Stoves in an airtight house: extractor hood, ventilation and negative pressure monitor
If the extractor hood runs while the stove is burning, flue gas can flow back into the room in an airtight house. Our explainer film shows why this happens and which two solutions reliably help – with chapters to jump to directly.
Modern houses are well insulated and built airtight. That keeps the warmth inside, but it has a downside: air that an extractor hood blows outside hardly finds its way back in through cracks and joints. This applies to every stove, whether wood-burning stove, tiled stove or fireplace.
What happens in an airtight house
A powerful hood in extraction mode quickly moves 600 cubic metres of air per hour outside. The same amount has to come back in. If it finds no way, the pressure in the room drops and negative pressure builds up. In an airtight house a strong hood easily reaches 60 pascals and more. A wood-burning stove, on the other hand, is usually designed for a chimney draught of only around 12 pascals. The chimney then becomes the easiest way in: the flue gas reverses, and flue gas containing poisonous carbon monoxide flows into the room. Carbon monoxide can be neither smelled nor seen.
Two solutions against negative pressure
- Window contact: A wireless contact reports whether the window is open. The hood is plugged into an adapter plug and only gets power when the window is tilted open. There are also controls with a sensor on the flue pipe: if the stove is cold, the hood may run; if it is warm, only with the window tilted.
- Negative pressure monitor: As soon as a fire is burning in the stove, it continuously compares the pressure in the flue pipe with the pressure in the room. If the difference becomes too small, it switches off all extraction devices via a contactor in the distribution board, i.e. extractor hood, bathroom fan and ventilation system. The stove then draws safely again.
Ventilation systems
A central ventilation system needs a negative pressure monitor. If the supply air fan fails there, the extract air keeps running and the negative pressure rises. With decentralised fans in the outside wall, too, a negative pressure monitor is the rule. A CO detector is a good addition, but it does not replace a negative pressure monitor.
Think about the cables early
For a negative pressure monitor to work, the stove location needs the right cables: a 5-core cable to the stove and separate conduits for the sensor line and the pressure measuring line, ideally up to just below the wall rosette. A wireless connection to the distribution board is also possible, but it is more expensive, and reinforced concrete ceilings weaken the signal. It is easiest if we plan this together with your electrician before the walls are closed.
Combustion air from outside is a separate topic
A duct that supplies the stove with combustion air from outside does not protect against the negative pressure caused by the hood. The film nevertheless shows the three ways to do it: from the cellar, through the outside wall or via an air-flue chimney. Even with room-sealed stoves we coordinate the stove, ventilation and hood with the chimney sweep beforehand.
We plan with you
Stove, ventilation and electrics belong in one joint plan. Talk to us, ideally before the cables are laid. You can book a consultation directly online.
Short answers to read up on can be found under Frequently asked questions.
