Although the combustion analyzer doesn't measure them, proper ventilation/infiltration air, sufficient combustion air, and a safe heat exchanger are critical to proper operation of a gas- or oil-fired appliance. No combustion test or appliance inspection is complete without addressing these concerns.

To get us all on the same page, let's start with some definitions.

Ventilation air is the total air, which is a combination of the air brought inside from outdoors and the air that is being re-circulated within the building. Sometimes, however, the term is used in reference only to the air brought into the system from the outdoors; this document defines ventilation air as indoor and outdoor air ventilation.

Infiltration air is the outdoor air that enters a building through cracks or unintentional openings.

Combustion air is the air supply brought into the furnace's combustion chamber, supplied from within the basement or from outdoors. Combustion air is necessary to burn fuel.

Dilution air is the air that enters a draft hood or draft regulator and mixes with the flue gasses.

Natural- and induced-draft gas appliances use air from inside of the building to burn the fuel. If air used during the combustion/ventilation process from inside the building is not replaced by outdoor air:

  • There will be insufficient air in the building for proper combustion.

  • There is a very high probability that CO will be generated due to the lack of O2 inside of the building.

  • Poor ventilation/infiltration can cause a negative pressure inside of the building.

  • If the building is under a negative pressure, the chimney will not draft. The higher pressure outdoors will force air down the chimney or flue vent spilling the flue gasses into the building.

  • Vent spillage increases the probability that the flue gasses will contain CO due to poor combustion.

    Performing a three-part heat exchanger inspection, including the O2 test, is an important part of any furnace inspection. On atmospheric furnaces, the only test that should be done in the stack is draft. All of the combustion and heat exchanger testing must be done directly in the outlet of each cell.


    The ventilation air test is a worst-case test used to determine whether or not enough indoor air and infiltration air is coming into the building. It is used to test for proper ventilation/infiltration under actual operating conditions.

    It should be performed on every gas appliance installation and every gas appliance service call prior to servicing the appliance. It also should be performed prior to any attempt at modification of the appliance or of the installation, including service, clean and checks, and/or mechanical changes.

    The ventilation air test procedure is outlined in the International Fuel Gas Codeâ„¢ (ANSI Standard Z223.1).

    If it is determined that there is a condition that could result in unsafe operation, the appliance should be shut off and the owner advised of the unsafe condition. (Note: If conditions can be changed to temporarily correct the condition - like removing the door from the adjoining space, cracking a window in the basement, or locking out another appliance that is not deemed critical for heating the structure - the heating appliance can be left in operation provided the corrections to the combustion/ventilation system are incorporated prior to returning to normal conditions and you have the owner's written permission.)

    Any changes made should be noted on the work order and signed off by the customer before those changes are made. Any appliance left in operation must not show any signs of combustion/ventilation problems. If there isn't sufficient air for combustion and/or ventilation, the homeowner or technician will be at risk by operating the appliance under worst-case conditions.

    This test is performed to make sure that the building into which a fossil fuel appliance is to be installed, has enough ventilation/infiltration air to replace the air used in combustion and venting.

    Homeowners are constantly making changes to the home - caulking windows, adding weather stripping - so this test should be performed annually. Modern buildings are much tighter than older buildings; some do not allow enough leakage for infiltration air to enter the building from outdoors. In addition, winterization practices on older homes have sealed many of the openings that provided combustion and ventilation air.

    If the building does not have enough infiltration air, provisions must be made to bring in outdoor ombustion and ventilation process. This could mean the installation of natural or mechanical combustion air provisions.

    This test should be performed even when installing a 90-plus-AFUE furnace that takes all of the combustion air directly from the outdoors. The International Fuel Gas Code recommends this test as necessary because of the changes being made in the venting system by removing the old appliance. It should be performed on all furnaces, boilers, hot water tanks, or other fuel-burning appliances during inspections or installations, including the installation of woodstoves or other fossil fuel-burning appliances.


    (Note: If the appliance fails this test, do not proceed until repairs are made.)

    The following procedure, as outlined in the 2003 International Fuel Gas Code, and used here with permission, is intended as a guide to aid in determining that an appliance is properly installed and is in a safe condition for continuing use. It should be recognized that generalized procedures cannot anticipate all situations. Accordingly, in some cases deviation from this procedure may be necessary to determine safe operation of the equipment.

    If you discover a condition that could result in unsafe operation, shut off the appliance and advise the owner of the unsafe condition.

    How to conduct a test for gas leakage:

    1. Visually inspect the venting system for proper size and horizontal pitch; check for blockage, restriction, leakage, corrosion, and other deficiencies that could cause an unsafe condition. This will require removal of the vent from the chimney in most cases.

