Draft measurement example (C) D Friedman Draft 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 FriedmanWe 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

  • - 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").

Correcting Inadequate Chimney Draft - extending the flue height

Draft measurement instrument (C) D Friedman Replacing a too-short chimeny with a proper one (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. 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 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.

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.

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