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 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 downdrafts.   

Draft pressure is critical to the design of the particular heating system and generally falls into four categories:

Ř                  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. 

Ř                  Power 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 barometric control. 

Ř                  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 and to allow for the exact amount combustion air to be introduced to the flame. 

Ř                  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.

 

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. 

To 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 tested.

 

While 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.   

Theoretically, 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.   

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 continuous negative pressure in the combustion chamber to allow for the additional introduction of air and fuel.

 

Generally Acceptable Draft Measurements

 

Type of Heating System

 

Overfire Draft

 

Stack Draft

 

Gas, Atmospheric

 

Not Applicable

 

-.02 to -.04 WC”

 

Gas, Power Burner

 

-.02 WC”

 

-.02 to -.04 WC”

 

Oil, Conventional

 

-.02 WC”

 

-.04 to -.06 WC”

 

Oil, Flame Retention

 

-.02 WC”

 

-.04 to -.06 WC”

 

Positive Over Fire

Oil and Gas

 

+.4 to +.6

or PMI

 

-.02 to -.04

or PMI

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. 

In situations where a barometric control has been installed and subsequent testing determines high draft levels still exist, additional barometric controls can be added.

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.

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 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 velocity of a certain volume of flue gases and excess air passing the sampling tip of the instrument.  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 the additional introduction of the proper fuel air mixture for combustion. 

Also, 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.

 

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