Guide to Barometric Dampers & Draft Regulators on Oil Fired Heating Equipment
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Draft Regulators & Barometric Dampers: A Guide to Barometric Dampers on Oil Fired Boilers, Furnaces, Water Heaters: inspection, adjustment, cleaning, troubleshooting
How automatic draft regulators work
How barometric draft controls are set and adjusted
Draft inducers & draft boosters for low-draft problems
Definitions of draft regulator, automatic vent damper, automatic duct damper, automatic fire dampers, & draft inducers
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Here we discuss Draft Regulators or Barometric Dampers and we provide A Guide to Barometric Dampers on Oil Fired Boilers, Furnaces, Water Heaters: inspection, adjustment, cleaning, troubleshooting
This website answers most questions about central heating system troubleshooting, inspection, diagnosis, and repairs. We describe how to inspect
residential heating systems to inform home owners, buyers, and home inspectors of common heating system defects.
The articles at this website describe the basic components of a home heating system,
how to find the rated heating capacity of an heating system by examining various data tags and components, how to recognize common heating system operating or safety defects, and how to save money on home heating costs.
We include product safety recall and other heating system hazards.
Details about draft control on oil fired heating systems (such as the oil fired heater shown in the photo above), including furnaces or boilers, are discussed right here at Draft Regulators barometric dampers on oil fired equipment. (Details about draft control for gas fired heating systems, including furnaces or boilers, are discussed at Furnace Draft Hood on gas fired equipment. Readers should also see CHIMNEY INSPECTION & REPAIR GUIDE and Flue Vent Connectors - Boilers, Furnaces for inspection, defect identification, and repair suggestions for chimneys and flues.
Guide to Inspecting Barometric Dampers or Draft Regulators on Oil Fired Heaters, Furnaces, Boilers, Water Heaters
Barometric dampers are devices used to regulate the draft on oil-fired heating equipment such as furnaces, boilers, or
water heaters. The barometric damper or draft regulating device we are discussing here is normally used only on oil-fired heating equipment, not on gas-fired equipment. The equivalent draft control on gas fired heating systems is discussed at Furnace Draft Hood on gas fired equipment.
On oil fired equipment the barometric damper, or draft regulator is typically a round Tee inserted in the flue vent connector between the heating appliance and the chimney. The face of the tee contains a round "door" with an adjustable weight.
The service technician adjusts the weight to control the swing or opening of this door which in turn controls the amount of excess air that can enter the flue and chimney when the oil burner is operating.
The draft regulator in our photo is not working - and has been deliberately jammed - indicating that something else is wrong, probably a chimney problem or an oil burner operating problem: the technician was unable to get enough draft, so s/he defeated the regulator - this is not a good idea, as we'll explain below.
Why we need the Barometric Damper or Draft Regulator on a Boiler or Furnace
During oil burner operation, air moves into the oil burner and combustion chamber (as combustion air)
and continues onwards as combustion gases moving out of the
combustion chamber, up through the boiler or furnace heat exchanger and on to the chimney where these gases are vented outside.
The force with which this air or combustion gas moves is the "draft" inside of the heating appliance.
Where Do We Measure Chimney Draft & How Much Draft do we Want at an Oil Burner Fired Heating Boiler, Furnace, or Water Heater?
Normally we measure
draft at two locations: over the fire or in the combustion chamber where typically we may see -0.02 to -0.03 inches of water column pressure,
and in the breech or at the stack pipe (properly, the flue vent connector) measured just a few inches above the boiler or
furnace top, and before the barometric damper itself.
Notice that we're using negative numbers for draft measurement - that's because gases in the flue are moving up, up, and away, like superman, and onwards out of the building - away from the heating equipment. The gas pressure in the chimney needs to be less than atmospheric pressure in the boiler room for gases to leave.
This sketch of a barometric damper used on oil fired heating equipment (heating boilers or water heaters) is provided courtesy of Carson Dunlop.
