Gas Water Heater Installation
42
Step 2: The Common Vent
A “combined” vent is a venting system for two or more appliances at one level that are
attached to a common vent.
Least Total Height
is the vertical distance from the highest appliance flue outlet in the system
to the top of the vent. This is one fixed dimension for any one system, regardless of the
number or placement of appliances in the system.
Connector Rise
for any appliance is the vertical distance from its flue outlet to the point where
the next connector joins the system.
Using the Common Vent Tables
Determining each vent connector size:
1. Determine the least total height for the system.
2. Determine the connector rise for each appliance.
3. Enter the vent connector tables (refer to 43 page) at the appropriate least total height.
Continue across to the right on a line for the first appliance Connector rise to the rating
plate BTUH rating (or the next higher rating) for that appliance. Read the connector vent
size for that appliance at the top of the column.
4. At the least total height, repeat the procedure for the connector rise and BTUH rating for
each appliance.
Caution
: Never use a connector smaller than draft hood outlet size
.
Determining the size of a common vent:
1. Add together all appliances BTUH input ratings to determine the total BTUH rating.
2. Enter the common vent tables (refer to 43 page) at the least total height.
3. Continue across to the right, stop at the first value which is equal to or greater than the
total BTUH rating.
4. Read the size of the common vent at the top of the column.
Caution
: Regardless of the table results, the common vent must always be at least as large
as the largest connector. If both connectors are the same size, the common vent must be one
size larger.
Intake
[Room Sealed Different]
Exhaust
[Room Sealed Same Area]
Exhaust
Intake
Condensate
Drain
Condensate
Drain
Blind
Blind
