the fuel/air mixture (A) is delivered to the boiler, it is necessary that an
equal volume of flue gasses (B) enter boiler breach area.
As such, the draft rate is critical in maintaining this balance.
Draft diverters/hoods and barometric controls are designed to provide
draft control and allow dilution air to mix with the flue gases to reduce the
potential for condensation. A single
acting barometric control is recommended for fuel oil fired equipment while a double acting barometric
is only approved for gas fired systems. Both
types of barometric dampers are capable of providing a constant over fire
draft regardless of poor or excessive chimney draft conditions.
The double acting control door swings both directions to relieve
Draft pressure is critical to the design of the
particular heating system and generally falls into four categories:
systems are very common and depend entirely on the slightly negative stack
pressure (due to the heated flue gases being lighter than air and naturally
rising) to safely exhaust flue gases to the outside, while at the same time
pulling in sufficient combustion air. These
systems have draft diverters or hoods located immediately downstream from the
heat exchanger which allow room air to be pulled in and mixed with the
products of combustion before entering the vent system.
Burners have a mechanical blower which delivers combustion air to the
flame but also rely on an even more precisely controlled natural draft to
maintain consistent combustion air intake and requires the installation of a
Draft boilers which are designed to operate under a positive pressure in
the combustion chamber generally have a breach damper (either manually or
automatically controlled) which maintains the boiler combustion chamber and
flue gas passageways under a positive pressure to maximize efficiency.
Manufacturers’ positive pressure requirements vary widely.
However, a precisely controlled negative draft in the stack is still
required to remove the products of combustion and to allow for the exact
amount combustion air to be introduced to the flame.
Draft systems also have a mechanical
combustion air blower but are designed for a positive over fire pressure
created, in part, by resistance to flue gas flow in the stack which also
operates under a positive pressure.
remember that the amount of positive or negative pressure in the firebox (over
the fire) of a gas fired system will influence the fuel input as well.
check draft, a mechanical/digital draft gauge or inclined manometer is
necessary. As with combustion
testing, draft sample locations will vary depending on the type of equipment
it is of utmost importance to follow the equipment manufacturer’s
recommended draft readings, typical overfire draft measurements are in the
-.02 Water Column Inch (WC”) range on both oil and gas power burner systems.
when a -.02 WC” is measured over the fire, stack draft will be in the -.02
to -.04 WC” range for gas fired systems and -.04 to -.06 for oil fired
units. Stack draft has been
commonly used to set up barometric controls and evaluate draft conditions,
however, it does not necessarily guarantee correct over fire pressure which is
actually the main factor influencing combustion air intake.
combustion air intake on atmospheric equipment is so diffuse that overfire
draft readings generally cannot be obtained.
Consequently, draft must be measured immediately downstream from the
draft hood and should be in the -.02 to -.04 WC” range.
This will insure that there is continuous negative pressure in the
combustion chamber to allow for the additional introduction of air and fuel.
Generally Acceptable Draft Measurements
of Heating System
to -.04 WC”
to -.04 WC”
to -.06 WC”
to -.06 WC”
manufacturers recommend barometric controls be installed only when high draft
conditions exist. However, field
experience has shown that almost all vent systems are capable of producing
excess levels of draft during certain periods of the year.
situations where a barometric control has been installed and subsequent testing
determines high draft levels still exist, additional barometric controls can be
multiple boilers are vented into the same chimney, attempts should be made to
balance the draft of each individual boiler with separate draft controls as
opposed to one draft control in the main breaching.
For example, in the following diagram, draft controls should be installed
in location A or B. A draft control
in location C would not provide for sufficient draft control of each
boilers run a positive pressure from overfire, through to the stack termination.
The entire system must be welded or otherwise sealed tightly to prevent
flue gases from escaping. Also, the
height of the vent termination must be limited to prevent a negative draft.
As the vent system is operating under a positive pressure, barometric
controls are not appropriate for installation on this type system.
advantage of the potential for most efficient and reliable operation requires
more extensive testing be completed to properly set up this type equipment,
particularly when multiple units are commonly vented. Each needs to be tested under all conceivable operating
remember that some manufacturers of barometric controls require that a spill
switch be installed on the barometric and wired to shut the burner down in the
event of an extended period of backdrafting.
The spill switch generally called for has a manual reset button.