For proper operation and efficient fuel consumption in oil, gas, and/or coal-fired heating appliances, draft must remain constant.
Field Draft Controls maintain consistent draft by counteracting the negative forces caused by changes in temperature and barometric pressure, as well as the effects of wind. When it is, combustion is more complete, fuels are utilized efficiently, and money is saved. How Draft Controls Work
Static pressure of the cool air (1) Illustration A exerts pressure on the outside of the furnace or boiler, the breaching, and stack. The pressure difference between the room air and heated gas (air) causes products of combustion (2) to flow (draft) through the unit and rise through the breaching and chimney.
Room temperature air (3) enters through the barometric draft control (4) in the precise amount needed to overcome the excess drafts caused by temperature variations, wind fluctuations, and barometric pressure changes.
Combustion of fuel is complete and the process is stabilized. The velocity of combustion gases through the heat exchanger is slowed so more heat is extracted. The unit operates more efficiently, reliably, and requires less maintenance. When to use a Draft ControlDraft 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 Burner
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 down-drafts. 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 down-drafts.
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. Type RCOil or Coal - Residential and Commercial
The Field RC is furnished as standard equipment on many leading brands of oil or coal-fired heating equipment. It is calibrated to allow for easy adjustment to the furnace or boiler manufacturer's specifications. Designed for settings from .02" to .08", this control is so sensitive that instrumentation should be used when adjusting the unit during installation.
Type MOil or Coal - Residential
The M control lends itself ideally to conditions requiring a great deal of stability and accuracy. Designed for settings from .01" to .1", the Field M Control is recommended for oil or coal-fired residential heating applications. The Type M features an infinitely variable screw adjustment, permitting an extremely fine instrument setting. The M employs side wings to control air direction with gate curvatures designed to compensate for differences in horizontal and vertical settings.
A double-acting control for gas-fired furnaces and boilers is widely used for conversion burner installations, gas draft-induced appliance operation with mechanical draft inducers, or sidewall power venters. It is also recommended for use on gas atmospheric appliances where a draft hood cannot be installed, and can improve combustion stability and draft on many gas atmospheric installations with venting problems. The MG-1 provides precise, accurate control of drafts st levels higher than permitted by a standard draft diverter, which is a frequent requirement with gas. Because it is double-acting, it opens out to relieve positive vent system pressures as low as .01". Draft adjustments using weights are simple and accurate from .01" to .1".
Type M+MG2Solid, Oil or Gas Commercial/Industrial
This is a series of compact, rugged, heavy-duty controls for use on large residential, commercial and industrial applications. The Field Type M+MG2 Draft Control provides precise draft regulators for solid fuels, oil, gas, or oil/gas appliances requiring only the simplest, on-the-job adjustments depending on which fuel is to be utilized. In a gas installation, the double-acting Type M+MG2 is specified instead of a draft hood to give the appliance the assistance of the chimney. In a duel fuel oil/gas or a gas-fired only application, use the M+MG2 as a double-acting draft control with the optional Field Thermal Switch accessory. Use the M+MG2 as a single-acting draft control for oil or solid fuel applications. For any installation with 10" or larger diameter vent pipe, specify our standard M+MG2 Draft Control, the unit that can be adapted to any fuel. The moving part (gate) rests on a long, thin, stainless steel knife edge which, in turn, is supported by self-aligning and self-cleaning bearings. When the heavy gauge gate moves, only the knife edge rests on the bearing for minimum friction and maximum sensitivity to draft changes.
Recommended Locations for Field Draft Controls
For gas-fired equipment, the preferred location of the control is on the bull head tee. This location provides maximum relief of downdrafts with minimum positive pressure. (See Figure 1, Diagram A-C)
With oil or solid fuels, the bull head tee is not recommended, so locate the control as shown. (See Figure 1, Diagram D-J) These locations are acceptable for gas units as well. Except on forced draft systems, locate the control as close as possible to the furnace or boiler, at least 12" beyond a stack switch on oil-fired units, and at least 18" from a combustible ceiling or wall..
Multiple Appliance Location Recommendations
Commercial and industrial furnaces and boilers are frequently installed in multiples. (See Figure 2) Use a draft control for each boiler located on the uptake between the smoke outlet and the breach (location A). When this uptake is too short to permit the installation of a control, locate a seperate control for each boiler on the main breach (location B). If neither of these locations is possible, use a single large control in the breaching between the chimney and the nearest boiler (location C). Where several units are vented into a common breaching, the most draft-critical should vent highest in, or be placed closest to the chimney. Incinerators should generally be placed farthest from, or vented lowest in the chimney. Choosing The Right SizeSimple rules of thumb to guide size selections:
- For most application, the draft control should be the same size as the flue pipe. In other words, use a 6" control for a 6" pipe.
- It the vent pipe or breach is square, use the round equivalent. For example - on a 14" x 14" breach use a 14" control. Little flow occurs in the corners of a square pipe, so its capacity is approximately the same as a round pipe of the same diameter.
- If the breach is rectangular or oval, compute its cross-sectional area and select a draft control having the same or a greater nominal cross-sectional area. A breach 14" high x 10" wide would have a cross-sectional area of 140 square inches. From Table 1, select a 14" control with a cross-sectional area of 154 square inches.
- Where a control larger than 32" is required, use more than one draft control with combined cross-sectional areas equal to or greater than that of the breach.
- When chimneys are of an unusual height, or if the draft to be maintained is either very high or very low, it is advisable to deviate from the rules of thumb outlined here and refer to the sizing chart.