How many screws should be used at each joint of connector pipes?

Using three screws at each joint of connector pipes is the recommended practice because it ensures a secure and stable connection that can withstand the thermal expansion and contraction that occurs during heating and cooling cycles. The additional fastening provided by the third screw helps to minimize the risk of disconnections or leaks, which can lead to dangerous situations, including the potential for carbon monoxide escape or fire hazards.

It's also beneficial from an installation standpoint, as having a greater number of screws allows for better alignment of the pipes and improved stability in the overall structure. This practice is part of ensuring the integrity and safety of the chimney system, contributing significantly to the effectiveness of venting exhaust gases from fuel-burning appliances.