Draft

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 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 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 pressure.

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 manufacturer’s 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.

Generally Acceptable Draft Measurements

Type of Heating System	Overfire Draft	Stack Draft
Gas, Atmospheric Fan Assist (80%)	Not Applicable	-.02 to -.04 WC”
Gas, Power Burner	-.02 WC”	-.02 to -.04 WC”
Oil, Conventional	-.01 to -.02 WC”	-.02 to -.05 WC”
Oil, Flame Retention	-.005 to -.02 WC”	-.02 to -.04 WC”
Positive Over Fire Oil and Gas	+.4 to +.6 or PMI	-.02 to -.04 or PMI

Always check with the manufacturer of a particular appliance to determine the recommended overfire/stack draft requirements.

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.

Manufacturers of 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 individual boiler.

When 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.