In Condensing Boilers, this component is placed between the primary heat exchanger and the drain line, and it prevents the exhaust gases from discharging into the condensate drain pipe.
Compared to the early models of Rapid Boilers, which featured a flow switch and thus required a certain water flow rate to produce domestic hot water, manufacturers have equipped modern boilers with a new, more precise mechanism for detecting DHW demand.
In Storage Boilers of any type, there is a reserve of sanitary water already at temperature and ready for use. The boiler keeps it hot, thus reducing ignition times and increasing the responsiveness of the domestic hot water system.
In the low-efficiency type C boiler, the air pressure switch detects the operation of the flue fan (exhaust fan). It consists of two tubes connected to the venturi ports of the extractor, functioning as pressure and vacuum sensors.