These Halstead devices are fitted into the fan to detect the airflow, when a fan runs up to speed a volume of air is moved and this air is pulled across the venturies. The venturies connects the fan with the fan proving switch which is called the air pressure switch, they are connected by heat resistant silicone tubes which allows the air flow from the fan to operate the air pressure switch.
They are fit to older non condensing Halstead boilers that have 240V fans, these fans only have 1 speed. The newer energy efficient Halstead fans are driven by a DC voltage which means the fan speed can be controlled and different speeds can be achieved. In these boilers venturieses aren?t used or required.