In the breech we want to see about -0.05 inches WC pressure. If the
breech draft is too low the combustion process and venting process may be inadequate, and if the draft measured in the
breech is lower than the draft measured over the fire, the oil burner and combustion chamber are operating under
pressure - which is often a problem on residential heating systems since few of them are designed to work this way. Thanks to L. Steinke for correcting our WC pressure data.
We do not measure draft in the flue vent connector past the barometric damper since the damper is regulating the draft and we're not seeing what the oil burner is seeing at the fire. Lots of companies make draft gauge measurement devices, including Bachrach™, and including nice little pocket units that anyone can carry.
In sum the draft we typically see on oil fired heating equipment is
- 0.02" to 0.03" water column (w.c.) in the combustion chamber just over the fire
- 0.04" to 0.06" w.c. in the breech - the flue pipe area between the top of the boiler and the bottom of the barometric damper. Some oil burner models require higher draft than these numbers, and other oil burner models are actually tolerant of back-pressure in the combustion chamber (positive draft, or draft in the "wrong" direction").
In the heating industry, traditionally draft measurements around -0.02" w.c. are considered "low", and around-0.06" w.c. are "high" draft levels.
How a Barometric Draft Control Works
It's simple, as Carson Dunlop's sketch above shows. The service technician measures draft over the fire and in the breech, and she moves a little weight on the hinged barometric damper door to cause the door to open wider or less wide to let more or less room air into the chimney as needed.
As long as the gas pressure inside of the flue and chimney is less than room air (that is, it's "negative" as we explained below), air from the room wants to enter the chimney through the barometric damper opening. Like the Goldilocks and the Three Bears, there are three possibilities:
The draft as sensed at the barometric damper is too strong (maybe a wind is blowing over the chimney top, increasing the draft): in this case the draft inside the flue, at the breech at the draft regulator is more negative - maybe -1.2 " w.c.. Since the air in the room is at normal pressure it will be stronger than the pressure inside the flue, so air in the room will push the draft regulator door "in" and make it open, letting room air flow into the chimney until, balanced by the setting of the weight on the moving draft regulator's door, the incoming room air enters the flue in enough volume to drop the draft pressure back to its desired setting, maybe to -0.04" w.c.
The draft as sensed at the barometric damper is too weak (maybe a wind is blowing down the chimney flue because we left off our chimney cap): in this case the draft inside the flue, at the breech and thus at the draft regulator is less negative, maybe -0.01" w.c. than we wanted - we're having trouble sending those flue gases up the chimney. The little weight on the draft regulator door causes the door to close, reducing the inflow of room air into the flue, and thus increasing the draft in the flue back to the desired number, maybe to -0.04" w.c. once again.
The draft as sensed at the barometric damper is just right: it's hovering where we set it at -0.04" w.c. In this case the barometric draft regulator is probably showing its little door a little bit open, with a little room air flowing into the flue. (Otherwise we wouldn't have any room to close the door to increase the draft when we need to do so.) And the draft in the flue is staying at the desired number, maybe -0.04" w.c.
How is the Barometric Draft Control Adjusted?
By moving a weight along a scale. You can see a weight and scale in our photo of the Field Type AF Draft Control.
While the heating equipment is operating at normal temperature, the draft is set to a number specified by the oil burner manufacturer, so we can only give approximate settings in this discussion. To find the proper weight setting to control the draft regulator, the heating service technician will make three measurements:
draft over the fire
draft in the breech
CO2 measurements (which tells us how complete is the heating oil combustion process) - adjusting the draft affects the rate of combustion air movement into the combustion chamber.
It is the position of the weight along a moveable scale, usually by screwing the weight in or out, or by sliding the weight along a scale (see our photo), that adjusts how far the draft regulator door will open in response to these three conditions described above. It's basically a principle of leverage -the weight is moved closer to or farther out from the axis of rotation of the moving draft regulator door.
So do not change the barometric draft control's weight setting unless you're a trained service technician who knows when, where, how, and why to measure draft at an oil fired heating appliance.
Why a draft regulator is needed:
While above we described how we measure draft inside of heating equipment and on the way to the chimney where (we hope) combustion gases are to be vented safely outside, the "draft" that the oil burner and furnace or boiler experience are not constant.