    2. Shut off all gas to the appliance. Shut off any other gas-burning appliance within the same room. Use the shutoff valve in the supply line to each appliance.

    3. Inspect burners and crossovers for blockage and corrosion.

    4. Inspect the heat exchanger for cracks, openings, or excessive corrosion (applicable only to furnaces). Inspect for evidence of water or combustion product leaks (applicable only to boilers).

    5. Close all building doors and windows as much as practical, and close all doors between the space in which the appliance is located and other spaces of the building. Turn on clothes dryers. Turn on any exhaust fans, such as range hoods and bathroom exhausts, so they will operate at maximum speed. Do not operate a summer exhaust fan. Close fireplace dampers.

    6. Check to see if there is sufficient combustion air for the heating appliance. If not, refer to Section 304 of the Code for guidance.

    7. If there is sufficient combustion air, place the appliance being inspected into operation. Follow the lighting instructions. Adjust the thermostat so the appliance will operate continuously.

    8. Determine that the pilot (where provided) is burning properly and the main burner ignition is satisfactory, by interrupting and re-establishing the electrical supply to the appliance in any convenient manner. If the appliance is equipped with a continuous pilot, test the pilot safety device to determine if it is operating properly by extinguishing the pilot when the main burner is off and determining, after three minutes, that the main burner gas does not flow upon a call for heat. If the appliance is not provided with a pilot, test for proper operation of the ignition system in accordance with the appliance manufacturer's lighting and operating instructions.

    9. Visually determine that the main burner gas is burning properly, i.e., no floating, lifting, or flashback. Adjust the primary air shutter as required. If the appliance is equipped with high and low flame controlling or flame modulation, check for proper main burner operation at low flame.

    10. Test for spillage at the draft hood relief opening after five minutes of main burner operation. Use a match flame, candle, or smoke.

    11. Turn on all other gas-burning appliances within the same room so they will operate at their full inputs. Follow lighting, flame inspection, and spillage inspection instructions for each appliance.

    12. Return doors, windows, exhaust fans, fireplace dampers, and any other gas-burning appliance to their previous conditions of use.

    13. Applicable only to furnaces: Check both the limit control and fan control for proper operation. Limit control operation can be checked by blocking the circulating air inlet or by temporarily disconnecting the electrical supply to the blower motor and determining that the limit control shuts off the main burner gas. Applicable only to boilers, determine that water pumps are in operating condition. Test low-water cutoffs, automatic-feed controls, pressure- and temperature-limit controls, and relief valves in accordance with the manufacturer's recommendations to determine that they are in operating condition.

    Here are some notes:

  • To properly burn 1 cubic foot of natural gas, 15 cubic feet of air is needed for combustion and to meet excess air requirements.

  • If the gas appliance is a natural-draft appliance, an additional 15 cubic feet of dilution air are vented through the draft hood.

  • A mid-efficiency furnace will consume 15 cubic feet of air/1 cubic foot of natural gas.

  • A natural-draft appliance will need 30 cubic feet of air/1 cubic foot of natural gas burned.

    This means a 100,000-Btuh furnace would require 3,000 cubic feet of ventilation/combustion air for each hour of continuous operation. If not, makeup air provisions are made. The air must be replaced through infiltration air openings.

    The flame in this photo was disrupted at about the same time as the blower started. A cracked heat exchanger, however, was not the problem. Blockage (in this case, from a bird in the heat exchanger) was causing a lack of combustion air in the cell. An oxygen-starved flame will travel toward a source of air.


    (Note: If the appliance fails this test, do not proceed until repairs are made.)

    The Gas Appliance Manufacturers Association (GAMA) funded a study to determine the most effective method for checking a heat exchanger. The study determined that:

  • Three-part testing is necessary to determine the integrity of a heat exchanger.

  • The International Fuel Gas and Oil Codes state that the heat exchanger should be checked on all service calls. The codes do not specify a method.

  • The three-part procedure is as follows:

    1. Watch the flame when the blower comes on. The blower operation should not affect the flame pattern.

    2. Perform a visual inspection of the heat exchanger. This may be limited by the shape of the heat exchanger and by visual obstructions, such as an evaporator coil.

    3. Perform a chemical test on the heat exchanger. Introduce a chemical that can be detected inside the heat exchanger, then use an instrument that can sense that chemical in the supply air stream.

    The chemical test was performed with a tracer gas, nitrogen and methane, and a calibrated detector, usually calibrated to 1,200 cc of tracer gas. The study was performed in 1986, and was validated and recommended by GAMA. However, no method has been officially adopted into the code.

