5 Common Combustion Problems With High-Efficiency Furnaces

Service Man Working on Furnace
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A high-efficiency condensing furnace is a little more complex than a conventional furnace and troubleshooting is somewhat more complicated since there are more potential problems. The main difference between a conventional furnace and a 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 furnaces. Often, you will see two PVC pipes extending from the furnace through the sidewall or roof of your home—one is the air intake vent, the other the exhaust vent. This indicates a direct-vent system. 

There are also single-pipe vent systems on some condensing furnaces, where the combustion air is drawn from the indoor air supply, and a single PVC pipe serves to vent combustion exhaust gases. Some furnaces can be installed either with single-pipe or two-pipe direct-vent systems. 

When a condensing furnace has a weak flame or will not ignite at all, there are several conditions you can check for relatively easily: 

  • Obstructed intake air vent
  • Improper exhaust gas recirculation
  • Clogged condensate drain
  • Clogged flue vent
  • Faulty pressure switch

Weak Flame Caused by an Obstructed Intake Air Vent

An obstructed air intake vent pipe 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. This will provide free airflow to the 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 the vent with a plumber's tool known as a sink auger

Weak Flame Caused by Improper Exhaust Gas Recirculation

When a direct-vent system has its air intake and exhaust vents improperly installed on the outside of the home, it can create a problem of "short-circuiting," allowing exhaust gases to flow back into the combustion air intake pipe.

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. The result is that air reaching the furnace does not have enough oxygen for proper combustion.

This problem can be avoided by use of a concentric vent kit or by properly installing both vents in a two-pipe system. Fixing this problem is probably a job for your HVAC contractor. 

Failure to Ignite Caused by a Clogged Condensate Drain

In addition to the ignition failures common with a conventional furnace, such as a faulty igniter, ignition problems on a condensing furnace may be related to 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 or frozen condensate, or it simply drains improperly, the pressure switch may open, preventing ignition. 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 can again flow 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 unless the problem is fixed, the symptoms will reappear once the condensate backs up.

Failure to Ignite Caused by Clogged Flue Vent

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, and make sure the exhaust pipe is supported every 5 feet of its horizontal run. The minimum upward slope from the furnace is 1/4 inch of rise for every foot of horizontal run. A sagging or improperly sloping exhaust vent pipe can collect condensate water and restrict flow, resulting in a tripped pressure switch.

Failure to Ignite Caused by a Faulty Pressure Switch

A furnace pressure switch is a safety device designed to sense the negative pressure created by the draft inducer at startup. It makes sure the inducer fan is running and shuts down furnace ignition if there is not enough airflow to maintain proper combustion.

As with a conventional furnace, the pressure switch on a condensing furnace sometimes can fail or 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 to call for heat.
  2. Disconnect the power leads attached to the pressure switch terminals.
  3. Set the multimeter to ohms (resistance test).
  4. Place each multimeter lead on a separate terminal on the pressure switch. The meter should read 0 or close to 0 (indicating no resistance). A resistance reading of infinity or a large number means the pressure switch is faulty.

In addition, you should check for cracked or broken hoses running from the pressure switch because this can also cause a tripped switch.

Furnace pressure switch examples.
Home-Cost.com 2011