Indoor installation and venting. Positive and Negative Pressure Venting Systems.
The heater is designed such that it may be vented using either a negative-pressure or a positive-pressure venting system. The appropriate system of venting for a particular site will depend on many factors such as vent termination needs (horizontal/vertical), and the cost of venting system. Table 2 lists the indoor venting kits available and the limitations of each system.
Heaters come stock with outdoor top. These tops are for all indoor venting applications.
Negative or Positive?
Negative is for a vertical or straight up or almost straight up vent pipe where the heat rise creates lift and minimal condensation will form in the pipe.
Positive is for a more horizontal application venting application where the heat rise is less and there is a chance of more condensation in the pipe that will cause rusting and damage.