The chimney draft

Whether a chimney "pulls" or not depends on some physical conditions. What plays a role and how to regulate the chimney draft, explained in detail this post. In addition also some remedy measures, if the chimney once simply does not pull.

Requirements for a sufficient chimney draft

Through the chimney effect warm air rises in the cold air of the chimney upwards, causing a natural suction effect. This suction - an upward negative pressure - "pulls" the exhaust gases through the chimney.

Important factors for this physical effect are:

• the exhaust gas temperature
• the chimney dimensions
• the direction of the temperature difference

exhaust gas temperature

The upward flow (natural convection) in the chimney is caused by density differences in the air. The warm air coming from the stove is lower in density than the colder air in the chimney and outside. This increases the warm air inside the colder (also in the fireplace) upwards.

If the exhaust gas temperature is too low, or if the exhaust gases cool down too quickly on the way through the chimney, this flow decreases with decreasing exhaust gas temperature until it breaks down and the exhaust gases therefore no longer rise upwards.

Sense of three-shell (insulated) chimneys

Especially with low temperature heating systems, this risk exists if the way through the chimney is too long, or there is too much heat loss of the exhaust gases on the way through the chimney.
For this reason, modern chimneys are often designed with three shells with an insulation. This prevents excessive cooling of the exhaust gases and maintains the convection upright.

fireplace dimensions

The chimney must be properly sized in size to provide enough "pull". If the clear width is too large, the pressure of the upward flowing, warmer masses of air on the exhaust gases and they are no longer completely sucked up.

Conversely, if the width is too small, the exhaust gas pressure inside the chimney may theoretically become too large, but in practice this is rarely the case.

The dimensions of the fireplace must therefore always be matched to the respective heating system, the exhaust gas temperature and the exhaust gas pressure.

Direction of the temperature difference

On very hot summer days, it may happen that the sun warms the top of the chimney so much that the air in the chimney head is warmer than the exhaust gases. She then presses down and squeezes the fumes back into the oven. In this case, the convection has reversed.

The only possible remedy here is to ignite a paper fire in the cleaning area of ​​the furnace connection in order to reverse the temperature difference. Officially this is prohibited - but it is the only effective measure.