Williams 509622A Owner's Manual & Installation Instructions page 9

If the furnace is installed in an area with another gas
appliance(s), the total input rating of all appliances must be
considered when determining the free area requirements
for combustion and ventilation air openings.
Ducts must have the same cross-sectional area as the free
area of the openings to which they connect. The minimum
dimension of rectangular air ducts must not be less than
3-inches in length or height.
WARNING: Danger of property damage, bodily
injury or loss of life. Even when a house meets
requirements for unconfined space with adequate air
infiltration, it is recommended that a fresh air intake be
installed to lessen the possible dangers from any future
changes on the home.
LOUVERS / GRILLES AND SCREENS COVERING FREE
AREA OPENINGS
If a screen is used to cover the opening(s), it must not be
smaller than 1/4-inch mesh. Use the free area of a louver or grille
to determine the size opening required to provide the free area
specified. If the free area is not known, assume a 20% free area
for wood and a 60% free area for metal louvers or grilles.
INFILTRATION AIR
If your furnace is in an open area (unconfined space), the air
that leaks through the cracks around doors and windows
may be enough for combustion and ventilation air. The doors
should not fit tightly. The cracks around windows should not
be caulked or weather stripped.
Spillage means air starvation. A fresh air duct or air intake
opening must be installed to provide air directly to the
furnace or other gas appliances.
If spillage exists or when the furnace is in a building of tight
construction where the windows and doors are weather
stripped, air for combustion and ventilation must be
obtained from outdoors or space open to the outdoors.
To determine if infiltration air is adequate, perform the
following checks:
1. Close all doors and windows. If you have a fireplace, start
a fire and wait until flames are burning vigorously.
2. Turn on all exhausting devices, i.e., kitchen and bathroom
exhaust fans; water heaters (gas and electric).
3. Turn on all vented gas appliances, i.e., heating equipment
(includes any room heaters), water heaters.
4. Wait ten (10) minutes for drafts to settle.
5. Check for draft hood spillage at each appliance. Hold a lit
match two-inches from the draft opening. (Figure 2).
A. No Spillage
If the match flame pulls toward draft hood, this indicates
sufficient infiltration air. Return exhausting devices and
appliances to the condition you found them.
B. Draft Hood Spills If there is spillage at a draft hood
(match goes out or flame wavers away from draft hood),
check for plugged flue connectors and chimneys. Repair
blockage and test again.
FIGURE 2 – DRAFT HOOD SPILLAGE
VENT PIPE
DRAFT HOOD
OPENING
(match or
smoke pen)
TYPICAL WATER
HEATER
If you have a fireplace, open a window or door near the
fireplace and then check for spillage. If spillage stops, do not
use the fireplace without a nearby window or door open until
you can supply fresh air by a permanent duct.
If you have kitchen and bathroom exhaust fans, turn them
off and check for spillage. If spillage stops, do not use exhaust
fans (circuit breakers for fans should be turned off) until you
can supply fresh air by a permanent duct.
Provide opening(s) having a total free area of one-square-
inch per 4,000 BTU/hr. of the total combined input ratings
for all appliances in the area. The required free area is shown
in Figure 3. This fresh air opening or duct must terminate at
a point not more than one-foot above the floor. It must have
at least one-square inch of free area for each 4,000 BTU/hr. of
input of all appliances in area.
FIGURE 3 – FREE AREA
Free Area in Square Inches.
Each opening is based on one square inch per 4,000 BTU/hr
Square inches
BTU/hr input
of opening
30,000
7.5
35,000 8.75
40,000
10.00
45,000 11.25
50,000
12.50
60,000 15.00
MONTEREY TOP VENT GAS WALL HEATER
VENT PIPE
DRAFT HOOD
OPENING
(match or
smoke pen)
WALL
FURNACE
Required number of holes
sill or header plates
1 - 1/2ʺ 2ʺ 3ʺ
7 4 2
8 5 2
9 5 3
10 6 3
11 8 3
13 8 4
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