High-efficiency furnaces, also known as condensing furnaces, recirculate hot combustion gases to cause them to condense before venting. Surplus heat released naturally in the condensation process is recovered by a secondary heat exchanger, boosting energy efficiency in these models to an AFUE (Annual Fuel Utilization Efficiency) rating of over 90 percent. The technology generates up to a gallon of condensate a day as a byproduct. In residential applications, this liquid may be conveyed out of the house through a PVC discharge pipe that terminates on the outside of an exterior wall. If the condensate drain outlet is blocked, your high-efficiency furnace may fail to start.
Because condensate leaves the furnace warm, in most climate conditions, it flows freely to the outdoors. In very frigid temperatures, however, condensate may get cold enough to freeze as it approaches the end of the discharge pipe. If ice blocks the tube completely, condensate backs up into the furnace and trips a safety sensor that prevents the unit from starting until the blockage is cleared and free flow of condensate is restored. Usually an error code appears on the thermostat LCD screen indicating a condensate blockage. When a high-efficiency furnace fails to operate only when outdoor temperatures are very low, a frozen condensate line is a prime suspect.
Do It Yourself — Maybe
Several DIY methods can thaw a frozen condensate line. Normally, ice forms in the external portion of the pipe that extends down the exterior wall to the ground. Pouring warm — not boiling — water along the pipe is the preferred method for melting an ice blockage. Commercially available heat packs for muscular pain that are heated in a microwave can be placed on the frozen portion of the pipe to thaw it out, too. Heat from a hair dryer set on "low" may be used by moving the dryer up and down the frozen span rapidly to warm the pipe without melting the PVC. Take precautions to guard against electrical shock hazard when utilizing a hair dryer outdoors in wet conditions.
Fending Off a Freeze
The best strategy for a frozen condensate line is, of course, prevention. Pipe insulation available at any home center and installed on the exterior portion of the drain pipe usually is enough to prevent freezing in moderately cold temperatures. In an extremely hard freeze, electrical heat tape designed for exterior use can keep the pipe from freezing. Use of electrical pipe tape on the condensate drain line should be limited to cold snaps when unusually low temperatures are expected. Continuous use of electrical tape on PVC pipe may make the pipe brittle.
The ultimate answer to recurrent condensate drain freezing is to have a plumber reroute the condensate drain line to an indoor household drain so it flows directly into the sewer. This keeps the entire drain line within the envelope of the home and prevents freezing. Local plumbing codes stipulate certain methods for joining a condensate line to a domestic drain in order to prevent sewer gas emitted from the drain from infiltrating the furnace.