    A more modern alternative to the trace gas test is the O2 test using a combustion analyzer. The O2 test has several advantages over the trace gas test, including:

  • Testing during normal operation under normal temperatures.

  • Locating leaks below the burner.

  • Multiple tests can be performed at once, including combustion air testing, heat exchanger testing, CO testing, and efficiency testing.

  • A combustion analyzer can be used to determine unacceptable leakage in atmospheric draft appliances and draft-induced appliances using O2 as the trace gas (which will be below 21 percent and stable during normal operation), and an analyzer as the calibrated measuring device.

    If you perform this test method, note that as of this writing, there has been no formal field study to document the correlation of minimum leakage rates and O2 changes. During lab testing, however, we were able to determine leakage through a single, 1/8-inch test hole drilled in several locations in the heat exchanger. After each test, the hole was plugged with a screw and an additional location was tested. Multiples of holes were also tested to simulate pin holing in a heat exchanger. Locations were specifically selected that avoided air blowing directly into the heat exchanger, and rather depended upon system static pressure to force additional air from the supply side into the heat exchanger.

    If the O2 level changes when the blower starts, there is leakage into the heat exchanger and further investigation is warranted. Closing main dampers will increase supply-side static pressure and further amplify the leakage rate. If you can't find cracks or holes, you still need to have the customer officially sign off that they have been notified that there is a potential problem with the operation of the furnace.


    You can use a flue gas combustion analyzer effectively for finding leaking cracks or holes in a furnace heat exchanger. Keep in mind that not all cracks or holes will leak. Smaller cracks and holes, found only by a thorough visual inspection, may not leak during the time of testing. However, they still pose a potential danger to your customer. Cracks will continue to open over time due to the tremendous forces on the metal at the ends of the crack.

    On all furnaces, the static pressure achieved by the system blower can overcome any positive pressure in an atmospheric draft appliance. On draft-induced appliances, the pressure within the heat exchanger is always negative, causing them to leak in rather than out. Any leakage in a heat exchanger, in or out, poses some danger to the customer. Leaks out can allow flue gasses that may contain CO into the living space; pressurization of a draft-induced appliance's heat exchanger can result in a rollout and possible fire.

    With all combustion appliances, readings on the analyzer should be stable after several minutes. When the stack temperature stabilizes, all other gas readings on the analyzer should also remain stable. Readings that change during operation, after stabilization has taken place, indicate a problem due to combustion air, venting, or a mechanical source, such as a cracked heat exchanger.

    (Note: Cleanouts on oil furnaces and older gas appliances can test positive for leakage. This is not a heat exchanger failure. Inspection gaskets should be replaced and properly sealed following the manufacturer's recommendations. If those recommendations are not available, an industry-approved method should be used to seal the cleanout.)

    O2 Test Procedure:

    1. Follow the manufacturer's instructions to properly zero the combustion analyzer.

    2. Insert the combustion analyzer in the appropriate test position in the furnace. For atmospheric draft appliances, this would be directly in the heat exchanger cell. For 80- to 90-AFUE furnaces, this would be in the stack.

    3. Start the furnace and observe the oxygen reading for stability (one to three minutes).

    4. When the blower starts, watch for a change in the O2 reading. If the blower starts prior to stabilization of the O2 reading, insert and remove a piece of cardboard during operation to observe if any changes in the combustion readings take place.

    Corrective action:

    1. Attempt to visually find the crack or hole.

    2. If the defect is found, show it to the customer.

  • Write, on the service invoice, that the testing indicates a leak in the heat exchanger, even if the leak is not found.

  • Inform the customer, in writing, that the heat exchanger has a defect and poses a potential danger even if the leak is not found. Keep a copy of the written message for the office's files. Using the word potential is also very important. If a service person tells customers that they will get CO poisoning, or that they are going to get sick, the service person may be accused of trying to use scare tactics.

  • Explain the potential health risk:

    a. A defective heat exchanger allows flue gases to enter the building.

    b. If poor combustion takes place, carbon monoxide may enter into the structure. Carbon monoxide is a deadly gas. It is colorless, odorless, and it displaces oxygen in the blood stream. High levels of carbon monoxide can cause brain damage and/or death.

    c. Ask the customer's permission to shut down the furnace for their safety. Open the safety disconnect at the furnace. Record that request on the service invoice.

    3. Possible solutions: Replace the heat exchanger, or replace the furnace. Never attempt to repair a heat exchanger.

    Visit for a supplement to this article providing specific troubleshooting procedures and typical readings for various types of furnaces.

    Publication date: 04/17/2006