It was surprisingly cold in North Carolina this past winter, with temperatures breaking records by plunging into the single digits for days on end. Some homeowners who purchased condensing furnaces were also in for a surprise when they went to turn up the heat and found their furnaces were not operating.

In most cases, it was due to frozen condensate, which occurs when condensation and exhaust gases generated from the heating process are discharged outside via a vent pipe and freeze in the cold air. If ice continues to build up, the vent pipe can become blocked, causing the furnace to shut down. Frozen condensate can occur in many different areas — not just in North Carolina. With proper installation and maintenance, the chances of it happening decrease dramatically.


Furnace shutdown due to condensate freeze is not terribly common, said Derek Cole, general manager of Simmons One Hour Heating and Air in Laurinburg, North Carolina, but it’s more likely to occur when an area has multiple days of freezing temperatures. “If it does happen, contractors should thaw out the frozen line, rerun the drain, and then insulate both to keep this from happening again. It can be expensive to fix, but it has the possibility of causing a lot of damage, so it needs to be done.”

Frozen condensate only occurs when temperatures dip below 32°F and the equipment is not protected from Mother Nature, said Jeff Preston, product manager, unitary products group, Johnson Controls Inc. “Contractors must be aware that whenever water and low temperatures mix, there is a chance the condensate system will freeze. Typically, it’s due to an installation issue, because the furnace is installed in such a way that the condensate system is exposed to low temperatures.”

In order to prevent frozen condensate, installers must follow the manufacturer’s installation instructions and pay close attention to local codes and practices, said Terry Stern, product manager, residential heating products, Rheem Mfg. Co. “For installations where the furnace may reach temperatures below 32°F, such as in an alcove or in an attic, the installer must take precautions to ensure the drain trap and connected drain pipe do not freeze.”

When the drain trap is mounted outside or partially outside the cabinet, noted Stern, installers must protect exposed drain piping by using heat tape or a heating cable. “A UL- or CSA-listed heat tape or UL- or CSA-approved heating cable with a rating of 3-6 watts per foot is acceptable protection when installed and maintained in accordance to the manufacturer’s instructions. If the drain trap is installed within the furnace cabinet, no freeze protection is required.”

Ron Staley Sr., owner of Staley Mechanicals in Dillsburg, Pennsylvania, agrees that the single most important thing that can be done to minimize condensate freeze problems is to read the installation and operating instructions that come with the equipment. “The instructions are very comprehensive and easy to follow if you take the time to read them. Remember, a proper installation is always less expensive than repairing an improper venting installation.”

That being said, if a furnace stops working due to frozen condensate, the solution depends on the cause, said Staley.

If a furnace is vented through a sidewall, it should be terminated as short as possible. Sometimes, an installer will run the vent laterally after it penetrates the wall to avoid an obstacle, such as a window to avoid a code violation. Depending on the length of the lateral run, the condensate can freeze in near- or sub-zero temperatures. Lateral runs should be done on the interior of the building or a different termination location should be chosen, if possible. Vertical runs outside the structure should only be long enough to rise above the snow line.

If a vent must pass through a vented crawlspace or attic, there is a possibility of freezing if ambient temperatures drop low enough. In such cases, the vent should be insulated and/or a self-regulated heat tape installed.

If a furnace is installed in a vented crawl space or attic, the condensate lines and trap can freeze if temperatures drop low enough. These types of installations should have a self-regulated heat tape installed.


Frozen condensate can occur in both new installations and retrofits, although, in new construction, the home or attic design sometimes exposes a furnace to low temperatures, and if the furnace is not properly insulated, freezing can occur, said Preston. “In a replacement situation, condensate freeze was never an issue with a standard-efficiency furnace, so where the furnace is located is not an issue. However, a condensing furnace introduces water to the system, which can freeze when winter arrives. So, the location of the standard-efficiency furnace may not be ideal for the condensing furnace that replaces it.”

Freezing can occur in one- or two-pipe systems; however, two-pipe systems offer an advantage because they harness the combustion air outdoors and bring it from the intake to the burner compartment, said Preston. “The choice of systems can affect condensate freezing indirectly, but any low-temperature source can freeze the condensate system.”

In addition to condensate freeze, there are other venting problems that can occur with condensing furnaces. Staley noted that wasps and rodents often build nests in vent piping during the summer, and, while screens will stop the rodents, the wasps, spiders, and other insects may find their way in. “In some cases, when the nests are further in, a line must be pulled through the entire vent system with a foam ball (purchased through an electrical distributor) attached to clear the debris. We also sometimes see rocks, toys, and bark that children stuff into the vents where they terminate. In these cases, the vent piping must often be cut to clear the obstacles. We’ve found that termination screens foil these attempts.”

Another problem that Staley often comes across is vent systems in which the pipes are too small when considering the input of the equipment and the length of the run. “This causes cycling of pressure switches, lockouts, and other problems. In one case, on a furnace only a few years old, it caused a yellow flame that sooted up the furnace and the venting system. This allowed deadly carbon monoxide into the home, so the furnace and venting system had to be replaced.”

Venting problems can not only result in unsafe operating conditions, they can affect the longevity of the furnace, as well. “System failures and component failures will have a direct effect on the reliability of the product,” said Preston. “The best advice is to follow the manufacturer’s installation instructions and be aware of the environment in which the furnace will be installed, especially the temperature range.”

Publication date: 11/30/2015

Want more HVAC industry news and information? Join The NEWS on Facebook, Twitter, and LinkedIn today!