have a set venting capability that cannot be adjusted, which limits their usefulness in regulating output. Increasing the size of the radiator will increase the heat output but does not provide a means for controlling or modulating the output, which might not be suitable for varying heating requirements.