For example wind blowing over a chimney
top can increase draft, as can a second appliance using the same chimney as the heater. Since the force of draft is not normally
constant, and since we want the draft to be constant for optimum oil burner operation, the barometric damper is installed.
If the oil burner sees flue draft that is too low the combustion gases will not vent safely out of the building and the heating equipment may suffer from backpressure in the combustion chamber, causing overheating or other malfunctions.
If the oil burner sees flue draft that is too high combustion gases will vent out of the building just fine, but we're sending too much heat up the chimney by moving combustion gases too fast through the heater, thus we're sending our oil dollars up the chimney as heat rather than into the building as heat.
The service technician adjusts the barometric damper to maintain a continuous draft in the range we described above. Then if
local conditions change, the barometric damper can open or close to let in more or less additional air into the flue and chimney,
keeping the draft constant.
Are Barometric Draft Controls Used on Gas-Fired Equipment?
Incidentally, draft controls might be found on gas-fired heating equipment too, but the specifications are quite different. Gas fired heaters such as domestic gas fired furnaces are usually designed to operate at very low over-fire drafts - which means almost zero draft will be measured at the flue vent connections. That's why you usually don't see a hinged-door barometric draft regulator on gas fired equipment. Take a look at DRAFT HOODS - gas fired for details.
Field Corporation, and surely other draft regulator producers, provides different model draft controls for gas fired equipment, such as the Field Type MG1 and MG + MG2 regulators which use double swinging gates that open inward under normal up-draft conditions and outward in case of blocked flues, thus relieving internal pressures. Since improper venting of gas fired appliances easily produces very dangerous, potentially fatal Carbon Monoxide (CO), it is critical that these appliances are vented properly.
Do not ever install an oil-fired appliance draft regulator such as the Field Type AF shown here onto gas-fired equipment.
List of Defects & Signs of trouble with a barometric damper or draft regulator and what they mean
No draft regulator or barometric damper is installed: in this case the heater may work but it is impossible to tune it for optimum performance. We may be wasting fuel and money.
The barometric damper is shared: if the damper is connected so that it is shared by two different heaters, say an oil fired furnace and an oil
fired water heater, it is impossible to tune the system for optimum performance since the two appliances will prefer different adjustments each.
The barometric damper is broken or missing parts: it cannot do its job and needs repair or replacement.
The barometric damper is stuck or has been wired shut or covered with foil or tape: we love this clue. This is what a service tech does when
s/he simply has been unable to get adequate draft for the oil burner. The tech thinks that since draft is always inadequate on this system
there is no reason to let any air in at the damper. Well that's true, but we're treating the symptom, not the cause. The root cause might
be, for example, a blocked chimney flue - which is unsafe and needs to be discovered and cleared. We've also see this silly "repair" when the
underlying problem leading to inadequate draft was that someone had left the cleanout door to the chimney open.
The barometric damper is stuck in the "open" position - usually due to a mechanical defect that is easily repaired, but sometimes because the weight has been misadjusted or lost. An always-open damper cannot properly regulate draft.
Barometric damper location: a barometric damper will work properly to regulate draft and reduce backdrafts in a variety of locations in relation to the heating appliance and the chimney. [Images at left are courtesy Field Controls.]
Field corporation and other equipment manufacturers provide an instruction sheet with their product, showing the appropriate locations for a draft regulator. According to Field,
"The control should be located as close as possible to a furnace or boiler and positioned as shown in [the figure at left]. The draft regulator should be 18" from a stack switch and at least 18" from a combustible ceiling or wall. Do not locate the draft regulator in a room separated from the appliance."
If the draft regulator is not located properly it won't work properly and the heating system may also be unsafe. Further, if you see a draft regulator installed at the far end of a very long flue vent connector, say 12 to 25 feet or more, there is a more basic problem with excessive flue length and operating problems related to that condition, independent of the draft regulator. See FLUE VENT CONNECTORS for details.
"The barometric damper should be adjusted (by adjusting the weight position) to maintain as low a draft as will give good combustion and meet the requirements for heat. The bracket is marked "Lo", "Med" and "Hi" which correspond to draft settings [if draft is measured inside the flue immediately before the regulator] of 0.2", 0.4", and 0.6" w.c."
