When a new furnace system is constructed or an existing one modified, somebody must certify that the exhaust gases are safely transferred to the outside.
Normally, a chimney calculation is carried out for this purpose. EN 13384 is the European norm for chimney calculation which describes the calculating method for chimneys with single and multiple appliances.
During the chimney calculation, at least two conditions are always checked: the pressure and temperature requirement.
The compliance with the pressure requirement ensures that the pressure in the chimney is sufficient to safely transfer the exhaust gases to the outside.
In addition, the compliance with the temperature requirement ensures that the chimney in the long run is protected from damages caused by condensation or that the outlet does not freeze if the outside temperatures are very low.