The cross-sectional area of a flue serving a solid fuel burning appliance must not be greater than what multiple of the cross-sectional area of the appliance flue collar?

The correct multiple of the cross-sectional area of the appliance flue collar is three times. This guideline is crucial in ensuring proper draft conditions and efficient operation of the solid fuel burning appliance.

When the cross-sectional area of the flue is too large relative to the collar, it can lead to insufficient draft, causing smoke and combustion gases to back up into the appliance or the living space. A flue that is excessively large can impede the natural upward movement of combustion gases, which rely on a difference in temperature and pressure to flow effectively.

Using three times the cross-sectional area as a maximum allows for adequate draft without compromising the efficiency of the system. It strikes a balance between ensuring proper ventilation and preventing issues related to draft failure. Therefore, adhering to this guideline is important for safety and efficiency in solid fuel burning appliances.