Draft measurement example (C) D FriedmanDraft Measurement Guide for Chimneys & Flues

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How to measure draft at heating equipment or at chimneys.

This article explains the methods of measurement and proper adjustment settings for draft regulators or barometric dampers on oil fired heating equipment. 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.

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Where & How Do We Measure Chimney Draft or Heating Appliance Flue Draft?

Photograph of a draft regulator

As we explain at our home page for this topic, DRAFT REGULATOR, DAMPER, BOOSTER, a barometric draft control, also called a "damper" or barometric damper, is a hinged, weighted door on an opening at a heating flue.

Barometric dampers are used on oil-fired heating appliances (furnaces, boilers, water heaters) to assure constant draft and thus uniform combustion.

[Click to enlarge any image]

Without this regulation (hence the term "draft regulator") as building and outdoor conditions vary (wind, doors open or shut, temperature etc), the draft seen by the heating appliance will vary, making maintenance of proper combustion condtions at the oil burner impossible.

A typical draft regulator is deceptively simple: The hinged 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.

Details about draft control for gas fired heating systems, including LP or natural gas fueled furnaces or boilers, are discussed separately at DRAFT HOOD, GAS HEATER On gas fired equipment.

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

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

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.

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.

Draft measurement example (C) D Friedman

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

Draft measurement instrument (C) D Friedman


Draft measurement instrument (C) D Friedman

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

Correcting Inadequate Chimney Draft - extending the flue height

Draft measurement instrument (C) D Friedman

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.

Replacing a too-short chimeny with a proper one (C) D Friedman

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.

What are the recommended draft settings at a draft regulator / barometric damper?

Field Type AF Barometric Draft Control Adjustment (C) Daniel Friedman

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 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.

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.

Barometric Damper Weight location & adjustment on 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.

Because chimney defects also can have a severe effect on draft seen by the heating appliance, readers should also see CHIMNEY DRAFT & PERFORMANCE .

And at FLUE VENT CONNECTORS, HEATING EQUIPMENT we discuss inspection, defect identification, and repair suggestions for the "stackpipe" that connects heating appliances to chimneys and flues.

Antique Draft Indicator Instruments

Oil burner draft gauges (C) & Dustin Cohen

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 windbox 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" oon 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.

Reader Question: can draft be measured with the burner off?

(Mar 4, 2012) Donald Horvath said:

Can the draft at the breech be measured with burner shut off or running and is that measurement (-5) always the same/

Reply: yes but ...

Sure, you can measure draft in a flue or chimney (provided there is a suitable opening) any time, but let's keep in mind just what you are measuring.

If you measure chimney flue draft when the heating equipment is OFF you are measuring natural convection in the chimney, air leaks or heat loss up the flue, or a temporary condition. The draft we see at a chimey is affected by temperature, barometric pressure, the chimney top or cap design, chimney condition: leaks, damage, obstructions, and other factors that indeed have some impact on the heating equipment when the burner is running.

But what you are NOT measuring is what the oil burner is seeing when it is running, nor can you properly adjust the oil burner air intake nor the barometric damper itself when the oil burner is off. The burner needs to run and the system reach operating temperatures to make those measurements and adjustments.

Reader Question: how do I measure fireplace draft?

(Oct 22, 2012) fireplace chimney draft said:

How do i measure the draft in my chimney for a fireplace?


It is possible to measure fireplace chimney draft by making a suitable test opening into the chimney flue (not a usual practice and not something I recommend). Measuring by waiving an instrument at the fireplace opening or chimney top is in my opinion a waste of time, inaccurate, misleading, not useful.

Question: I adjusted my oil burner's draft myself

(3/3/2014) Steve Harris said:

I don't have a Damper. it's a Rheem with a Becket .I was having soot problems an started opening the shutters an it got better. I have no gages,is there such thing as to much air.6 techs couldn't figure it out I went on line an did it my self.

Reply: DIY draft adjustment on an oil buner fired heating appliance is not a great idea, may be unsafe, wasting fuel

Yes Steve - too much combustion air will have these effects:

1. system runs too hot - potentially dangerous, as a fire hazard

2. system efficiency falls - more and more heat goes up the chimney instead of into the building

"techs couldn't figure it out" is odd; give your service manager a call, discuss the concern, ask for help from an experienced technician. No heating company likes to hear that you messed with the system. Their completely reasonable fear is that you made the system unsafe but that they'll get blamed if something happens.

In the old days we could guesstimate oil burner adjustment (with slow speed 1725 rpm) motors: we'd blow cigar smoke at the air shutter and see it go in = adequate draft. We'd spit on the flue vent connector and see it sizzle (about 450 F which was OK if a teensy hot) - but today's equipment, running at higher speed and higher efficiency, also requires training and tools to set up properly.

And Steve, if there is NO draft regulator installed, that alone indicates trouble, a compromise, a draft problem in the equipment or chimney, and it means that it is impossible to keep the system running at optimum and correct settings all the time.

Question: Using a Dwyer 2300 Draft Gauge to measure chimney draft

(Apr 7, 2015) Chris said:
I have a Dwyer 2300 draft gauge to measure draft in my chimney. Do I do this with the burner running, and hot gasses going out, or cold? I don't want to destroy the gauge.


To measure draft the burner must be on and should have run long enough that the flue, chimney, and heater are up to full operating temperature.

The temperatures can be quite hot - depending on your heating fuel. Oil fired heaters can produce temperatures anywhere from 300 to 1200 degF. (normal would be around 450F measured in the breech)

The draft gauge should have a metal probe that can tolerate high temperature.

Otherwise you've got something wrong. In this case you are not using the proper instrument. Simple draft gauges sold for use with heating equipment are probably less precise than the Dwyer 2300 but are designed to include an appropriate stack or flue vent connector probe. Also their scale is optimized for the range of draft for which heating equipment normally operates.

Give Dwyer Instruments a call at 1 800.872.9141

Articles relating height of a column of water or gas, building height, water pressure, chimney draft, gas pressure include

Chimney Draft Articles


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