Gas Furnace Combustion Efficiency Quiz

Ten questions on combustion, flue gas analysis and carbon monoxide. Pick an answer to see whether you are right and why. Related reading: The Code: gas heat.

1. Complete combustion of natural gas produces mainly:

   1. Carbon monoxide and soot
   2. Carbon dioxide and water vapor
   3. Hydrogen and oxygen
   4. Nitrogen only

   With enough oxygen and good mixing, natural gas burns to carbon dioxide and water vapor, releasing heat. Carbon monoxide is a sign of incomplete combustion.

2. A combustion analyzer typically measures which values directly in the flue gas?

   1. Oxygen, carbon monoxide and flue gas temperature
   2. Refrigerant pressure
   3. Supply air humidity
   4. Gas meter flow

   From oxygen, CO and flue temperature, the analyzer calculates carbon dioxide, excess air and combustion efficiency.

3. Excess air in combustion means:

   1. Air beyond what is needed for complete combustion
   2. Air leaking from the ductwork
   3. Air used to cool the cabinet
   4. The blower's airflow

   Burners need some excess air to make sure all the fuel burns. Too much carries heat up the flue and lowers efficiency. Too little causes incomplete combustion and CO.

4. A high flue gas temperature on a furnace usually indicates:

   1. More heat is leaving through the vent instead of going into the home
   2. Better efficiency
   3. Too little gas
   4. A cold heat exchanger

   Heat in the flue gas is heat that never reached the air. High flue temperatures can come from overfiring, low airflow or poor heat transfer.

5. Why do condensing (90 percent plus) furnaces produce water?

   1. They burn propane only
   2. They cool the flue gas below its dew point, condensing water vapor and recovering its latent heat
   3. Their heat exchanger leaks
   4. The blower pulls in humid air

   A secondary heat exchanger takes so much heat out of the flue gas that its water vapor condenses. That recovered latent heat is where the extra efficiency comes from.

6. Condensing furnaces are typically vented with:

   1. Single-wall metal pipe
   2. Plastic pipe, such as PVC or CPVC, as specified by the manufacturer
   3. A masonry chimney
   4. No vent at all

   Flue gas from a condensing furnace is cool and acidic. Manufacturer-approved plastic venting resists corrosion, while chimneys and standard metal vents can be damaged.

7. A high carbon monoxide reading in the flue usually points to:

   1. Complete combustion
   2. Incomplete combustion from too little air, overfiring, a dirty burner or flame impingement
   3. A clean heat exchanger
   4. A correctly sized vent

   CO forms when fuel does not burn completely. Find and correct the cause rather than adjusting until the number looks better.

8. Manifold gas pressure on a natural gas furnace should be:

   1. Whatever the gas meter supplies
   2. Set to the value on the rating plate or in the installation instructions, measured with a manometer
   3. Always 1 psi
   4. The same as for propane

   Manifold pressure controls the firing rate. Too high overfires the furnace and too low underfires it. Natural gas and propane use different pressures.

9. Temperature rise on a furnace is:

   1. Supply air temperature minus return air temperature
   2. Flue temperature minus outdoor temperature
   3. The thermostat setpoint minus room temperature
   4. The change in gas pressure

   The rise must fall within the range on the rating plate. Too high points to low airflow or overfiring. Too low can cause condensation in a non-condensing heat exchanger.

10. The best way to protect occupants from carbon monoxide after servicing a furnace is to:

    1. Rely on smell
    2. Test combustion with an analyzer, check the venting and make sure working CO alarms are installed
    3. Open a window
    4. Turn the gas pressure down

    CO has no color or smell. Measuring it, checking the vent and having alarms in the home are the layers of protection.