12
Indoor Installations
Installations with Ambient Combustion Air Supply
In this type of installation, duct furnaces will draw their combustion air from the space surrounding the heating unit.
Combustion air openings must be provided in the access panel to the burner vestibule area with openings properly sized as
outlined in Section V. Installation must be in an unconfined space with sufficient volume for all appliances located in the
space. An unconfined space is a space containing not less than 50 ft.
3
(1.42m
3
) per 1,000 Btuh (0.2928 kW) input rating for
all appliances installed in that area. A single, properly sized pipe from the duct furnace vent connector to the outdoors must
be provided.
Furnaces may be vented vertically or horizontally as shown in Figures 6 and 8
.
Vent pipe diameter and maximum vent length
are determined by furnace input rating as shown in Table 2
.
Flue gases must be directed downward.
Table 2- Vent Pipe Diameter and Maximum Vent Length
The maximum vent length is the equivalent linear length of pipe. The equivalent length for 3” and 4” elbows is 6’.
Figure 6 – Vertical One-Pipe Vent
a) Side Discharge
Model
Size
Input (Btuh)
Vent Pipe Diameter
Max. Vent
Length
Diameter
Material
090
90,000
3"
PVC
70'
135
135,000
3"
PVC
70'
180
180,000
4"
PVC
70'
225
225,000
4"
PVC
70'
270
270,000
4"
PVC
70'
315
315,000
4”
PVC
50’
360
360,000
4”
PVC
50’
400
400,000
(2) 4”
PVC
70’
450
450,000
(2) 4”
PVC
70’
540
540,000
(2) 4”
PVC
70’
630
630,000
(2) 4”
PVC
50’
720
720,000
(2) 4”
PVC
50’
