Tall chimneys contain a taller column of warm, rising gases. The movement within the flue of this taller column of gases also increases the pressure difference (or draft) at the bottom of a chimney. The actual chimney height of the venting system is the critical factor, not the volume of gases contained in it.
The rule of thumb states that the total system, from the floor where the appliance is mounted, to the top, should never be less than 15 ft. Most normal installations exceed this.
Installations in cottages with shallow-pitch roofs may not. If draft problems are experienced with shorter systems, consider adding to the length to ensure adequate draft under all conditions.
Chimneys must be at least three feet higher than the highest point at which they contact the roof and two feet higher than any point within ten feet of horizontal distance from the chimney. These are minimum guidelines and may be increased if necessary.
Some draft problems have to do with inadequate gas temperature. They say that the flue should be the same size as the appliance flue collar. Venting systems that are over sized for the appliance they serve are common, partly because people used to think that bigger is better. Now it is clear that bigger is not better when it comes to sizing. A given volume of flue gas flows faster and has time to lose heat in a smaller flue than in a larger one.
Chimney height and other safety tips that should be considered in your project.