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Where’s the Draft? – HVAC Insider

Last updated: 10-09-2020

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 Where’s the Draft? – HVAC Insider

A condition that concerns many technicians is when they measure no draft pressure in a flue. The display on their draft gauge reads zero. Let’s look at three common reasons that draft pressure in a flue disappears and ways to determine what you’re dealing with.

Building depressurization is the most common reason for no draft pressure. Depending on the source, depressurization can occur randomly or continuously. The pressure needed to allow flue gases to exhaust as designed are fragile and easily influenced.

Air competes for ways to make it in or out of a building. When depressurization occurs, air pulls in from the easiest location to compensate for air being exhausted. Unfortunately, the easiest location is often a duct intended for the removal of flue gases. That would be the flue.

Room pressure testing provides clues to these issues, but draft pressure testing is the most effective way to determine the cause. It is important to assure no fans are operating and measure draft pressure with only the equipment operating to establish a baseline value.

As you measure draft pressure, start sequentially turning on different fans, starting with the largest. Watch the draft pressure. When it drops, you found the guilty fan. If you turn on the appliance and measure zero draft from the start, you might have a restricted flue instead of depressurization.

Equipment with no draft from the beginning could have a restricted or blocked flue. Conditions like this are why it’s so important to measure ambient carbon monoxide (CO) for your safety. If the equipment produces high CO levels and the flue is restricted, you might not walk out of the mechanical room.

In this situation, debris could block the flue because of deterioration or an animal nest. These are clues that much bigger issues exist. They are often symptoms, not causes. Digging a little deeper with a visual inspection will uncover this problem quickly.

Another restriction rarely considered is a flue liner. They are a code requirement in many parts of the country and can cause more issues than they solve when sized or installed incorrectly. If there is a kink in the liner or it’s undersized, you can expect to encounter flue gas spillage as you would with a blocked flue, except everything looks great with a visual inspection.

To determine if you’re dealing with a restricted flue, shut off gas to the equipment and watch for an increase in draft pressure. If it moves upward within a minute after you turn off the fuel, there is a restriction in the flue that you need to investigate.

One of the most deceptive situations you can encounter with no draft is water heater operation in the summer. You’ll often find no signs of spillage when this condition exists, even though the ball of the draft gauge is sitting at the bottom, not moving.

It may be hard to believe, but this condition could be acceptable even though it looks quite the opposite. When the summer heat turns up, there is less temperature difference between the flue gases and outside than in the winter. Because of this, draft pressure can drop so low that it is virtually undetectable.

If you encounter this situation, measure multiple CO readings in the flue gas to tell you if the water heater is safe or not. If CO readings are stable during the run cycle, the equipment is venting correctly. If CO readings continue to rise during the run cycle, flue gases are not venting properly. Your next step should be to investigate one of the previous reasons for no draft.

While this isn’t an all-inclusive list of reasons for no draft, it’s a starting point for the most common ones. It’s important to remember draft pressure is not the determining factor in whether flue gases properly leave the equipment.

You can use some subtle differences between all three reasons to help you uncover the source. Reasons number one and two often show signs of spillage around the drafthood and need a draft interference test and/or restricted flue test to distinguish between them. Stable CO readings are the only indicator that number three is a reason. Without them, you’ll probably misdiagnose the problem.

David Richardson serves the HVAC industry as a curriculum developer and trainer at the National Comfort Institute, Inc. (NCI). NCI specializes in training focused on improving, measuring, and verifying HVAC and Building Performance.

If you’re an HVAC contractor or technician interested in learning more about adding carbon monoxide testing to your services, contact David at davidr@ncihvac.com or call him at 800-633-7058. NCI’s website www.nationalcomfortinstitute.com is full of free technical articles and downloads to help you improve your professionalism and strengthen your company.


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