When there’s not enough air to breathe
What happens when an extractor hood and a fireplace are running at the same time? Quite simply: the fireplace or stove draws in combustion air and directs the exhaust gases outside through the chimney. Under normal conditions, there is an atmospheric overpressure in the room, which is generally unproblematic.
The issue arises when a mechanical ventilation system (such as an extractor hood) operates in a closed and airtight room. It can extract up to 1,000 m³ of air per hour without allowing fresh air to flow back in. This creates a negative pressure of up to -40 Pa, posing a risk both to the occupants and to the fireplace.
Protecting fireplaces
The simplest and most cost-effective way to protect both people and the fireplace is by installing a contact switch on the window. The extractor hood only operates when the window is tilted open. This way, the hood can draw in the required air through the open window.
Another alternative with DIBt approval is the BRUNNER negative pressure safety switch (USA). Its function is very simple: when the stove or fireplace is “cold,” the monitoring remains in sleep mode. As soon as the temperature sensor reaches a preset temperature, a pressure sensor begins measuring the difference between the chimney draft and the room pressure.
In most cases, this does not lead to a malfunction. However, if a ventilation system or extractor hood does not operate as intended, a negative pressure affecting the burning fireplace is detected, and the source of the disturbance is automatically shut off for safety reasons.
The BRUNNER USA safety device ensures that no toxic CO gases are unintentionally drawn from the fireplace into the living space.
Conclusion
When an extractor hood and a fireplace are operated in the same room, safety measures against negative pressure are mandatory. The BRUNNER negative pressure safety switch (USA) measures the room pressure and automatically switches off the source of interference if negative pressure occurs.
Expert advice
A window contact switch or the negative pressure safety switch is required and useful to prevent or detect negative pressure. With the window contact switch, the window is tilted open when the extractor hood and fireplace are operated at the same time. For those who prefer not to do this, especially during the winter months, the negative pressure safety switch is the better option.
