Photograph of a draft regulatorGuide to Barometric Dampers & Draft Regulators
on Oil Fired Heating Equipment, Wood Stoves, Coal Stoves

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Draft regulators / barometric dampers on oil fired heating equipment:

Here we explain the inspection and adjustment of draft regulators or barometric dampers on oil fired heating equipment:

A Guide to Barometric Dampers on Oil Fired Boilers, Furnaces, Water Heaters: inspection, adjustment, cleaning, troubleshooting.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Guide to Inspecting Barometric Dampers or Draft Regulators on Oil Fired Heaters, Furnaces, Boilers, Water Heaters

Photograph of a barometric damper (C) Daniel Friedman[Click to enlarge any image]

Article Series Contents

Draft Regulators for oil fired heaters: 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 in this article just below.

Draft Regulators for gas fired heaters: Details about draft control for gas fired heating systems, including furnaces or boilers, are discussed separately at at DRAFT HOOD, GAS HEATER for gas fired equipment.

Draft Regulators for oil fired heaters: 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 in this article just below.

Draft Regulators for gas fired heaters: Details about draft control for gas fired heating systems, including furnaces or boilers, are discussed separately at at DRAFT HOOD, GAS HEATER for gas fired equipment.

Define Barometric Damper or Draft Regulator

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.

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 shown just above 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

Metal chimney too short (C) Daniel Friedman

During oil burner operation, and also on some gas fired equipment, combustion air moves into the burner are and combustion chamber (as combustion air).

As combustion continues (the fuel is mixed with air and burned), a mix of air and combustion gases continues onwards, moving out of the combustion chamber, up through the boiler or furnace heat exchanger, through the flue vent connector ("stack pipe or flue pipe" and on into the chimney where these gases are finally vented outside, usually above the building roof.

The force with which this air or combustion gas moves is the "draft" inside of the heating appliance.

Too much draft increases heating appliance operating cost by venting heat out through the chimney instead of transferring the heat into the building where it was wanted. Too much draft can also increase chimney temperatures to an unsafe level.

Too little draft can result in incomplete combustion, soot-clogging of heating equipment (dangerous), and more dangerous heating appliance malfunctions such as oil burner puffbacks and in some cases dangerous production of carbon monoxide gas that leaks into the building (a potentially fatal problem).

So virtually all fossil-fuel-fired heating appliances provide some sort of draft control or draft regulator to keep the draft at required levels both in the combustion chamber and out through the chimney.

Details of Why is a draft regulator is needed ?

Metal chimney too short (C) Daniel FriedmanChimney draft is not constant. 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.


Our photo (above left) shows a chimney that extends less than two feet above a flat roof on a one story home, resulting in inadequate draft and sooty burner operation. The home suffered recurrent oil burner sooting, puffbacks, high and repeated heating service repair bills, and ineffective attempts by heating service techs to "fix" the problem by running the oil burner at high temperatures - a trick that mostly served to increase the heating bills for the home.

At DRAFT MEASUREMENT, CHIMNEYS & FLUES we illustrate how this particular short chimney and inadequate draft problem were finally fixed.



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?

Draft controls are 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 HOOD, GAS HEATER for details.

Field Corporation [1], and Tjernlund [2] 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.

Where Should the Draft Regulator (Barometric Damper) be Installed?

Field draft control installation position (C) Field Controls - D Friedman

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." [1]

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.


"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."

Measure Chimney & Flue Draft when adjusting the barometric damper / draft regulator

Draft regulator, barometric damper schematic (C) Carson Dunlop Associates

Details about when, where, how and why to measure draft at oil or gas fired heating equipment as well as measuring draft for fireplaces, coal stoves, wood stoves etc. are at DRAFT MEASUREMENT, CHIMNEYS & FLUES.

Excerpts on typical draft settings or targets for oil fired heating equipment are given just below.

Normally we measure draft at two locations: over the fire or in the combustion chamber. The draft we typically see on oil fired heating equipment like water heaters, boilers and heating furnaces is

The sketch at left showing a barometric damper on oil fired heating equipment (heating boilers or water heaters) was provided courtesy of Carson Dunlop Associates.

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. - for correcting our WC pressure data.

How Does a Barometric Draft Control or Draft Regulator Work?

Draft regulator, barometric damper schematic (C) Carson Dunlop Associates

It's easy to understand how a draft regulator work: 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 porridge in 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? What are the recommended draft settings?

Field Type AF Barometric Draft Control Adjustment (C) Daniel FriedmanBy moving a weight along a scale. You can see a weight and scale in our photo of the Field Type AF Draft Control.

