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Carbon Monoxide: How to Perform a Draft Pressure Test

By David Richardson

David Richardson, NCI Trainer

Draft testing is an essential skill you need to diagnose carbon monoxide (CO) problems. Draft is the force that controls combustion air to the burner. Your job is to assure draft pressure is kept within specific boundaries. Be careful, however. Certain assumptions about draft can lead you to misdiagnose a combustion problem.

For example, industry standards assume that equal amounts of flue gas and room air always move through the venting system. This leads many to believe draft testing guarantees proper venting.

Field conditions often vary from the assumptions made by the standards and can result in unintended CO problems. Let’s look at how you can perform a draft pressure test and keep this valuable troubleshooting skill in context.

Test Instruments and Accessories

Before you can test draft pressure, you need the right test instruments and accessories. These include:

  • Dwyer 460 air meter / draft gauge kit
  • 1/4-inch drill bit
  • High-temperature tapered silicone test port plug to seal test ports in double-wall vent pipe
  • High-temperature silicone to seal draft test ports on single-wall vent pipe

These accessories are essential to properly diagnose CO issues. Fortunately, they are inexpensive and fit right inside most combustion analyzer cases.

Draft Gauge Basics

The Dwyer 460 is an easy test instrument to learn to use. All you need is a little practice. There are two sides on the gauge – one side is red; the other is black. Use the black side of the gauge for draft testing.

It’s important to understand the draft gauge only measures negative (-) pressure. Positive (+) pressure in the flue will force the ball into the clean out port at the bottom of the gauge. Draft is measured in inches of water column. The design draft pressure range is -.01″ to -.02″. This range is located on the black, left-hand side of the draft gauge.

There is a small white ball inside the draft gauge that should move up and down freely. Occasionally the inside of the gauge can get damp and must be cleaned so the ball moves freely.

A short rubber tubing section connects the draft gauge to a curved metal tip that is inserted into the flue for draft measurement.

Test Port Installation

You’ll install a test port in the equipment flue to obtain a draft reading. Test ports are installed 18 to 24 inches above a drafthood, barometric damper, or draft inducer. Draft testing only applies to natural draft and fan-assisted (80%+) equipment, Condensing equipment (90%+) doesn’t need a draft pressure measurement. It’s a positive pressure flue.

Once you find an appropriate location, drill a 1/4-in. hole in the flue pipe to create a test port. Be sure you install the test port where the draft gauge hangs freely and doesn’t lean against anything.

When you complete your test, seal the test port with the appropriate material. Drilling into double-wall pipe requires a tapered high-temperature silicone plug to maintain the one-inch clearance to combustibles. You can seal single-wall pipe with high-temperature silicone if concerned about the hole. Don’t go too crazy over this – the venting systems on natural draft equipment all leak and should be under a negative pressure.

Measure Draft Pressure

Once the test port is installed, insert one end of the curved metal tube into the flue pipe. The rubber tubing transfers draft pressure from the flue to the gauge.
If there is a positive room pressure you might see a reading without the equipment running. This is a good situation to encounter as many mechanical rooms are depressurized from return duct leakage.

Next, turn on the equipment in heating mode and allow it to run for five minutes after the burners ignite. Then, read the draft pressure on the gauge and record it.
Common vented equipment also must be tested with both pieces of equipment operating together. This allows you to see if the common flue can handle both running at the same time.

Diagnose Draft Pressure

Once you record the reading, determine where it falls in relationship to design draft. Draft pressure lower than -.01” may indicate combustion air issues, a flue restriction, or airflow interference issues. Draft higher than -.02” can create issues with natural draft appliances.

At this point you are more concerned with the range and movement of the draft gauge ball than a specific draft pressure reading. The ball should never drop during equipment operation. If it does, this is a sign of flue gas spillage or backdrafting.

Now that you know how to perform a draft test, I want you to be aware that you should combine draft testing with carbon monoxide readings to determine if the equipment is venting correctly. Draft pressure by itself is limited in the information it provides. However, when combined with CO readings, you get essential information necessary to track down combustion air and venting problems. It is difficult to differentiate between the two without it.

Next month we will look at four draft tests you can use to identify specific CO problems that range from flue restrictions to inadequate combustion air supply.



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