What do extractor fan and fireplace have to do with each other? Quite a lot, if you look at the physical effect. If the extractor hood and fireplace are operated at the same time, negative pressure can occur in the room. Since 2006, safety measures for the joint operation of fireplaces and ventilation systems have been mandatory in DIN 1946 Part 6 - 2006 to provide more safety.
What happens when the extractor hood and the fireplace are operated at the same time? Quite simple: The fireplace or the stove draws in its combustion air and leads the flue gases through the chimney into the open air. Under normal conditions, there is an atmospheric overpressure in an installation room. In and of itself everything is unproblematic.
An issue only arises when a mechanical "ventilation system" (extractor hood) operates in a closed and tight room. It consumes up to 1,000 m³ of air per hour into the open air without any possibility of letting the room air flow in. This creates a negative pressure of up to -40 Pa and thus a danger for the occupants and the fireplace.
The simplest and cheapest solution to protect people and the fireplace is a contact switch on the window. The switch only releases the function of the extractor hood when the window is tilted. Only then can the extractor hood serve its "air hunger" via the tilted window.
Another alternative with DIBt approval is the negative pressure safety control (USA) by BRUNNER. The function is very simple: When the stove or fireplace is "cold", the monitoring is set to sleep mode. But as soon as the temperature sensor reaches a fixed temperature, a negative pressure measuring cell begins to measure the difference between the negative pressure in the chimney and the positive pressure in the room. As a rule, this does not cause any malfunction. If a ventilation system or the extractor fan does not work as intended, a negative pressure for the burning fireplace is set and the source of interference is switched off for safety reasons.
The BRUNNER USA as a safety device thus ensures that toxic CO gases are not unknowingly draught into the room via the stove.
A contact switch on the window or the negative pressure safety control are indicated and useful to prevent or detect negative pressure. With the contact switch on the window, the window is set to tilt when the extractor hood and the fireplace are in operation at the same time. If you do not want this, especially in the winter months, you are better off with the negative pressure safety control.