Combustion Technology (Pty) Ltd
|Combustion||Perfect Combustion||Incomplete Combustion|
|Efficiency Types||Testing||O2 versus CO2|
|Three T's of Combustion||NOx||Draft|
|Flue Gas Test Procedures||Acceptable Test Results||Accurate Testing|
|Savings Potential||Training Centre|
As 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 varying degrees of draft control and allow dilution air to mix with the flue gases to reduce the potential for condensation. A single acting barometric control is designed for fuel oil fired equipment while a double acting barometric is only approved for gas-fired systems. The double acting control door swings both directions to relieve downdrafts. Both types of barometric dampers are capable of providing a constant over fire draft, which is necessary to insure combustion air intake remains consistent under varying chimney draft conditions.
Draft pressure is critical to the design of the
particular heating system and generally falls into one of four categories:
Ø Atmospheric 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.
Ø Power Burners have a mechanical blower, which
delivers combustion air to the flame, but also rely on a precisely controlled overfire
draft to maintain consistent combustion air intake. This
generally requires the installation of a barometric control.
Ø Balanced 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
positive pressure requirements vary widely. However, a precisely controlled negative draft in the stack
is still required to remove the products of combustion at a controlled rate and to
allow for the exact amount combustion air to be introduced to the flame.
Ø Forced 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
To check draft, a
digital/mechanical draft gauge or inclined manometer is necessary. As with combustion testing, draft sample locations
will vary depending on the type of equipment tested.
While it is of utmost importance to follow the equipment manufacturers recommended draft readings, typical overfire draft measurements are in the -.005 to -.02 Water Column Inch (WC) range on both oil and gas power burner systems.
Typically, when a -.005 to -.02 WC is measured over the fire, stack draft will be in the -.02 to -.04 WC range for gas fired power burners and -.04 to -.05 for oil fired burners.
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.
The 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 stable, continuous negative pressure in the combustion chamber to allow for the
controlled introduction of air and fuel.
of Heating System
to -.04 WC
to -.04 WC
to -.02 WC
to -.05 WC
to -.02 WC
to -.04 WC
Always check with the
manufacturer of a particular appliance to determine the recommended overfire/stack draft
In the past, many
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 and that even slight variations in stack draft may influence
combustion air intake.
In situations where a
barometric control has been installed and subsequent testing determines high draft levels
still exist, additional barometric controls can be added.
barometric controls generally require that a manual reset spill switch be
installed on the barometric and wired to shut the burner down in the event of an extended
period of backdrafting
However, keep in mind
that a spill switch will most likely only trip when a chimney is obstructed. If flue gas spillage is caused by depressurization
of the mechanical room , sufficient cold air may be drawn down the stack to dilute flue
gases sufficiently to not activate the spill switch.
Where 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
common venting residential fan assist (80%) furnaces/boilers with atmospheric hot water
tanks (as is allowed by many manufacturers), great care must be taken to insure the common
vent functions under the wide variety of operating conditions.
Forced draft 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 (generally 15 feet from the breech) 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.
Taking 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 conditions.
Note: A draft reading only measures the difference in pressure
between the inside and outside of the vent.
Combustion testing verifies that these gases are being drawn from the combustion chamber/heat exchanger as designed and at a sufficient rate to allow for additional introduction of the proper fuel air mixture for combustion.
Combustion and draft
testing also verifies that the heating equipment is operating as designed and
engineered by the manufacturer.
Also, 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.