This vent pipe extension is meant for use with Rinnai RCi series tankless gas water heaters. It is engineered for gas appliance venting.
 
For Rinnai RCi series tankless gas water heaters
Positive fit and lock
Two layers of pipe
Seals at every joint and expands and contracts with weather and temperature changes
No flammable, mess, harmful glues, cements, or solvents needed