Concentric vent terminations are specialized conical vents, having a pipe within pipe design, with the exhaust terminating from the front of the cone and the intake drawing fresh air from behind the cone. These types of vents are known for frequent exhaust infiltration events since the opening resembles that of a birdhouse. As pictured below, we manufacture seasonal concentric vent caps to guard the exhaust which prevents the most common infiltration events being serviced by HVAC and plumbing professionals in the field. However, some do question whether they should also use a vent screen on their concentric vent’s intake and our short answer to this question is no.

Concentric Vent’s Intake Restriction

The length of PVC pipe used in any ventilation system, along with every elbow utilized in each vent run, contributes to a condition called friction loss. Draft inducer motors have a CFM (Cubic Feet Per Minute) rating, which determines how much air it can draw and how much exhaust it can discharge. It’s this CFM rating that determines how much friction loss is acceptable to remain within OEM specifications. Since concentric vents have a pipe within pipe design, where the intake pipe enters the concentric vent assembly is already severely restricted (see the image below). It’s because of this existing restriction that we do not recommend using a screen on the intake side of a concentric vent.

Heeding the Do Not Block Intake Warning

Most concentric vents have the words “do not block intake” embossed on the cone in very large lettering and for good reason. It’s because of the existing intake restriction that consumers should heed these words of caution. Failure to follow these cautionary words may result in lost appliance efficiency, unwanted safety lockouts and potentially pose a hazard to the occupants of the dwelling.

Protecting Intake Vents Correctly

The best way to protect the intake on a concentric vent is to remove the concentric vent termination entirely and use standard vent termination elbows for both the intake and exhaust. Doing so not only eliminates the concentric vent’s existing intake restriction, but also eliminates other known problems with this style of vent. Other problems with concentric vents include exhaust being drawn in by the intake, howling or reverberating from wind entering the exhaust on windy days and the obvious easy entry animals and insects have when concentric vents are terminated horizontally. Unfortunately, converting a concentric vent termination to standard intake and exhaust termination hubs is not without the expense of labor and materials to drill another hole in the dwelling to accommodate a separate intake vent terminated outside. Furthermore, condominium bylaws and home owner association regulations may prohibit permanent changes in a home’s exterior which may include changes in the vent terminations. Fortunately, most boilers, furnaces and hot water heaters utilizing a concentric vent are located in large enough spaces where fresh air can be supplied from inside the dwelling.

Terminating an Intake Inside

Providing there is enough square footage in the room where the appliance is located, a concentric vent’s intake can often be terminated inside the dwelling for as little as $15 without the need to make any changes to the vent on the outside. In the conversion pictured above, the PVC intake pipe was cut about 18″ above the furnace and a 90° elbow added at the termination. Another cut was made on the pipe about 12″ away from where it attaches to the concentric vent assembly. The no longer needed large section of pipe/elbows was discarded. Then a cap was cement welded onto the end of the PVC pipe still attached to the concentric vent assembly, making it impossible for anything to gain entry into the dwelling from the intake pipe.

Special Conversion Considerations

Direct vent systems, which have both intake and exhaust terminations outside, are by far more superior to a non-direct vent system having an intake terminated inside the dwelling. When a high efficiency boiler, furnace or hot water heater draws fresh air from the inside, it creates negative air pressure. Though this negative air pressure is often negligible, it can adversely impact other appliances that rely on atmospheric venting for exhaust discharge. Additionally, negative air pressure inside the dwelling will help to pull in air from the outside through cracks and crevices not properly sealed. For these reasons, we recommend consumers consult a locally licensed HVAC or plumbing company for further assistance in determining suitability in terminating any intake pipe inside the dwelling.