Draft Measurement Guide for Chimneys & Flues

How to measure & how to set or adjust chimney draft

Where do we Measure Chimney or Flue Draft & What is the Correct Draft Measurement at Oil Fired Heaters?

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.

[Click to enlarge any image]

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 Associates, a Toronto home inspection, education & report writing tool company [ carsondunlop.com ].

In the breech we want to see about -0.05 inches WC pressure for typical oil fired heating equipment.

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 reader L. - for correcting our WC pressure data.

Photo above: draft gauge at left (arrow 1) and measuring draft in the breech before the barometric damper (arrow 2).

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.

Our photo (left) shows a traditional Bachrach kit draft measurement gauge in use (green arrow) and its connection to the flue vent connector at the top of a boiler (red arrow) that in this case had no barometric damper installed.

This boiler had a poor draft, a history of sooting problems and a too-short chimney that we were in process of replacing when these photos of draft measurement procedure were made.

At this New York home (photos just above and below) the draft in the breech was running at about -0.02" w.c. - it should have been at least twice that amount.

Above you can see that the draft was hovering around 0.02" and below you can see our connection of the draft gauge sensing probe into the flue vent connector just above the boiler top.

How Much Draft do we Want at an Oil Burner Fired Heating Boiler, Furnace, or Water Heater?

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").
  

Note: wood stoves, coal stoves, pellet stoves, and catalytic wood stoves have different draft requirements.

See details at

• WOODSTOVE DRAFT CONTROL
• WOOD STOVE DRAFT TEST
• WOOD STOVE, CATALYTIC PUFF-BACKS

More on recommended draft settings at a draft regulator / barometric damper?

By 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, for oil fired heating equipment 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 - for oil fired heating equipment

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.

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.

Watch out: other heating fuels and other heater types such as woodstoves, coal stoves, and pellet stoves each will have its own draft setting requirements. Be sure to read the proper instructions that fit the device whose draft you are regulating.

Example: a woodstove using a catalytic combustor may require a draft setting of 0.05" WC

See details at WOOD STOVE, CATALYTIC PUFF-BACKS

See also PELLET STOVE DRAFT CONTROL

Correcting Inadequate Chimney Draft - extending the flue height

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.

After we replaced the too-short chimney with one of proper height, and with a draft regulator (barometric damper) now installed at the boiler top, our measurement showed that we had a good draft in the flue vent connector - almost 0.06" w.c. in the breech (photo above left).

In our chimney replacement photo (above right) the new chimney extending 24" above the roof of a new addition (green arrow) is much taller than the original 20" chimney (red arrow). That's how we got good draft in the new chimney set-up.

See details of diagnosing and fixing chimney draft problems found at

• CHIMNEY DRAFT & PERFORMANCE - diagnose chimney different chimney draft problems
• CHIMNEY HEIGHT EXTENSIONS

Barometric Damper Weight location & adjustment on draft controls

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.

The 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.

Because chimney defects also can have a severe effect on draft seen by the heating appliance, readers should also

see details at

• BAROMETRIC DAMPERS
• CHIMNEY DRAFT & PERFORMANCE
• FLUE VENT CONNECTORS, HEATING EQUIPMENT inspection, defect identification, and repair suggestions for the "stackpipe" that connects heating appliances to chimneys and flues.

Antique & Other Draft Indicator Instruments

This multi-dial gauge provides critical information used to assure safe and proper function of the oil burners used to heat water to steam in the Pratt Engine Room steam boilers.

From left to right, this gauge measures wind-box pressure, furnace draft and uptake draft. Here the term "furnace" is being loosely applied as these are boilers. (Furnaces heat air, boilers heat water.)

The center gauge or "furnace draft" on most oil burners is measured in the combustion chamber right over the burner and is a critical data point.

Photo provided courtesy of NY photographer / director Dustin Cohen.

At STEAM BOILERS GENERATORS CONTROLS, PRATT we describe the multiple-readings of the draft gauge shown above.

Using the Bachrach Draft-Rite Gauge

Above: one of my favorite draft tools is this little Bachrach Draft-Rite that fits into a shirt pocket. Here the tool's probe has been inserted into the flue vent connector above a heating boiler.

When the hole is much larger than the probe Bachrach advises sticking the probe well into the flue (6 inches).

Below: a view of the Draft-Rite showing its zeroing opening covered by my thumb. With the hole covered and the gauge held upright it is turned until its pointer is at zero. At that point you're ready to take a draft measurement.

Below: view of this draft measuring tool showing the zeroing opening on its back and the detachable probe that, once assembled and screwed into the gauge back opening, is inserted into the flue vent connector opening or over the fire to measure draft.

Watch out: you would not actually make a measurement with the tool held as shown, as it's got to be upright and leveled with its pointer zeroed first, as shown above.

Below: The Bachrach DraftRite tool assembled, ready for use

Below: an example measurement of draft using the Draft-Rite showing - 0.04" of water or "negative" draft or "updraft. This gauge can measure updraft (on the right-hand scale) as negative numbers or updraft, or the tool can measure downdraft (on the left-hand scale) as positive numbers or pressures.

