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Common Venting CAT II: Flue Sizing

Mechy82

Mechanical
Joined
Apr 22, 2025
Messages
1
Problem: What size vent is needed to vent 3 boilers?

Information:
(3) lochinvar Knight XL boilers 650,000 Btu.
  • All 3 units are inline and going to a common vent.
  • Natural draft application
  • Rise of each unit is roughly 6' and 20' lateral with 1 elbow 2 tees.

Installation Manual calls out common vent application move to a category II increaser size from 6" to 8". I believe this to be slightly undersized.

Attached design and manual for clarity.
1745333849045.png1745333877254.png
 
A common vent header for fired equipment is a BAD idea - always insist on individual vents.
 

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