In general the draft regulator is set to the lowest draft that gives good combustion and proper oil burner operation. Higher wastes energy.

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:

  1. Draft over the fire (typically set to 0.02" to 0.03" WC over the fire)
  2. Draft in the breech (always higher than the draft over the fire, and typically around 0.04 - 0.06" WC).
  3. 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.
  4. Stack temperature measurements made at any heating appliance give another indication of system operating efficiency and proper draft settings. A fire that is too hot, measured in the breech or flue vent connector is sending too much heat up the chimney rather than into the building, while a temperature that is too low risks fouling the equipment.

    And very high stack temperatures also indicate a fire or safety hazard. Typical stack temperatures for oil fired heating equipment will be around 450 °F. And for burning dry wood in a wood stove, 350-450 °F. Proper coal stove operating temperatures vary by appliance and are specified by the manufacturer. Improper temperatures may also be an indicator of dangerous, potentially fatal carbon monoxide production.

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.

When the barometric draft regulator door is more open, it is admitting more building air into the flue and reducing the draft seen by the heating appliance. When the draft regulator door is more closed, it is admitting less building air into the flue and is thus increasing the draft seen by the heating appliance.

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.

Weight location & adjustment on barometric draft controls

Draft regulator installation instructions (C) Field Controls - D Friedman

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.

Why we Measure Carbon Dioxide CO2 as Well as Draft when Adjusting the Draft Regulator

Field controls also points out that

It is essential that CO2 readings be taken to determine proper [draft regulator] adjustments. (This test and others should be conducted by a qualified fuel oil dealer or appliance installer for your safety). [1]

While we and the draft regulator manufacturers give typical draft measurement numbers for draft over the fire and at the damper itself, some oil burner manufacturers may require specific draft settings other than those standard ones.

In addition, variations in chimney and building details from one installation to another may affect how the oil burner, heating appliance itself, flue vent connector, chimney, chimney cap, and site wind conditions all interact. Ultimately we need to know these effects on combustion. We want not only proper draft and [usually] no backpressure in the combustion chamber, we also want efficient combustion - that's where the CO2 measurement comes in.

Draft Regulator Inspection Points, List of Defects & Signs of trouble with a barometric damper or draft regulator and what they mean

Photograph of a draft regulator (C) Daniel Friedman

For a detailed guide to inspecting draft regulators and barometric dampers and for example of additional draft regulator defects, failures, mis-adjustment, please

Field draft controls showing different weight positions (C) & Field Controls Field draft controls showing different weight positions (C) & Field Controls

Notice that as illustrated in the two images of draft controls from Field Controls in our first photo above, the weight may be positioned "up" or "down" depending on the draft control model.

The RC draft control (designed for oil or coal heaters) has the weight and hinge "up".

The Model M draft regulator is shown with the adjustable weight "down". For draft controls whose hinge is in the lower third of the opening (the lower control in our image), it's might appear that this control will work properly only as shown - with the weight at the bottom of the control door. That's not so. The Field MG1 and MG2 Draft Controls have the hinge in the upper third of the opening and the weight above the hinge.

But our second illustration above shows two different Field Draft Controllers both of which have the hinge in the upper third of the opening and the adjustable weight is above the hinge.

Some draft controllers work with the weight above or below the hinge, but be sure to check the installation manual for your specific draft regulator as well as to measure the draft to adjust it properly for the fuel and burner type and chimney properties at your installation. With no other information such as an "up" arrow embossed on the draft regulator, I would install it with the label and writing properly oriented "up".

All of the draft regulator manufacturers provide installation instructions - look in the box in which your product came. If your draft regulator is a Field Controls device, the company provides installation guides for each draft regulator at the company's website but they can be tricky to find. Use this link and enter the draft regulator model number such as MG1.

Loose or Damaged Draft Regulator (Barometric Damper) Parts or Improper Draft Regulator Installation Points

Field-installed draft regulator tee vs. using a factory tee

Is the draft regulator a field-installed retrofit unit (below left) or does it connect to a factory-built Tee (below right)?

Photograph of a draft regulator (C) Daniel Friedman

Retrofit draft regulator opening size: Why do we care if the damper is mounted on a field built tee or a factory tee? The draft regulator at above left is a field add-on that is installed using a two-piece eared collar provided by the manufacturer.

Photograph of a draft regulator

The components are installed by assembling the two halves of the collar and by cutting a a hole in the existing length of flue vent connector pipe. The eared collar is mounted over the hole in the flue vent connector and the draft regulator is mounted to the collar opening.

The size of the hole to be cut is determined by making a trace-mark inside of the mounting collar when the collar is held against the face of the flue vent connector in its destined location. On occasion we find that the hole cut in the pipe was too small (improper draft regulation) or too large (leaky connections).

