Field Controls SWG-3 Product Manual page 30

TYPICAL NATURAL VENT SYSTEM
ROOF
CHIMNEY
A
B
HEATING
A
APPLIANCE
WHEN TO USE A DRAFT CONTROL
• Draft Inducers/Power Venters
With these devices, draft is increased or created, causing
fluctuations in air flow through the combustion chamber.
These fluctuations can be negated by the use of a
barometric draft control located between the draft inducer
or power venter and the furnace, boiler, or water heater it
services. Use a single-acting control for oil and gas-fired
equipment with a power vented system. Use a single acting
control for oil, and a double-acting control for gas-fired
equipment with a draft induced system.
• Power Burners
A power burner is designed so that a fan delivers negative
air pressure to the combustion chamber. A single-acting
draft control for oil maintains that negative pressure. A
power burner designed to burn natural or LP gas operates
in the same manner. While a draft hood (diverter) is often
used on gas units fired with an atmospheric burner, a
double-acting barometric draft control should be used for
furnaces or boilers fired with power burners.
• Forced Draft Burners
Forced Draft installed with a stack height in excess of 30'
will probably develop excessive natural draft, reducing
the amount of pressure within the furnace or boiler. A
barometric draft control will help eliminate this undesirable
stack action and permit the unit to be pressurized.
Technical Service: 800.742.8368 •
CHIMNEY CAP
CHIMNEY CAP D
ROOF
E
D
CHIMNEY
E
C
OUTSIDE
WAL L
B
C
OUTSIDE
HEATING
WAL L
APPLIANCE PRESSURE
A
fieldcontrols.com
DRAFT CONTROL
Illustration B demonstrates how pressure changes as air/
gases flow through the appliance (A), past the draft control
(B), through the vent pipe (C), and outside the building (D/E).
( + )
PRESSURE
( + )
B
A
( - )
B
C
( - )
ILLUSTRATION B
C
• Dual Fuel Appliances
Burners capable of burning either gaseous fuels or
oil should be equipped with a barometric draft control.
We suggest using a double-acting control on units where
fuels are frequently changed. The double-acting feature
is important for gas-firing appliances because it allows
spillage of combustion products in case of blocked flues
or downdrafts. To detect flue gas spillage on dual fuel
installation, a Field Thermal Safety Switch is recommended.
• Gas-Fired Appliances
Gas-fired furnaces and boilers generally require a
double-acting draft control. Like a single-acting control, it
opens inwardly to maintain a uniform draft. But, unlike a
single-acting control, it is also free to open outwardly to
spill the products of combustion, in case of blocked flues
or downdrafts. National codes often mandate the use
of a draft control. Usage is generally limited to furnaces
or boilers designed for use with power burners and
incinerators. Draft controls are generally used when oil-
fired units are converted to gas.
D
D
E
AREA OUTSIDE
BUILDING
E
AREA OUTSIDE
BUILDING
29