Modified Stack Venting

modified stack vent


Definitions:

modified stack venting means a stack venting arrangement where the stack vent above the connection of the highest stack vented fixture is reduced in diameter.

OBC section 1


Installation Requirements:

(7.5.2.1.) see also stack vent installation requirements

  1. The stack must be a soil stack of minimum 3" size and it must extend as a stack vent which is reduced in diameter.
  2. All modified stack vented fixtures must be located at the same floor.
  3. The maximum number of modified stack vented fixtures  is 6. The lowest fixture in a modified stack vented group is a water closet. The maximum number of water closets in a modified stack vented group is 2. The maximum number of other fixtures connected above the water closet is 4.
  4. Where  two water closets installed, they must be connected  at the same level  to the stack. All other fixtures must be connected directly and independently to the stack above the center line of water closets and the uppermost fixture must be connected to the vertical portion of the stack.
  5. Where there is only one water closet in a modified stack vented group, it may connect to the vertical stack or horizontal continuation of the stack and remaining fixtures upstream of the water closet must be connected directly and independently to the stack and the uppermost fixture  must be connected to the  vertical portion of the stack.
  6. The total number of fixture units  connected above the water closet is 8.
  7. Maximum size of a fixture drain connected above the water closet is 2".
  8. No siphonic trap connected above the water closet is permitted.


Size of the stack vent:

the size of the stack vent is calculated as a branch vent according Table 7.5.7.2.A.


sizing


Connection of the Stack Vent

The vent begins at the connection of the highest fixture to the soil stack. Diameter of the stack is determined according to Table 7.5.7.2.A.


reducing-TY


The stack vent in a modified stack vented group terminates in a vent stack above the flood level rim of the highest fixture connected to the soil stack.


connection


The connection may be done with a TY or a Y and 450 elbow.


TY




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