A field-installed damper mounting "tee" like the one at above left is secured to the flue vent connector by two sheet metal screws, one on either side (red circle). If these screws are loose, stripped, or if the vent connector itself is corroded and rusty, the whole regulator assembly can fall off of the system - an unsafe event for obvious reasons.

The factory tee shown in our photo at above right (orange arrow) is secured to the boiler top and the flue vent connector (green arrow above the tee) using three sheet metal screws.The draft regulator assembly(blue arrow) is also secured to the opening of the tee (at right) by one or in some cases more than one sheet metal screws. Inspect to make sure that the SMS are all in place and that the parts are not loose nor rattling..

Loose-rattling draft regulator?

Interesting is that someone must have had trouble with this damper falling out or rattling, as they used adhesive-backed foil tape (blue arrow) to secure the regulator assembly to the field-installed tee.

The adhesive tape shown by our arrow is a clue to heating system operating history, or it might mean that the installer didn't know how or where to install the sheet metal screw (at the top and bottom of the regulator mounting ring) to secure it to the tee. Or maybe s/he was out of sheet metal screws.

Any clue suggesting unusual installation like this is an orange alert flag: look further for odd or improper workmanship.

Soot & Debris inside the flue vent connector

Photograph of a draft regulator

This photo shows how, simply by pushing open the draft regulator swinging door to inspect inside the unit you can reliably identify a field-installed cut-in draft regulator instructions - notice the bent-over tabs (green arrows) that were cut and adjusted by the technician.

As we elaborate
at DRAFT REGULATOR SOOT INSPECTION, this heater is also dirty (red arrow) and has not been properly maintained.

Screws Required to Secure Tee to Flue Vent Connector & Draft Regulator Assembly to Tee

The flue vent tee photo (below left) shows two of three sheet metal screws required to secure the tee assembly to the boiler top; three more screws are used to secure the tee to the flue vent connector ("stack pipe", not shown here) as well as three more (some instructions permit two) screws to secure the draft regulator assembly to the tee opening.

Photograph of a draft regulator

Our photos above and below show one of the screws required to secure the draft regulator assembly to the flue vent tee .

Photograph of a draft regulator (C) Daniel Friedman

Usually we find that it's a tight fit jamming the draft regulator assembly into the tee opening, but we still install the factory recommended screws.

Vibration during heating equipment operation or the banging caused by rough heating boiler starts (mini "puff-backs") or other wear and tear can otherwise loosen the whole assembly.

Photograph of a draft regulator (C) Daniel Friedman

Above we show where the draft regulator is missing one of its securing screws. There are several reasons why a screw might be missing:

The mounting screw was forgotten at initial installation

The mounting screw was removed to adjust the damper to proper level position, then the tech dropped the screw, it rolled under the boiler, and s/he didn't have another screw handy;

The service tech removed screws during cleaning (more likely at a flue vent connector not at the damper) and was in a rush to leave the job.

A close look at this photo shows that the damper position has been adjusted (to correct an out-of-level condition. We can just see the originally-drilled screw hole in the damper assembly (red arrow) just above the original screw insertion hole (blue arrow) in the flue vent tee. Our orange arrow points to a 3/16" gap between the draft regulator fully-in raised stamping and the edge of the tee opening - not a serious concern as long as the device is properly secured.

Check the Draft Regulator Door Hinge Pins

Our photos below (and my red pencil) point to the hinge pin that allows the flapper door of the barometric damper (or draft regulator) to swing freely. For the unit to operate properly the door needs to move freely in response to changes in draft in the flue.

Photograph of a draft regulator (C) Daniel Friedman

If the pins are bent, rusted, jammed, or field-modified (say a lost pin is replaced by a finishing nail) the draft regulator may not work properly and may even be unsafe.

Photograph of a draft regulator

Check the Draft Regulator Weight Assembly

Check that the weight assembly is present. You should see the bracket and adjustable weight on the outside of the draft regulator door (above my red pencil in the photo at below left). Now tip the hinged door open and check that the interior weight (red arrow in our photo at below right) is also in place.

Photograph of a draft regulator

If the inside weight has been lost it may have fallen into the boiler or furnace. If the outside draft control weight is missing look for it on the floor next to those missing sheet metal screws we mentioned earlier. Without the weights it is unlikely that a barometric damper can do its job properly.

Photograph of a draft regulator

Note: some draft regulator controls do not use weights on both sides of the control door.

Photograph of a draft regulator (C) Daniel Friedman

Now use your finger or a tool to gently push the barometric flapper door open and let go - it should swing freely and should not jam open nor jam shut.

