Specification

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Draft
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.
• 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.
When to Use a Draft Control
A
B
C
E
D
E
CHIMNEY CAP
ROOF
CHIMNEY
HEATING
APPLIANCE
OUTSIDE
WALL
PRESSURE
AREA OUTSIDE
BUILDING
( + )
( - )
D
B
A
C
A
B
C
E
D
E
CHIMNEY CAP
ROOF
CHIMNEY
HEATING
APPLIANCE
OUTSIDE
WALL
PRESSURE
AREA OUTSIDE
BUILDING
( + )
( - )
D
B
A
C
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).
Illustration B
Typical Natural Vent System