Although a properly installed and maintained high efficiency furnace will operate for many years without any problems, we do occasionally speak with customers that have icing problems at their furnaces exhaust vent. Since our PVC vent screens are designed to work in all seasons on a properly vented high efficiency furnace, we felt that it was necessary to help educate home owners about winter ice accumulation problems around exterior PVC vents, its cause and how to fix the problem.

For many of our newly installed high efficiency furnace customers, seeing ice around an exhaust vent can be troubling. To understand why ice is forming around their vents, we must first understand the difference between high efficiency furnaces and how their different combustion processes impact the formation of ice. Below we will discuss the primary differences in conventional and high efficiency condensing furnaces, with follow-up information on how this impacts the creation of ice and what remedies home owners can take to resolve the problem.

Conventional and Condensing Furnace Differences

A conventional furnace, such as an 80% efficient model, relies on a single heating stage to produce warm air for home comfort. The heat produced from the furnace combustion exhaust discharged is typically hot and can travel far enough away from a home before it ever has an opportunity to create a problem with ice. On the other hand, modern day high efficiency condensing furnaces utilize a special heat exchange technology that extracts more heat from the combustion of natural gas before it is discharged. By extracting more heat from the combustion of natural gas, high efficiency furnaces are able to utilize less natural gas to heat homes and businesses. The problem with extracting more heat from the combustion process is twofold. First, so much heat extraction from the natural gas combustion process occurs that the warm air cools and condenses. This heat extraction process creates moisture, which is managed by discharging it through the furnace’s condensate pipe and down into a floor drain or exterior location for some regional systems. Secondly, high efficiency furnaces extract so much heat from the combustion process that the temperature of the exhaust gases is much cooler than conventional furnaces.

Formation of Ice from Condensed Moist Air

When exhaust gases discharged from a high efficiency furnace exhaust vent meets the cold air, the air condenses faster than conventional furnaces because it is cooler. This condensing process, along with the one that occurs within the high efficiency furnace itself, creates moisture that can collect and freeze around and inside exhaust pipes since it can’t travel far enough away from the home before condensing. As the furnace continues running through its heating cycles, to maintain the thermostat’s set temperature in the home or business, more condensing occurs and ice continues to build up.

Eliminate Ice Accumulation Nears Vents by Solving Problems

Most vent icing problems involving high efficiency furnaces are not a direct result of the weather, but an improperly installed venting system. Below you will find some of the most common problems involving PVC intake and exhaust vent pipes and how these installation deficiencies allow the formation of ice to occur.

Improper Slope of PVC Vent Pipes

High efficiency condensing furnaces require that PVC vent pipes be discharged to the outside of a home or business. The PVC pipes must be sloped at a minimum pitch of ¼” per foot BACK to the furnace. This is very important and something many do it yourselfers overlook. High efficiency furnaces utilize their condensate pipe during operation to discharge water down a floor drain and not outside where it can quickly freeze.

Sagging PVC Vent Pipes

PVC intake and exhaust vent pipes must be supported all the way from the furnace to the discharge points outside. The diameter of pipe will determine what most manufacturers recommend. However, as a general rule of thumb, 2” piping runs should utilize PVC support hangers every 4’ at a maximum and 3” piping runs should utilize a PVC support hanger every 5’ to prevent sagging pipes. If pipes do sag, condensate can accumulate in the areas where the pipe is sagging, restrict the airflow and cause the furnace to shutdown in a lockout safety mode or create a pool of liquid that adds moisture to the air being vented to the outdoors. The added moisture to air being discharged during winter can contribute to ice accumulation.

PVC Vent Pipe Runs that are Too Long

Each furnace manufacturer provides guidelines on how long PVC vent pipe runs can be. These guidelines should always be followed as they are model specific and impact the performance of the high efficiency furnaces they manufacture. If a PVC vent run is too long, the force at which exhaust gases are discharged may not be enough to push it far enough away from the home. Long runs of PVC vent pipes also allow the exhaust temperature to cool down even more before it exits the home, which will allow the exhaust to freeze even faster once it reaches the outdoors.

Too Many Elbows Used in PVC Ventilation Systems

Though most professional HVAC technicians do not make this mistake, many do it yourselfers fail to consider how 45° and 90° elbows impact the maximum length of their PVC vent pipes. Each elbow used in a ventilation system adds a certain amount of restriction that a blower motor must overcome. As a general rule of thumb, a 45° elbow will add 2.5’ to the length of a PVC vent run while a 90° elbow will add 5’. When calculating how long your PVC vent runs are, make certain to include the additional feet for any elbows used so that your ventilation system is within the furnace manufacturer’s specifications.

PVC Vents are Positioned Too Close to the Ground

Intake and exhaust vents for high efficiency furnaces must be high enough off of the ground to avoid any restrictions or being too close to snow that may melt and form an ice blockage. Because intake vents pull in outdoor air, an inadequate intake vent height may also allow the vent to pull in snow where it then melts and freezes. Each manufacturer will have their own recommendation, but as a standard rule of thumb vents should be positioned 12” above snow grade. Notice that the phrase “snow grade” was used in the prior sentence. Snow grade, which is also referred to in the industry as the snow line, is the point at which the snow is expected to reach its highest point above ground level. Those in the Northeast, for example, would want their vents at least 3’ off of the ground as 24” of snow may accumulate near the vents. Though the accumulation of 24” of snow may be rare for many regions, drifting must also be considered when determining the venting height of any high efficiency furnace.