Check the Operating Environment for the Heating Equipment

Even a perfectly-installed draft regulator cannot overcome other combustion air, draft, chimney, or boiler/furnace operating defects from other causes.

Check the service tag for the maintenance history of the equipment and for clues about a history of problems (repeated service calls for the same complaint).

Photograph of a draft regulator (C) Daniel Friedman

Check the area around the heater for soot, oil spills, or an obvious lack of combustion air (tiny room, no air inlets).

Photograph of a draft regulator

In a forensic investigation of a heating system problem I'd be worried about why the bottom portion of the older (blue) service tag has been torn off (photo above left). The service company doesn't want to leave the equipment with no service tag - that itself is a red flag. But what information did someone want me to not-see?

Our boiler top dust photo (above right) and my finger show that 6 1/2 months of operation and dust (July 1 to January 15) of this oil fired boiler has produced almost no soot blow-out. In fact the system is almost too clean - I wondered if it was running with excess combustion air - staying clean but maybe giving up on efficiency, thus increasing the building's heating cost.

More draft regulator defects are described in detail at FAQs Questions & Answers about inspecting and adjusting the barometric damper below in this article .

Also see DRAFT REGULATOR SOOT INSPECTION where we provide details about what you may see when inspecting the draft regulator interior or the flue that it serves.

Typical Draft Regulator - Barometric Damper - Sizes & Costs

Barometric dampers or draft regulators (synonyms) are sold in a variety of sizes for residential and larger commercial heating systems. Draft regulator sizes are specified in inches, representing the diameter of the moving regulator door assembly.

Draft regulators are sized to match the flue vent connector to which they are mounted - in turn implying that the draft regulator will also be properly sized for the operating requirements (and draft requirements) of the heating appliance as well. That is, if the chimney and flue are too large or too small for the heating appliance the installation is improper and may be unsafe as well.

Sizes of draft regulators range from as small as 3-inch in diameter up to at least 32-inches in diameter. Prices for draft regulators range from under $50. U.S. to over $600. U.S. Larger sizes of barometric dampers, such as the Field 20-inch unit, retail (in 2013) for about $530. U.S.

Types of Air Regulators, Draft Controls, Fireplace & Stove, Dampers, Vibration Dampeners

This topic has moved to a separate article now found at DAMPERS & DRAFT REGULATOR TYPES covering all types of building and HVAC system air controls, dampers and dampeners such as automatic and barometric flue dampers used on heating equipment, coal stove, pellet stove, woodstove draft controls, draft inducer fans, HVAC duct air flow controls (manual register controls, duct dampers, zone dampers, automatic fire dampers, vibration dampeners), fireplace dampers, and fresh air supply dampers and controls.

Significance of Soot production or soot blow-back stains at the Damper or Draft Regulator

Soot production or soot blow-back stains at the combustion chamber inspection port or burner mounting tube (or soot in general).

Oil fired water heater with a backpressure sooting problem (C) Daniel Friedman

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.

Oil fired water heater with a backpressure sooting problem (C) Daniel Friedman

Our photo at above left shows 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.

At above right, a different heating system appliance shows soot beginning to blow back out of the draft regulator itself. And notice the sloppy installation? The draft regulator was not installed level. The weight system does not work properly when the damper door is out of level.

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.

When the oil burner is not operating, should the damper door be closed?

How to open and inspect a barometric damper and flue vent connector (C) Daniel Friedman

I am fascinated by and appreciative for your publication "Guide to Barometric Dampers..." Many thanks for the complete explanations.

I am getting the run-a-round from the 'service' people from the service company; I'm not sure they know much more than I.

I have one question which I hope you will answer: when the oil burner is not operating, should the damper door be closed? Logic would suggest to me that it should be closed but I am terribly uninformed. - R.D.B.

Reply: Generally, yes, at rest, the damper door is shut. But here are some things that might make it open:

A competent onsite heating or chimney and flue inspection by an expert usually finds additional clues that help accurately diagnose a problem with the heating appliance or the flue and chimney that vent its combustion products.

That warning made, yes, in general, a properly adjusted and balanced barometric "flapper" door is in the vertical or closed position when the heating flue to which it is connected is not in use. If the door is open you may be wasting warm air and heat from the area around the heating boiler or water heater that the damper is serving. Field Controls puts it this way:

Always set the [barometric damper draft] control to maintain as low a draft as will give good combustion and meet the requirements for heat. [1]

Why? Because if you set the draft higher than necessary, you are wasting money, sending heat up the chimney instead of into the building (or if it's a water heater, into the hot water tank). We need enough draft that the oil burner does not blow soot back into the building, and so that combustion is efficient, but we do not want excessive draft.

