Remodeling a Home

If you own a high efficiency furnace that won't start, a potential cause of the problem is a blocked condensate drain. High efficiency furnaces are designed to save homeowners money by reducing energy use, but they also produce more condensation during operation than their conventional counterparts. This can lead to the growth of bacterial slime and other clog-causing debris; as a consequence, clogs cause an increase in operating pressure, and the pressure switch responds by shutting down the furnace.

Fortunately, you can clean the blocked condensate drain yourself and get your furnace operating again in a short amount of time. Here is what to do.

Tools and materials needed

• Wet/dry vacuum cleaner
• Funnel with attached plastic tubing
• Water
• Laundry bleach
• Measuring cup
• Hose pliers
• Screwdriver

Step-by-step procedure

1. Remove the access panel to the furnace—Be sure to disconnect the electricity to the furnace before performing any work to protect yourself from accidental shock. Next, unscrew and remove the the sheet metal screws that hold the access panel to the front of the furnace. Set the screws aside in a safe location, then lift the panel up and away from the furnace.

2. Identify the condensate drain and pressure switch—The condensate drain to the furnace will be located near the bottom of the furnace and will consist of a tube attached to a plastic "trap". The trap is attached to dual drain tubes leading from the draft inducer and heat exchanger.

The pressure switch, of which there may be two if your furnace is a dual-stage unit, should be easy to find; pressure switches are cylindrical metal objects a few inches in diameter, and each will be attached to two pressure monitoring hoses. It is important to remove the pressure monitoring hoses before attempting to clean the drain; otherwise the negative pressure placed on the pressure switches by the vacuum cleaner could damage the switches.

3. Detach the drain hose and pressure hoses—To remove the condensate drain hose, unscrew the hose clamp fastening the hose to the condensate trap. Pull the end of the hose off the trap's drain nipple and lay it aside.

To detach the pressure hoses, grasp the ends of the pressure hoses with a pair of hose pliers and pull them away from the pressure switch nipples. Be careful not to tear the hoses during the removal process. Be sure to make note of where each hose is attached, so you can reattach them to their proper locations during reassembly.

4. Clean the condensate drain trap—Once the drain hose and pressure hoses are removed, you are ready to clean the trap. Insert the free end of a hose attached to a funnel into the drain trap opening. Next, mix one tablespoon of household bleach with a cup of hot tap water and carefully pour the solution into the funnel. Stop adding the bleach and water solution and remove the tubing once it begins flowing back out of the trap.

When the trap has been filled with bleach and water, allow it to sit undisturbed for five minutes. Place a wet/dry vacuum nozzle on the drain trap opening and turn on the vacuum power. Keep the power on for a few seconds to remove the water and bleach as well as any slime or other debris, and remove the vacuum nozzle once you have suctioned the trap clean.

5. Reinstall the parts and power on the furnace—After cleaning the drain trap, reattach the drain hose to the trap nipple, then push the pressure hoses back onto the appropriate pressure switch nipples. Once you have reattached the hoses, slide the access panel back into position over the front of the furnace. Restore power to the furnace and check it for proper functioning.

These tips should hopefully help you get the job done. For more information or assistance, you can contact local plumbers.