What is the consequence of having too small of a vertical vent when using an induced draft motor?

The consequence of having too small of a vertical vent when using an induced draft motor is that pressurization will form inside the vent pipe. An induced draft motor is designed to create a negative pressure in the combustion chamber, effectively pulling combustion gases through the heat exchanger and out of the vent. If the vent is too small, it can restrict the proper flow of these gases, leading to back pressure within the venting system.

This back pressure can cause gases to not exit the system properly, which may, in turn, force some of the exhaust gases back into the combustion chamber or other parts of the heating system. Such a situation not only impairs the system's ability to vent properly, but it can also result in unsafe conditions, as harmful gases may enter the living space. Ensuring that the vent size is appropriate is critical for maintaining both the safety and efficiency of the heating system.