Automatic flue damper on oil fired heater (C) Daniel Friedman

Below we list reasons that your draft regulator door is staying open.

If you check all of these out and find that the installation is correct, the equipment is all operating normally, but that chimney and wind conditions are keeping the draft regulator open (and heat is thus wasted from your home), a solution could be the installation of an automatic flue vent damper that opens before the oil burner turns on and closes automatically when the oil burner has turned off. [See our photo at left].

Watch out: Installing an automatic flue damper can reduce building heat loss through the flue but we do not want to see this "solution" installed before you have an accurate diagnosis of just why your flue damper door is hanging open. 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.

Here are some things that might explain why the barometric damper is hanging open even when the heater is not running:

The heater is cooling down: If you are looking at the damper right after the oil burner has shut down, the heater is still hot and a mix of hot air and combustion gases are still zooming up the flue and chimney. If the draft effect of those rising gases is creating an updraft that is more than proper for optimum heater adjustment, the damper door will open to admit room air, thus reducing the draft seen in the flue.

The draft regulator is not properly adjusted. One of the basic tune-up steps performed on oil fired heating equipment, usually right after the system is serviced and cleaned, is to check for proper draft over the fire and in the flue. The in-flue measurement is usually made through a hole drilled in the space between the top of the heater and the under-side of the draft regulator. See What is the Right Draft Measurement at Oil Fired Heaters?.

Draft regulator with open door © D Friedman at

The draft regulator hinges or pivot pins are sticking. On occasion, especially if someone was trying to "adjust" a draft regulator or damper by bending or banging on it, we find that the regulator's door hinge sticks. The door may stick in the open or shut position, interfering with proper operation.

It's easy to check for this sticky hinge problem: just gently push the door open and shut with one finger - it should move freely.

When the oil burner is off and the system is cool, if you can push the regulator flapper door into a position in which the door sticks and does not return to the "closed" position on its own, the hinge is binding or the damper is not properly installed. Sometimes a little cleaning and a dab of oil on a hinge (I used pencil lead) is needed. If parts are badly rusted or smashed, just replace the unit.

The damper is installed on a shared flue or shared chimney: while it is not a recommended practice, if the damper is installed on a flue vent connector ("flue pipe" or "stack pipe" - the metal pipe connecting the heater to the chimney) that is shared with other heating appliances, then heat in the other appliance that is running or that has just shut down and is also "hot" can cause enough up-draft to cause the damper door to open.

In addition to the fire safety and code issues that severely limit any sharing of actual chimneys, the reason that manufacturers recommend that each heating appliance have its own flue damper is exactly this: you cannot adjust the barometric damper or draft regulator to optimize the performance of more than one heating appliance on a shared flue. Details are at SHARED CHIMNEY & FLUE HAZARDS.

The draft regulator is not properly installed - out of level or in the wrong location. A key reason that the manufacturers of draft regulators want the regulator face or "door" to be in the vertical position and the hinge axis of the door to be horizontal, is that the weight and calibration of the draft regulator adjustment and the response of the draft regulator to changing draft conditions depends on being in that position.

Even if the heating service technician adjusted the regulator for proper draft when the oil burner was up to temperature and running, if the damper is not properly installed it may not respond just right to changes in draft conditions such as wind over the chimney top.

In addition to out of level or wrong location, Field Controls also notes that the opening of the draft regulator should be pointing away from nearby walls or obstructions as these will interfere with its proper operation. Even if the installer placed the draft control properly plumb and level on the flue tee, if s/he forgot to set the locking screw (found at the bottom of the draft control assembly), the regulator may have rotated in its mount and may now be improperly positioned. [1]

Outdoor conditions are causing excessive chimney draft. For example at some building sites and depending on variables such as wind direction, nearby hills, chimney height, roof shape, height of chimney top above roof, chimney cap design, nearby trees or other obstructions (yep there are a lot of variables), wind blowing over the top of a chimney can actually increase the chimney draft to a too-high level, causing the barometric damper to open even if the oil burner is not running.

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

This article series answers most questions about central heating system troubleshooting, inspection, diagnosis, and repairs. We describe how to inspect, troubleshoot and repair heating and air conditioning 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.


Continue reading at DRAFT REGULATOR SOOT INSPECTION or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or continue reading at DRAFT REGULATOR, DAMPER FAQs




Or see DRAFT HOOD, GAS HEATER for draft regulation on gas fired heating equipment.

Or see DAMPERS & DRAFT REGULATOR TYPES for a complete list that includes draft regulation on wood, coal, oil, and combination wood-oil fueled boiler and furnaces.


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