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VZ-VZH-VZT-IOM (08-24) 1047121-A
Unvented Units
⚠ DANGER ⚠
• The use of optional outside combustion air is not recommended with unvented heaters. If exhaust
fans are used to supply ventilation air, an interlock switch must be used to prevent the heater
from coming on when the fans are off. This may be done using a pressure switch.
• Heaters installed unvented MUST BE interlocked with sufficient building exhaust.
• The heater may be installed unvented in accordance with building ventilation codes and with the Clearances section
if sufficient ventilation is provided in the amount of 4 CFM (US) or 3 CFM (Canada) per 1,000 BTUh firing rate.
• A turndown-type vent terminal (see Figure 18) with a screen and without a backdraft flap must be installed at the
exhaust end of the tube.
Figure 18. Vent Terminal for Unvented Unit
Vertical Venting
• Vertical venting must be installed in accordance with the Venting General Requirements, Vent Material
Recommendations, Vent/Combustion Air Duct Length Requirements, and Vent Terminal Requirements
sections.
• Refer to the Common Venting section for common venting (one vent for more than one appliance) requirements.
Table 13. Vertical Venting Specifications
Category I Category III
Unit sizes 40 and 60 Unit sizes 80–200
4-inch (10-cm) vent pipe 4-inch (10-cm) vent pipe
Not required to be gas tight—pressure inside vent system is
negative—leak test is not required
Vent shall not extend <2 feet (0.6 meter) above highest point where it
passes through flat roof of building
Length of horizontal portion of vent pipe must be <75% of vertical
portion of vent pipe (i.e., if vertical vent section length is 10 feet
(3 meters), horizontal vent section length must be ≤7 feet 6 inches
(≤2.3 meters))
Length of horizontal portion of vent pipe is >75% of vertical portion of
vent pipe (i.e., vertical vent section length is 10 feet (3 meters) and
horizontal vent section length is >7 feet 6 inches (>2.3 meters))
Horizontal portion of vent pipe can be Type-B double-wall or Type C
single-wall vent pipe with upward slope from heater of at least
1/4 inch (6.3 mm) per foot
Vent pipe must be single-wall corrosion-resistant (minimum
26-gauge)
Install field-supplied B-to-C adapter in place of vent adapter when
using Type B vent pipe for horizontal section
Continuous section of Type B vent pipe may be used only to pass
through roof or outside of building—not permitted to be used
inside space
Vent must terminate at least 5 feet (1.5 meters) above vent
connection on heater
Category l common vertical venting is not permitted 4-inch vent cap required for 4-inch single or common vent
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