Furnace Vent Screens and Flow Velocity

Occasionally we, along with HVAC technicians and plumbers, encounter customers who have enlarged their vent terminations to use a larger vent screen on their intake and/or exhaust vents. Thinking that a larger vent termination will provide better airflow, often to use a cheap and poor airflow rated screen, it’s understandable why some consumers go this route. Unfortunately, consumers fail to realize that enlarging a vent termination has a negative impact on the furnace’s flow velocity. Flow velocity is measured by Linear Feet Per Minute (LFPM) or simply Feet Per Minute (FPM). The CFM (Cubic Feet Per Minute) rating of a furnace’s draft inducer motor, along with the length of pipe and number and type of elbows used in each vent run, will determine a furnace’s gross intake and exhaust discharge flow velocity. Net flow velocity, at the termination, will also consider other factors such as existing debris inside ventilation pipes (bee/wasp nests, leaves, dead mice, etc.) along with the airflow rating of the vent screens used.

Enlarged Furnace Exhaust Vent

As you will see in the picture to the left, this customer enlarged a 3″ furnace exhaust termination to 4″. Once the 3″ enlarges into the 4″ adapter, the force (flow velocity) at which the exhaust is discharged is substantially reduced. With the reduced discharge flow, more moist exhaust gets trapped within the hub and aids in the formation of ice inside the hub and on the vent screen. Additionally, the force that helps to blow accumulated condensate off the screen’s wires, when a heat cycle completes, is greatly reduced which also accelerates icing during sub-freezing temperatures. Extending the exhaust vent, with a reducer coupling, also allows the warm exhaust to condense (freeze) faster before it ever leaves the termination. Because of the known problems with enlarged furnace vent terminations, along with the negative impact they can have on a furnace’s performance and safety concerns, they should be avoided. Instead, it is best to follow our best practices and utilize the correct vent screen for each existing and unmodified hub.

For winter use, 90% airflow rated PVS-RS series rodent vent screens should be used as they meet National Fire Protection Association (NFPA) specifications for use during sub-freezing temperatures. The NFPA is the organization that develops the standards and codes that many cities and states incorporate into their own building codes and are often adopted by ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers). During spring, summer and fall, PVS-IS series insect vent screens may be used to guard furnace intake and exhaust vents from bees, wasps and other insects in addition to animals. As you can see in the picture to the right, PVS series vent screens install inside the hub where they are not only shielded from debris but also positioned as close to the heat source (on an exhaust vent) as possible. All PVS series vent screens feature a 6061T6 aluminum housing which provides excellent thermal heat transfer qualities to assist during the coldest temperatures.

Unsafe Furnace Intake and Exhaust Vent Terminations

As you will see in the picture to the left, this consumer is utilizing a drain cover on the intake and left the exhaust unguarded. It was not the home owner’s original intent to leave the exhaust vent unprotected, but the formation of ice on the exhaust vent was so great the furnace locked out and quit working. The home owner had to go outside, in the snow, to remove the frozen drain cover and restart the furnace. In this particular case, the drain cover has an airflow rating of just 50% which is unsafe in any season. Had the home owner not enlarged the vent termination, and used a PVS-RS3 rodent screen in the 3″ hub, the net open area would have been nearly 3/4″ greater.

Pressure equalization is also important for a furnace’s draft inducer motor to operate properly. For proper operation, and to maintain a furnace’s energy efficiency, both intake and exhaust vents should use the same airflow rated vent screen. When the home owner removed the drain cover from the exhaust, he should have also removed the drain cover from the intake vent. Regardless, we are happy to report the problems this home owner encountered were resolved when he removed the 3″ to 4″ adapters and purchased the PVS-IRS3-2 3″ all-season intake and exhaust vent screen kit.

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