Revised 6/11/18 Copyright © 2018 Trade-Wind Manufacturing, LLC Proverbs 22:29 Page 11
5.
Roof Caps and Wall Caps
The roof cap or wall cap is the termination point of the venting system that allows the exhaust air to exit to the outdoors.
All sections of this fitting must have an equal or greater air path area than the ventilator’s discharge port. If any section
of the roof cap or wall cap is smaller than the ventilator’s discharge port, the entire ventilation system will lose efficiency
and the restriction will cause increased static pressure and decrease performance.
IMPORTANT:
Even though the intake side of the roof cap or wall cap may be properly sized, roof caps or wall caps with
built-in dampers must be made so that when the damper is fully open, the actual open area of the final air path is equal
to or greater than the discharge port of the ventilator. Any undersized portion of a roof cap or wall cap will cause
excessive static pressure that may result in rattling, vibration and air buffeting noises, as well as inadequate ventilation.
Do Not
use “goose-neck style” caps as they significantly restrict natural, heated air-flow.
Do Not
use caps with dampers
for BBQ Hoods.
Fig. 1
Fig. 2
The air exit opening on the roof cap used needs to be at or above 78.5 square inches. See illustrations above.
Never
use a
roof cap like the one shown in Figure 1. Instead use a style similar to the one shown in Figure 2.
6.
Attaching Duct to House Framework
The ventilation system should be attached to the framework in such a manner that the weight of the duct and fittings is
supported with no stress on the duct joints, fittings or on the ventilator. All ducting should be attached so as to avoid
any possible duct vibration from being transferred to the house’s framework.
7.
Duct Sizing Chart and Area Calculations
Ventilator Discharge Port
Types & Sizes
Duct Type Required
Duct Size
(in Square Inches)
Minimum Discharge Size
of Roofcap or Wallcap
Outside Opening
6” Diameter, Round
28.3”
6” Round Metal Duct
28.3”
28.3”
7” Diameter, Round
38.5”
7” Round Metal Duct
38.5”
38.5”
8” Diameter, Round
50.25”
8” Round Metal Duct or
6” x 9” Rectangular Metal Duct
50.25”
54”
50.25”
54”
10” Diameter, Round
78.5”
10” Round Metal Duct
78.5”
78.5”
3.25” x 10” Rectangular
32.5”
3.25” x 10” Rectangular Metal Duct
32.5”
32.5”
Formulas for Calculating Square Inches of Various Duct Sizes & Types
ROUND DUCT
RECTANGULAR DUCT
Radius
x
Radius
x
3.1416
=
Area (Sq.
Inches)
Width
x
Depth
=
Area (Sq. Inches)
x
x
=
x
=
The “radius” is one-half the diameter of a round duct, e.g.,
½ of a 10” round duct is 5”. 3.1416 is “Pi”, the “constant”
used when calculating the area of a circle.
8.
Best Smoke Capture Recommendations
Please see next
page for
drawing of
optimum
performance
roof cap.