The gauge is reading just over 0.04" of water negative draft or "updraft" in a flue or chimney.

See also these

• BACHRACH DRAFT RITE INSTRUCTIONS [PDF] [From our archives], Bachrach Corporation has offices world wide.
  
  Bachrach Canada: Bacharach, Inc. 10 West Pearce Street, Unit 4 Richmond Hill, Ontario. L4B 1B6, Canada Toll Free: +1 (800) 328 5217 Email: support@bachcan.ca
  
  Bachrach Europe: Bacharach, Inc. Unit D12 & D13 Santry Business Park, Swords Road Santry, Dublin, Ireland Telephone: +353 (1) 284 6388 Email: eutechnical@mybacharach.com eusales@mybacharach.com
  
  Bachrach USA: Bacharach, Inc. 621 Hunt Valley Circle New Kensington, PA 15068, United States Toll Free: +1 (800) 736 4666 Email: help@mybacharach.com
  
• An example of using the Draft-Rite draft gauge is
  
  at CHIMNEY RESERVE DRAFT MEASUREMENT

Wood Stove Draft Adjustment, Testing & Measurement

Checking the wood stove draft helps manage the fire's strength, heat, and assures proper exhaust venting.

Wood stove damper control: adjusts the rate of exhaust of combustion gases and smoke from the woodstove up into the chimney. More exhaust = hotter fire and less heat transfer into the room; Less exhaust = slower, smaller fire.

Shown here: our yellow arrow points to the draft outlet control on this wood burner.

A simple empirical test of woodstove draft adequacy is to watch the fire when you close-down or open-up the air inlet control to your woodstove.

Fire is brisk and the only combustion air inlet is through the normal stove's combustion air inlet: this shows that there is a good draft.

Fire doesn't change: If even after watching for a minute or two you see no change in the wood stove fire's height or level of activity when you change the air inlet settings, then look for draft problems and their cause

See details on how to measure, test or adjust the draft at a woodstove

at WOODSTOVE DRAFT CONTROL

and in that article also see

at WOOD STOVE DRAFT TEST

Draft Measurement & Management for Catalytic Wood-Stoves

What should the draft be on a catalytic wood stove?

Our reader asked:

During a "Back Puffing" event what would the draft measurement be above the flue collar?

And let me also ask, is it unreasonable to expect a catalytic wood stove to work properly with a draft at the collar between .04 and .08 inches of H₂O ?

Properly meaning no fumes being pushed out during operation. (2021-12-06) JR

Illustration, one of three models of the Blaze-King KE40 catalytic combustor woodstove cited in more detail below.

Reply by Inspectapedia Com Moderator - puff-backs or back-puffing woodstove using a catalytic combustor

@JR,

Short answer: check with what your manufacturer says, or start with 0.05" of water column. You also want to check the operating temperature of your stove and you want to assure that the catalytic combustor remains bypassed until the stove is at proper operating temperature.

In your initial question you asked about draft associated with a "puffback" and we thought you were referring to the explosive force that we describe below, associated with a malfunctioning oil burner.

But from the ensuing discussion it looks as if we're actually discussing a type of "puffing" or "puff-back" that occurs specifically in catalytic-type wood stoves - wood burning stoves that use a catalytic combustor to extract more heat out of the burned-wood than a conventional wood stove. 

To have room for a complete and detailed answer, we've moved your question and our reply to a new page now found at:

WOOD STOVE, CATALYTIC PUFF-BACKS

Please review those details and don't hesitate to post any new comments or questions.

Draft Measurement Research, Guides, Standards

• Watch out: because life-safety concerns apply when measuring or adjusting the draft on fossil-fuel appliances and because different fuels and heater types use different settings and even measurement procedures, be sure to see the specific draft measurement articles cited in the sections of the article above
• AHRI, INDUCED-DRAFT FURNACE HEAT EXCHANGER INSPECTION PROCEDURE [PDF], AHRI, Air-Conditioning, Heating, and Refrigeration Institute, 2111 Wilson Blvd, Suite 500
  Arlington, VA 22201 USA, Tel: (+1) 703-524-8800,
  
  Canada: AHRI, 2350 Matheson Blvd. East, Suite 101, Mississauga, ON L4W 5G9 Canada,
  Email: bteferi@ahrinet.org, Website: http://www.ahrinet.org/Home, Fact Sheet,
• API, Instrumentation, Control, and Protective Systems for Gas Fired Heaters [PDF](2013) API, American Petroleum Institute retrieved 2021/12/11 original source: https://docs.google.com/
• Beckett, UNDERSTANDING DRAFT [PDF] (1987) [Website] - local copy saved as Understanding-Draft-Beckett.pdf
  Beckett,
  U.S. Office 38251 Center Ridge Rd North Ridgeville, OH 44039 Toll Free: (800) 645-2876 Loca
  Canadian Office 430 Laird Rd Unit #3 Guelph, ON N1G 3X7 Toll Free: (800) 665-6972 Fax: (519) 763-5656- loc
  Excerpts:
  

  In the oil heating industry, “draft” describes the vacuum, or suction, which exists inside most heating systems. The amount of vacuum is called draft intensity.
  
