GasPellet Stoves/Corn/Biomass


Troubleshooting Direct Vent Products
Like Flying into the Bermuda Triangle!

By Ken Rajesky
Note: This article is technical in nature and intended for mechanics who are knowledgeable regarding the service and operation of gas and hearth appliances.

The call comes in from a longtime customer – his year-old direct vent starts up but within minutes the flames disappear. Your chest tightens because you know that severe weather is heading your way and his stove will be a major source of heat should the power go out. Your service guy is straight out with new installations and other service calls. And, if you leave the shop, it may take hours to solve his problem because you are just not sure about troubleshooting direct vented products. So, what do you? The answer is education.


Direct vented products are not the mystery that many service people feel they are. First, what is a direct vented product? Direct vented products are basically sealed systems. That means that combustion air is provided via a sealed intake duct. A separately sealed channel evacuates exhausts from the product. You may a vent system that has the exhaust channel within the combustion air channel. This is called a co-axial vent. Or, you may have two fully separate, sealed channels. A third option is a system that uses both a co-axial and co-linear system. Regardless of which system is used, the product must breathe in, and breathe out. If the product cannot breathe in, it will not have sufficient oxygen to sustain combustion. If the product cannot breathe out, the newly developed exhausts will contaminate the combustion zone not allowing combustion to continue. The advantage of direct vented products is that they are installation flexible, and supply their own combustion air. Another advantage of direct vented products is that they are unaffected by home negative pressures that may cause exhaust spillage.

Despite these positive advantages, you must still put in your time to learn about the product itself. That is paramount for there are several types of burners on the market today; Ceramic, steel, cast iron, tubular, square, rectangular, U-shaped, H-shaped, and many others. Each burner has different burning characteristics and operation. Each burner also utilizes different types of logs. These logs are available in different shapes, designed to be located in specific areas on and around the burner.

There are many types of valves available today. Most are fairly comparable in the manner in which you would begin to troubleshoot them. There are, however, still enough differences so that you should bring technical information with you during the service call for referencing. Again, get to know the valves on the products you sell and/or service.

When you are speaking to the customer, remember that YOU are the doctor so do what good doctors always do——ask questions. Ask many questions. Example: Is this a new appliance? Has anyone looked or serviced this product before or recently? When did this begin to occur? Has there been a distinct pattern (in this case, the stove works for 10 minutes before shutting down)? How is the product vented? Are there any elbows? You will find that by asking many questions, you can sometimes form preliminary diagnoses before leaving the shop. But never, ever, assume anything!

So, now you are on the way to the customers house armed with your service guide, manometer, voltage/ohm meter, and a pair of sharp eyes. You turn into the driveway and your heart starts to pound a little quicker because the customer is outside waiting for you. Now before you get started, note everything. You’ll find that these notes will be helpful later on.

Before you enter the home, first check the termination cap. A very high percentage of problems occur because of improper venting. Look for blockages such as bird’s nests. Look for an improper connection. Check for soot in and around the cap. Review the surrounding area. Are there any bushes, or other things that could potentially negatively influence the product’s performance?

Now, begin to establish a benchmark by comparing what you find to product specifications. Don’t assume that inlet and manifold pressures are okay. Confirm. Don’t assume that the venting conforms to the manufacturer’s requirements. Confirm. Don’t assume that the air shutter settings are proper and that the valve is indeed set up for NG or LP. Confirm!

Now, you are establishing a benchmark by ruling out various factors. This is important because if you require additional technical support from the factory, you will be asked for this information. Having this critical information eliminates wasted time and reinforces your credibility as someone who is competent in servicing this product. Keep in mind that troubleshooting is a procedure of reviewing the symptoms, and narrowing down the factors that could contribute to the problem. Some call it trial and error, but it’s not. It’s isolating, eliminating, and resolving.

After you’ve established a benchmark, begin to visually observe the product in action, particularly the flame pattern. Flame pattern in a direct vented product tells a good story. Observe the flame height, color, and movement. Above all, remember the Combustion Triangle. You need fuel, oxygen, and heat for proper combustion. Let’s look at some common problems and general solutions. Because every product is different, this information should be perceived as guidelines, not gospel.

Situation #1: If you observe a sooting condition, you’ll typically observe yellow/orange flames usually accompanied with black tips. First, check log placement on the burner as flame impingement, which cools the flame tip, can occur with improper location. Then again, compare the air shutter setting to specifications because it may be too closed. When the shutter is too closed, it affects the air-to-fuel mixture causing poor combustion.

Situation #2: The product burns fine without the glass installed, yet shuts down quickly after re-installing the glass. You observe erratic flame movement, concluding with flame lifting off the burner in a short span of time. In this case, you probably have a vent blockage. Or with a co-axial vent system, exhausts are being reintroduced to the combustion zone rather than leaving the vent through the termination cap. Flue gases will not support combustion twice. In either case, you are not receiving enough oxygen to sustain combustion. Remember the Triangle! Check the termination cap for blockage. Check each vent section to ensure a tight seal. It’s possible that you may need to apply silicone.

Situation #3: The pilot flame blows out during windy weather. Although termination caps are tested to high winds for AGA approval, gusty conditions can play havoc regardless. Confirm that the cap is designed for high winds. You may try shielding the cap from the prevailing wind side. You may also check with the manufacturer to determine if shielding the pilot assembly is necessary as well.

Situation #4: Short blue flames with little glow from the ember area. This is a case of too much air. Check the areas that affect air supply into the stove. Start with the air shutter setting. Check the glass seal——review the fit and gasket seal around the perimeter. You can use the old woodstove door trick——the dollar bill test! You may also have unrestricted airflow through the stove. Some models require a restrictor plate or baffle installed on taller venting systems, both vertical and horizontal terminations. Check to see if it’s required and if so, confirm that it is installed.

Situation #5: Flames become tall, stringy, little ember glow, and the flames begin to ghost (translucent) and lift off the burner. Product then shuts down after several minutes. During the first stages, the flames will be orange with black tips telling you that the air is restricted. As the flames become more blue and taller, then lifting, they are telling you that they do not have enough oxygen to sustain combustion. Check the termination cap for blockage. Then, check each section of vent as a poor seal may have occurred resulting in the re-circulation of exhausts. You may also wish to recheck the venting scheme to ensure that all horizontal runs are level or at least have a per one foot run rise. Also, too many elbows will restrict flow through the system.

Situation #5 best describes your customer’s problem. You’ve now reviewed the installation (basement rec. room, snorkel kit), confirmed pressures and settings, and observed the operation/symptoms. You haven’t checked the termination cap yet because the customer frantically met you in the driveway and ushered you in. So, you put on your winter cap, slide your boots on, grab your flashlight, and head out to the side of the house where the termination cap is located. Much to your chagrin, you find that the owner recently installed a plastic window casement shield. In doing so, he unsuspectingly sealed off the vent system. The product could not breathe in, or breathe out.

While this cause of this problem seems a bit far-fetched, it actually did happen to a colleague. It simply illustrates that if you ask questions, don’t assume anything, and educate yourself on the products you service, troubleshooting direct vent products is not a mystery as many may think.

And remember it’s the Combustion Triangle, not the Bermuda Triangle..