And it needs to enter exactly where we intended it to. That is essentially what airtightness is all about. For proper combustion, a wood stove needs oxygen. In an airtight wood stove, combustion air enters the stove only through the openings specifically designed for it. Not through unwanted gaps or other openings. That may sound obvious, but it places high demands on the design, construction and the way a wood stove is built.
Why is airtightness important?
In a wood stove, the air supply is carefully controlled. Both the amount of air and the point at which it enters the firebox affect the fire and combustion. When additional air enters the stove through unwanted openings, this is known as false air or uncontrolled air. This airflow is difficult or impossible to control and can therefore affect combustion. The more airtight the stove is, the more closely the airflow inside the stove corresponds to what we intended during its development.
How is the airtightness of a wood stove determined?
Airtightness can be measured. This involves measuring the stove’s leakage rate: the amount of air that can enter the stove through unwanted openings at a given pressure difference. For the Type BF classification, this leakage rate is measured at a pressure difference of 10 Pa. The measured leakage rate is then guaranteed by the manufacturer. It is a technical classification, but one that clearly demonstrates how carefully a wood stove has been designed and built.
From Type B to Type C
Different classifications are used for the airtightness of wood stoves. From low to high, these are:
Type B – Basic
The appliance is connected to an external air supply for combustion. With this classification, unwanted or ‘false’ airflows can be identified. There are no strict requirements for the leakage rate.
Type BE
With Type BE, the appliance is also connected to an external air supply for combustion. However, there are no strict requirements regarding the amount of air that may enter the appliance through leakage.
Type BF
A Type BF appliance goes one step further. The stove’s leakage rate is actually measured at a pressure difference of 10 Pa and guaranteed by the manufacturer. Airtightness is therefore not only a feature of the design, but also a measurable and guaranteed performance.
Type C
Type C sets the highest requirements for airtightness. This classification specifies a strict maximum leakage rate of less than 0.025 m³/h per kW, and the appliance must be able to withstand air pressure. This makes this class particularly suitable for highly insulated and airtight homes.
Type BF at Leenders
More and more Leenders wood stoves are classified as Type BF. As of 1 August 2026, these are the SORENNE, LYNNE, BOXER PLUS, LOBBE, LOBBE COMPACT, MATS, KILO and DAYNA. This means that not only are the airflows carefully determined during the design process, but the airtightness of the stove is actually measured as well. To us, that fits perfectly with the way we want to build a wood stove. The air a fire needs should enter exactly where it is supposed to.