  Draft volume specifies the cubic feet of gas that a chimney can handle in a given time.
  
  Draft intensity is measured in “inches of water”.
  
  Just as a mercury barometer is used to measure atmospheric pressure in inches of mercury, a draft gauge is used to measure draft intensity (pressure) in inches of water.

  Natural Draft is thermal draft. It occurs when heated gases expand.
  
  A given volume of hot gas will weigh less than an equal volume of the same gas at a cool temperature.
  
  Since hot combustion gases weigh less per volume than room air or outdoor air, they tend to rise.
  
  The rising is contained and increased by enclosing the gases in a tall chimney. The vacuum is then created throughout this column of hot gases.

• Brand, L., MEASURE GUIDELINE: COMBUSTION SAFETY FOR NATURAL DRAFT APPLIANCES USING INDOOR AIR [PDF] (2014) L. Brand Partnership for Advanced Residential Retrofit Gas Technology Institute 1700 S. Mount Prospect Road Des Plaines IL 60018 for U.S. Department of Energy, U.S. Department of Energy Office of Scientific and Technical Information P.O. Box 62 Oak Ridge, TN 37831-0062 phone: 865.576.8401 email: mailto:reports@adonis.osti.gov retrieved 2012/12/11, original source: https://www.nrel.gov/docs/fy14osti/61326.pdf
  
  Note: As the procedures in this document illustrate, some approaches to setting draft for heating equipment will use measurements of CO or CO2 or Oxygen levels as a measure of combustion levels - an indirect measurement of adequacy of combustion air and possibly of adequacy of draft.
  
  Abstract excerpt:
  This Measure Guideline provides guidance on combustion safety inspection procedures for appliances and heating equipment that use indoor air for combustion and dilution of flue gases in low-rise residential buildings.
  
  Appliances of interest in this Guideline include Category I and II gas-fired residential furnaces and other natural draft appliances that are not represented by a code-based categorization approach for venting at this time.
  
  Only natural draft (i.e., nonpositive vent static pressure) gas-fired combustion appliances installed in the living space or in an area freely communicating with the living space, vented alone or in tandem with another appliance are considered here.
  
  A separate Measure Guideline addresses combustion appliances located either within the living space in enclosed closets or side rooms or outside the living space in an adjacent area such as an attic or garage that use outdoor air for combustion and venting.
  
  This document is for building performance inspectors and auditors working in homes where energy upgrades are being conducted.
  
  Advanced steps are provided for auditors with special traning or trained heating and cooling technicians. Normal operation of combustion appliances can be affected by building alterations whether or not air infiltration control is included in the package of measures being applied.
• CSA/ANSI 3.21-2019 Industrial Gas-Fired Natural Draft Heaters For Installation In Non-Hazardous And Hazardous Locations In Oil And Gas Process Applications
• [1] "Instructions for Installing FIELD Type AF Barometric Draft Controls," Form No. 31 DC 30666, Field Corporation, Mendota, IL 61342, web search 04/02/2011, original source: http://www.fieldcontrols.com/pdfs/04592700.pdf, Field Controls, Kingston, North Carolina 28501, Tel: 919-522-3031
• [2] Tjernlund Draft Controls, A Series (single action for oil, solid fuel, and fan-assisted gas burners) and B Series (double action for gas heating appliances), web search 04/02/1011, original source: http://www.tjernlund.com/Tjernlund_8500490.pdf , Tjernlund Products, Inc., 1601 Ninth Street, White Bear Lake MN 55110-6794, Tel: 651-426-2993 or 800-255-4208 website: www.tjernlund.com Email: fanmail@tjfans.com
• ORNL, FURNACE AND WATER HEATER VENTING FIELD DEMONSTRATION [PDF] (2019) ORNL/TM-2017/756 Oak Ridge National Laboratory, retrieved 2022/02/11 original source: https://info.ornl.gov/sites/publications/Files/Pub106745.pdf -
  Excerpt:
  
  Replacing non-condensing furnaces with condensing furnaces can achieve a 10% or more reduction in the use of natural gas and propane, but the furnaces have different venting requirements that can complicate the replacement.
  
  When a non-condensing furnace that shares a common combustion exhaust vent with a natural-draft water heater is replaced with a condensing furnace in an existing home, the condensing furnace is typically installed with its own new vent system that vents through a side wall while the natural-draft water heater remains vented using the existing vertical flue or chimney.
  
  However, side-wall venting is not always possible, especially in an existing home.
  
  When side-wall venting is not feasible in a home, a condensing furnace must be vented vertically through the roof separately from the vertical vent used to vent a natural-draft water heater also installed in the home. In an existing home, a second vertical chase may not be able to be created, either based on structural or economic criteria.
  
  The purpose of this study was to determine if a technically viable and more economic option exists for these homes that require unique venting solutions.
• U.S. DOE, COMBUSTION APPLIANCE SAFETY & EFFICIENCY TESTING [PDF] U.S. Department of Energy, retrieved 2022/02/11 original source: https://nascsp.org/wp-content/uploads/2018/02/usdoe_combustionappliancesafetyefficiencytesting.pdf
• WOOD STOVE DRAFT TEST