Weight location & adjustment on barometric draft controls: the weight that is adjusted to regulate the operation of the draft control needs to be properly located as well as adjusted. T
he weight location switches on most regulators depending on whether the regulator is installed on a vertical flue or a horizontal flue. Field ships their draft regulators with the weight installed in position for a vertical flue.
The adjustment weight is in the right-hand slot when you are facing the control. If the damper is to be installed on a horizontal flue, the weight must be removed from the right-hand slot and attached to the left hand slot as shown in the illustration and sketches above.
Thanks to boiler expert Dirk Faegre for suggesting these additional details.
Soot production or soot blow-back stains at the combustion chamber inspection port or burner mounting tube (or soot in general).
Regardless of whether the draft regulator is serving an oil fired heating boiler or an oil fired water heater, soot coming out of the barometric damper or out of the flue vent pipe, or the presence of soot and burn marks on the heater, or even noises: stumbling, rumbling, noisy oil burners, as well as odors, are examples of improper oil burner operation that need prompt service.
Our photo at left show an example of improper oil burner operation on an oil fired water heater: both systems show soot blow-out at the water heater's combustion chamber inspection port.
Often these soot marks are a symptom of excessive pressure or "back pressure" inside the combustion chamber. Since this water heater is connected so closely to the chimney in a pretty new house, our first guess was that the water heater itself needed cleaning.
Oil fired appliance sooting problems can be caused by an oil fired water heater or heating boiler that is way past due for cleaning (soot blocks the exhaust flue), by a blocked chimney, by improper draft regulator adjustment, or other defects.
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Additional technical contributors & reference sources for this article are listed below.
Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.
"Instructions for Installing FIELD Type AF Barometric Draft Controls," Form No. 31 DC 30666, Field Corporation, Mendota, IL 61342
Thanks to Alan Carson and Bob Dunlop, Carson Dunlop, Associates, Toronto, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education, publications, report writing materials, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
Lonny Steinke, Worland, WY USDA kindly corrected erroneous data we published on the proper draft numbers. - October 2008.
Dirk Faegre, Camden, Maine (207) 232-9494
is a certified BPI energy auditor and certified Envelope technician who kindly suggested draft regulator and flue vent connector inspection defect additions 6 Sept 09
How to open and inspect a barometric damper for Inspection
How to open and inspect a barometric damper: it's easy if a barometric damper is actually installed where it should be. Just gently push the hinged door open with a finger, and shine a good flashlight inside to see what you can see.
Details of how to make this simple check, what you may see, and what it means, are discussed in detail at How to Inspect a Barometric Damper.
Automatic Vent Dampers on Heating Equipment Flues
Heating costs can be reduced by any measure that makes sure that as much as possible of the warmth produced by burning a heating fuel (such as natural gas) is sent into the occupied space rather than lost elsewhere.
An automatic vent damper, such as illustrated in Carson Dunlop's sketch here and in our photograph just below, reduces heat loss in buildings where gas-fired heaters are installed.
We discuss automatic vent dampers in detail at Automatic Vent Dampers where we explain how they work, how they save money, and what they look like.
There we also clear up some confusion among the terms: automatic flue vent damper, duct damper, fire damper, and draft inducers.
Draft Inducer Fans for Problem Chimneys & Vents
Draft inducers are special fans that are installed in the flue vent connector (or sometimes in the chimney) used to vent a heating boiler or furnace.
Most often we see these installed on oil-fired systems at which the technician was unable to obtain sufficient natural draft for proper oil burner operation.
If an oil burner lacks adequate draft, as we discussed above, it will not operate properly, leading to soot clogging, more costly heating bills, back pressure in the combustion chamber, and possibly unsafe heater operation.
We discuss draft inducer or "draft boosting" fans for heating systems (and maybe for some fireplaces) in detail at Draft Inducer Fans.
Definitions: What's the difference between a flue damper, duct damper, a fire damper, and a draft regulator?
