When a High-Efficiency Furnace Has Combustion Problems

  • 01 of 06

    Combustion Problems With High Efficiency Condensing Furnaces

    Service Man Working on Furnace
    Janice Richard / Getty Images

    A high-efficiency condensing furnace represents relatively recent home heating technology. As a result, this type has a few more troubleshooting considerations than conventional furnaces. How do you know if that's the type you have?

    The main difference between a conventional furnace and a high-efficiency condensing furnace is the heat exchanger technology used to extract heat from the combustion process and exhaust the combustion gases. Visually, you can usually identify a condensing furnace because the exhaust gases will exit your home through a relatively small PVC pipe, rather than the large metal exhaust flue found in conventional gas furnaces. Often, you will see two PVC pipes extending from the furnace through the sidewall of your home—one is the air intake vent, the other the exhaust vent.

    While there are a variety of furnace and ignition issues that can occur with this furnace type, this article focuses on issues related to the venting and condensate drainage of a direct vent (two-pipe) high efficiency condensing furnace, including:

    Weak flame or combustion problems caused by:

    • Obstructed intake air vent
    • Improper exhaust gas recirculation

    Failure to Ignite, caused by: 

    • Clogged condensate drain
    • Clogged flue vent
    • Faulty pressure switch

    You should also refer to the possible furnace and ignition troubleshooting issues discussed in the tutorial, "Gas Furnace Repair and Troubleshooting" but this tutorial will discuss additional troubleshooting and repair issues related to the venting and condensate drainage of a direct vent (two-pipe) high-efficiency condensing furnace.

    This Tutorial Will Cover the Following Problems and Issues

    Weak Flame or Combustion Problems:

    • Obstructed Intake Air Vent 
    • Improper Exhaust Gas Recirculation

    Furnace Will Not Ignite:

    • Clogged Condensate Drain 
    • Clogged Flue Vent.
    • Faulty Pressure Switch
    Continue to 2 of 6 below.
  • 02 of 06

    Weak Flame Caused by Obstructed Intake Air Vent

    Replacing Air Filter in a Furnace
    JaniceRichard / Getty Images

    Obstructed air intake vent pipes in a direct vent two-pipe system will cause serious combustion problems in a condensing furnace.

    1. Check to see if the furnace combustion problem is caused by an obstructed air supply pipe by removing the burner compartment cover which will provide free air flow to a combustion chamber. If combustion improves, then the dedicated combustion air supply vent pipe may be obstructed.
    2. Check for obstructions such as birds' nests or leaves in the air intake vent. If necessary, clean out with a plumber's tool known as a sink auger.
    Continue to 3 of 6 below.
  • 03 of 06

    Weak Flame Caused by Improper Exhaust Gas Recirculation

    Vent Inspection
    Imagesbybarbara / Getty Images

    When a direct vent two-pipe system has its air intake and exhaust vents improperly installed on the outside of the home, the problem of "short-circuiting," or entry of recirculating exhaust gas into the combustion air intake may occur.

    This can happen when the air intake and exhaust vents are too close together and exhaust vent gases—which consist mostly of water vapor, carbon dioxide, and carbon monoxide—are drawn back into the combustion air intake vent. Since the fresh air combustion intake vent has been compromised with exhaust gasses, air reaching the furnace does not have enough oxygen for proper combustion, resulting in combustion problems.

    • This problem can be avoided by use of a concentric vent kit or by properly installing both vents in a two-pipe system. This is probably a job for your HVAC contractor. 
    Continue to 4 of 6 below.
  • 04 of 06

    Failure to Ignite Caused by Clogged Condensate Drain

    Appliances in basement
    Furnace condensate lines are small diameter tubes running from the near top of the furnace cabinet to the floor drain. Jupiterimages / Getty Images

    In addition to the ignition failures common with a conventional furnace,  ignition problems on a condensing furnace may happen because of a clogged flue vent condensate line.

    • A partially clogged flue vent condensate line, or a clog at the condensate collector box near the inducer fan, will often trip the furnace’s pressure switch.
    • If the condensate drain is blocked by debris, improper draining or by frozen condensate, then the pressure switch will not allow normal operation. It will open the switch contacts and keep them open, preventing unit operation.
    • Because the pressure switch senses the accumulation of condensate in the furnace drain pan, the furnace will not operate until the condensate drain has been cleared and the condensate flows freely.
    • You may also notice excessive water coming out of the drainage hole of the smaller secondary heat exchanger when the drain line is disconnected.
    • Ignition problems caused by a condensate drain clog may be intermittent. They can start and stop as the restricted water flow drains away over time and allows the furnace to cycle back on, but will reappear once the condensate backs up if the problem is not fixed.
    Continue to 5 of 6 below.
  • 05 of 06

    Failure to Ignite Caused by Clogged Flue Vent

    Hot water heater and furnace in basement
    Vent pipes are large diameter and bring fresh outside air to the sealed combustion chamber and vent low temperature exhaust flue gasses to the outside. Jupiterimages / Getty Images

    The pressure switch can also be tripped by an obstructed exhaust flue gas vent pipe.

    • Similar to an obstructed air intake pipe, check for obstructions such as nests or leaves, but also check for inadequate sloping of the vent pipe (minimum upward slope from the furnace of ¼” per foot) and make sure the exhaust pipe is supported every 5 feet of its horizontal run.

    A sagging or improperly sloping exhaust vent pipe can collect condensate water and restrict flow resulting in a tripped pressure switch.

    Continue to 6 of 6 below.
  • 06 of 06

    Failure to Ignite Caused by Faulty Pressure Switch

    Furnace pressure switch examples. © Home-Cost.com 2011

    A furnace pressure switch is a safety device designed to sense the negative pressure created by the draft inducer at startup. It essentially senses to make sure the inducer fan is running and shuts down furnace ignition if proper combustion air flow is not maintained.

    As with a conventional furnace, the pressure switch on a condensing furnace sometimes will fail or will get stuck in an open position. To test for proper functioning of the furnace pressure switch, you will need a multimeter (voltmeter/ohm-meter).

    1.  Set your thermostat at a setting calling for heat.
    2. Disconnect the pressure switch’s power leads attached to the pressure switch terminals.
    3. Place the ohm meter leads on each of the terminals on the pressure switch and look at the meter reading. The pressure switch ohm meter should read 0 or close to 0 (indicating no resistance). A resistance reading of infinity or a large number means pressure switch is faulty.
    4. You should also check for cracked or broken tubes running from the pressure switch because this can also cause a tripped switch.

    TIP: A bad pressure switch will not function even when getting proper voltage. If the pressure switch does not work, there are a couple of tricks you can try in an emergency situation to get the switch functioning until you get the switch replaced. You can also try this if you do not have a volt meter.

    1. Give the pressure switch a hard flick with your finger.
    2. If that does not work, you can disconnect the tube at the end opposite of where the tube connects to the pressure switch (do not disconnect at the pressure switch itself), and gently blow and suck a few times until you hear the diaphragm/plunger in the switch move.
    3. The burners should then ignite if the thermostat is calling for heat. Since the pressure switch is a safety device, it must be replaced or the root cause problems fixed as soon as possible.