The increased use of direct vent appliances in the past five years has brought with it a variety of challenges/problems for the heating industry. A direct vent appliance uses neither a chimney nor a power venter. Flue gases are vented directly through the sidewall of the building using the burner to provide the necessary draft. Combustion air is delivered directly to the burner and the inlet must be located on the same sidewall as the flue vent termination.
Particular attention must be paid to the manufacturer’s specifications for the type of piping and the maximum distances allowed between the appliance and the vent/inlet terminations. If the connections are made with standard flue pipe, it is necessary to seal all of the joints with an appropriate compound. In many cases, one piece flexible piping is used as an alternative. In either case, the end result is what we have come to describe as sealed combustion.
Providing combustion air generally takes place in one of two ways. One method is to use a separate flue gas vent termination and a separate air intake hood. This type of installation requires that particular attention be paid to a required minimum distance of separation between the flue termination and the air intake hood. Objections to this type of installation have generally revolved around the need for two penetrations of the sidewall and the temperature of the combustion air during cold weather.
A later development provides for what we call a concentric termination. This method allows the flue gases to be vented through a pipe in the middle that is surrounded by an exterior pipe delivering combustion air to the burner. The obvious benefit of such an arrangement is one sidewall penetration and the tempering of cold combustion air.
The exterior termination designs of concentric units differ depending on manufacturer. These may vary from a simple vent hood style directed downward; an Aerocowl (round) or a straight horizontal vent with some type of deflector at the end to direct the flue gases.
Regardless of whether you use a separate or concentric system, each design generally has specifications as to their correct location above final grade and placement relative to windows, doors, walks, alcoves etc. Even more important is the direction of prevailing winds relative to the terminations.
Several years ago, we were plagued with complaints of nuisance lockouts on direct vent equipment. In many cases, total frustration developed after the replacement of numerous controls, nozzles, transformers and a variety of other burner components. The first description of cross contamination was described in an article written by the late Norm Petrin, a former Carlin representative. While troubleshooting several of these lockout calls, Norm had discovered that earlier concentric terminations had interior joints or connections sealed with silicone. Over time the sealant would separate and flue gases would recirculate into the combustion air. The resulting introduction of flue gases into combustion air has generally been described as cross contamination regardless of how the flue gases get there.
We know that the air in the atmosphere has 20.9% oxygen which we use in the combustion process. We know that the flue gas venting from the appliance has increased levels of carbon dioxide and lower levels of oxygen. The introduction of flue gases into the combustion air results in a decrease in the level of oxygen and the introduction of carbon dioxide and causes a change in the flame produced by the burner. This change in the flame is generally not detectable by eye. The decrease in the light that the cad cell observes can be so great, that the resistance in the cad cell increases, causing the control to lockout. The introduction of a 10% mixture of flue gases to the combustion air is enough to significantly influence flame signal.
Diagnosis of a cross contamination problem can be made in two ways. If an electronic combustion analyzer is available, a sample of the combustion air can be taken to verify that the air has 20.9% oxygen. A lower reading indicates the presence of flue gases. Another method is to use an ohmmeter to monitor the flame through the cad cell during burner operation. If a high or fluctuating reading is present through the cad cell, one need only remove the combustion air from the burner and observe if it results in a drop in the resistance to a normal operating range.
The problems we see in the field today are generally the result of a failure to follow or adhere to correct installation instructions and procedures. As stated earlier, there are manufacturer and code specifications that apply to the location of terminations. Specific clearance from final grade must also provide for clearance from anticipated levels of snowfall. The location of the appliance must also address prevailing winds which would direct the flue gases back into the combustion air intake.
The introduction of direct vent equipment has provided us with an alternative method of installation and a variety of service challenges. The diagnosis of what is causing a unit to malfunction can be made with the proper use of testing equipment. In too many cases, the only thing prolonging the resolution of a call is the failure to use the test equipment.