Don't confuse an automatic flue vent damper with two other "damper" devices: a heating system automatic duct damper or a duct fire damper.
What is an automatic flue damper?
An automatic flue damper is a device which closes the heating flue when the furnace or boiler is "off" so that we won't continue to lose building heat up the chimney - it's a device to reduce heating costs and save on heating oil consumption.
When the heating system has turned off at the end of an "on" cycle of burning fuel, the automatic flue damper electric (see sketch above and photo at left) motor turns a baffle inside of the flue vent connector pipe to a position "across" the pipe so that the airflow inside the pipe is blocked or stopped.
What is an automatic duct damper and how do they work?
An automatic duct damper is a mechanical device, usually controlled by a room thermostat, which opens or closes a metal baffle inside of a warm air (or cool air) heating (or cooling) duct in order to provide multiple heating zone control in a building.
You can see photographs of and read about manual and automatic heating and air conditioning zone dampers at ZONE DAMPER CONTROLS.
What is an Automatic Fire Damper in Ductwork and how do they work?
A fire damper is required in air conditioning and heating ducts in some commercial installations and possibly by local residential building codes in some jurisdictions.
A fire damper might work similarly to an automatic duct damper, but its purpose is quite different: in the event that a fire is detected in a building or in its mechanical systems, (by heat or smoke or other means of fire sensing), the fire damper closes off the air duct to avoid spreading smoke or fire rapidly through the building. The fire damper is otherwise normally "open".
You can see photographs of and read about fire dampers in ductwork at ZONE DAMPER CONTROLS .
What is a Draft Inducer Fan and how do they work?
A draft inducer is a booster fan that increases the flow of combustion gases up a chimney. They are used when there is a problem with the chimney or heating equipment installation that prevents natural draft from working adequately.
We discuss draft inducer or "draft boosting" fans for heating systems (and maybe for some fireplaces) in detail at Draft Inducer Fans.
What is a Barometric Draft Control and how do they work?
A barometric draft control, also called a "damper" or barometric damper, is a hinged, weighted door on an opening at a heating flue.
The door opens or closes to let extra air into the flue to assure that the draft in the flue remains constant at the proper setting needed for proper heating system operation.
Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.
Dirk Faegre, Camden, Maine (207) 232-9494
is a certified BPI energy auditor and certified Envelope technician who kindly suggested draft regulator and flue vent connector inspection defect additions 6 Sept 09
Domestic and Commercial Oil Burners, Charles H. Burkhardt, McGraw Hill Book Company, New York 3rd Ed 1969.
National Fuel Gas Code (Z223.1) $16.00 and National Fuel Gas Code Handbook (Z223.2) $47.00 American Gas Association (A.G.A.), 1515 Wilson Boulevard, Arlington, VA 22209 also available from National Fire Protection Association, Batterymarch Park, Quincy, MA 02269. Fundamentals of Gas Appliance Venting and Ventilation, 1985, American Gas Association Laboratories, Engineering Services Department. American Gas Association, 1515 Wilson Boulevard, Arlington, VA 22209. Catalog #XHO585. Reprinted 1989.
The Steam Book, 1984, Training and Education Department, Fluid Handling Division, ITT [probably out of print, possibly available from several home inspection supply companies] Fuel Oil and Oil Heat Magazine, October 1990, offers an update,
Heating, Ventilating, and Air Conditioning Volume I, Heating Fundamentals,
Boilers, Boiler Conversions, James E. Brumbaugh, ISBN 0-672-23389-4 (v. 1) Volume II, Oil, Gas, and Coal Burners, Controls, Ducts, Piping, Valves, James E. Brumbaugh, ISBN 0-672-23390-7 (v. 2) Volume III, Radiant Heating, Water Heaters, Ventilation, Air Conditioning, Heat Pumps, Air Cleaners, James E. Brumbaugh, ISBN 0-672-23383-5 (v. 3) or ISBN 0-672-23380-0 (set) Special Sales Director, Macmillan Publishing Co., 866 Third Ave., New York, NY 10022. Macmillan Publishing Co., NY
Installation Guide for Residential Hydronic Heating Systems
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