Ice Formation from Exhaust Vent Blockage

Though large restrictions in an intake or exhaust vent often results in the furnace shutting down in a safety lockout mode, smaller restrictions can also cause ice to form around the vents. Objects sticking out of a vent can allow snow to accumulate and ice over as snow is repeatedly thawed and frozen again. A dead animal, which may have gotten stuck entering a PVC vent, is one problem that home owners encounter. Sticks and other debris may be lodged into the vent, which is more common in homes where young children reside, also provide a base where ice can accumulate. In most cases, our vent screens can guard against these hazards even during winter (PVS-RS series models). However, regular inspection of the exterior PVC vents is also wise.

Intake Vent Ice Buildup Because of Improper Exhaust Discharge

Most ice problems involving intake vents are the result of improper exhaust discharge that is being pulled in through the intake vent when the appliance is running. This is more common in situations where vent pipes are sticking straight out of the dwelling and no effort has been made to control the direction of the exhaust. Another problem with pipes sticking straight out of a home or business is that snow can be warmed by the exhaust just before it is pulled into the intake pipe, where it then freezes. In many cases using directional control of the exhaust, with a hub, is enough to fix problems where ice is building up on or inside of an intake vent. If you are experiencing icing inside your intake vent, please consider the problem serious. If the ice is indeed caused by exhaust gases getting pulled into the intake, carbon monoxide is also being pulled in and discharged throughout your home or business. Carbon monoxide poisoning is a very serious and potentially fatal hazard. It is important to note that most intake vents enter the appliance inside the burner compartment and are not tied into the condensate drain. Because of this, never spray warm water into the intake vent to clear an ice blockage as it can enter the burner compartment and cause damage to the appliance or even an electrical fire.

Exterior Exhaust Vent Pipes Too Long

As was noted previously, the exhaust gases from a high efficiency furnace are much cooler than a conventional furnace. When these gases meet the cold air, they condense and create condensate. In a conventional furnace, the hot exhaust discharge has more time to exit a chimney flue and move away from a home or business before it condenses. That’s why icing problems with a conventional furnace are rare. But with a high efficiency furnace, which discharges exhaust at lower temperatures, the key to resolving exhaust vent ice buildup problems is to discharge the exhaust gas as fast as possible after exiting the home and using the directional control of a hub to point the discharge down. Ice problems in exhaust vents are more common where there is an excessive amount of PVC used to vent the exhaust, which allows the exhaust to condense and freeze before it even exits exhaust pipe. For example, some exhaust vents exit a home and an elbow is used to locate the vent higher than the intake pipe and then yet another hub is used to direct the exhaust away from the home. In this situation it is common for a significant amount of ice to buildup, and trigger a furnace fault code and lockout, simply because the exhaust gas is never given an opportunity to leave the pipe before it condenses.

Improper Vent Screen in Use

During the winter, home owners should not use fine mesh screens on their furnace’s intake and exhaust pipes. Wide mesh PVC vent screens, which have ½” openings and larger, are desired. Many plastic vent screens, for example, are more likely to freeze over because the thickness of the plastic mesh grid is thick and wide. Always consult the manufacturer of your furnace to determine what they feel is suitable to provide animal and debris vent protection during the winter months.

Improper Vent Hub (Elbow) Position

Some HVAC technicians prefer to vent straight pipe to the outside, without using a hub to direct intake and/or exhaust air. Failure to use hubs to terminate the vents can lead to a variety of problems, including vent blockages from icing. On straight exhaust terminations, condensate often exits the pipe and flows underneath the pipe back towards the home where it collects on the pipe and then freezes. Additionally, wind can push the moist exhaust back into the pipe and towards the home, where it too will freeze and collect. In many of these cases, where hubs are not used to terminate vents, large icicles can form that reach from the exhaust pipe all the way to the ground. On the other hand, some home owners may choose to put a hub on after a professional installation and position it in such a way that it collects debris and snow. For example, a 45° elbow should never be pointed up where debris can fall directly inside of it. Doing so would allow snow to enter the hub, melt and freeze. As the furnace cycles throughout the day and night, ice will quickly develop and the furnace would likely stop operating due to a lack of airflow. In most cases an exhaust hub pointed in the 5 o’clock to 7 o’clock position is desired, which allows condensate to drip directly to the ground and freeze instead of collecting on the pipe and freezing. Intake vents that terminate in the same 5 o’clock to 7 o’clock position also prevents rain, snow and debris from easily entering the ventilation system. Properly terminated vent hubs should be slightly pointed away from each other to prevent cross-contamination from the exhaust entering the intake vent.

Professional Help to Fix Ice Around Furnace Vents

Even though the conditions that create ice on PVC furnace vents may be easy for you to identify, after reading the information above, correcting the problem is best left to a qualified heating, ventilation and air conditioning technician. Unlike drains, which many home owners install themselves, gaps in PVC piping for furnace venting may allow carbon monoxide to accumulate in the home. Additionally, improper repairs to correct icing problems may make matters worse and force the high efficiency furnace to go into a safety lockout mode while it is very cold outside. When an unattended furnace automatically shuts itself down as a safety precaution, during extremely cold winter weather, the potential for freezing pipes is very real. When in doubt, contact a professional HVAC company to remedy your furnace’s PVC vent icing problem because no high efficiency furnace should cause ice buildup from the vents.