This article is part one of a three-part series on inspecting and servicing gas appliances.

First, it should be noted that, with every equipment inspection, there is an implied warranty. The service technician must be aware that it has been upheld in court that unless the consumer is notified in writing that there is a problem, or a potential problem, the person performing the inspection has determined that the appliance is operating safely and correctly.

The equipment inspections covered in this article series meet industry standards as outlined by:

1. The National Fuel Gas Code (ANSI Standard Z223.1 and NFPA 54);

2. State building codes;

3. RSES Publication 630-92 9/86;

4. The Gas Appliance Manufacturers Association (GAMA);

5. The International Fuel Gas Code; and

6. Many manufacturers' equipment installation, operation, and maintenance guides.

When each inspection is performed in its entirety, it will assure as much as possible the equipment is operating correctly and safely. Doing a proper inspection takes time and practice, but the benefits to your company and your customers will be paid back tenfold in increased comfort, lower utility bills, improved safety, lower callback rates, increased service sales, and higher professionalism for your company.

Lastly, it is not important that a service technician knows everything. What is important, however, is that as a technician you know where to find information and how to read and interpret an installation or service manual and properly test the appliance and its components to verify safe and proper operation. Check sheets should always be used as a service tool. They are as valuable for the senior technician as the junior man. Check sheets keep everyone on the same page by consistently performing the same inspection techniques and practices.

The checks outlined here are general in nature; however they do meet or exceed most appliance manufactures' guidelines. While this information is current today, technology and procedures are changing with the equipment. If available, use the manufacturer's recommended service guide along with the information provided here to perform a thorough inspection. As a technician, you are responsible for keeping up with technology and service and maintenance procedures. If in doubt, always consult the manufacturer.

The information contained throughout this article series follows the gas furnace check sheet provided in the PDF link below. (Click on the link "Equipment Check Sheet" at the bottom of this article.) I would recommend that you make several copies of this check sheet to use on jobs as a reference source.

Prior to testing any equipment at an existing installation to perform a clean and check or other maintenance, ambient CO levels should be checked prior to entering the home and the equipment should be run through a complete cycle (unless the serviceperson is specifically there for a no heat call) to determine if the call is in fact a clean and check rather than a no heat call.

Infiltration Air And The Ventilation Air Test

The following information is outlined in the International Fuel Gas Code, Appendix D.

Ventilation Air: The total air, which is a combination of the air brought inside from outdoors and the air that is being recirculated within the building. Sometimes, however, used in reference only to the air brought into the system from the outdoors. This article defines this air as indoor and outdoor ventilation air.

Infiltration Air: The term used to describe the outdoor air that enters a building through cracks or unintentional openings.

Combustion Air: 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: The air that enters a draft hood or draft regulator and mixes with the flue gases.

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:

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

B. There is a very high probability that carbon monoxide (CO) will be generated due to the lack of air (oxygen) inside the building.

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

D. 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 gases into the building.

E. Vent spillage increases the probability that the flue gases will contain CO due to poor combustion.

The Ventilation Air Test:
A. 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.

B. The procedure for the ventilation air test is outlined in the International Fuel Gas Code (ANSI Standard Z223.1).

C. It should be performed on every gas appliance installation and every gas appliance service call prior to servicing the appliance.

Background Information:
This procedure should be performed prior to any attempt at modification of the appliance or of the installation; this includes servicing, clean and checks, and/or mechanical changes.

If it is determined there is a condition that could result in unsafe operation, the appliance should be shut off and the owner advised of the unsafe condition. If there is not sufficient air for combustion and/or ventilation, the homeowner and or technician will be at risk by operating the appliance under worst-case conditions.

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. Any changes made should be noted on the work order and signed off on by the customer before the changes are made. Any appliance left in operation must not show any signs of combustion/ventilation problems.

This test is performed to ensure that the building into which you are going to install, or have installed, a fossil fuel appliance has enough ventilation/infiltration air to replace the air used in the combustion and venting process. As homeowners are constantly making changes to the home (caulking windows, adding weatherstripping), this test should be performed on an annual basis.

Modern buildings are much tighter than old buildings; some do not allow enough leakage for infiltration air to enter the building from the outdoors. Winterization practices on older homes have sealed many of the openings that provided combustion and ventilation air.

If the building cannot allow enough infiltration air into it, provisions must be made to bring in outdoor air to replace the air used in the combustion and ventilation process. This could mean the installation of natural or mechanical combustion air to assure proper combustion and venting.

This test should be performed even if you are installing a 90+ furnace that takes all of the combustion air directly from the outdoors. This test is recommended necessary (by the International Fuel Gas Code) since you are making changes in the venting system by removing the old appliance.

This test should be performed on all furnace, boiler, hot water tank, or other fuel burning appliance inspections or installations, including the installation of wood stoves or other fossil fuel appliances.

Recommended Procedure For Safety Inspection Of An Existing Appliance Installation As Outlined In The 2003 International Fuel Gas Code
Note: If appliance fails this test, do not proceed until repairs are made.

The following procedure is intended as a guide to aid in determining that an appliance is properly installed and is in a safe condition for continuing use. This procedure is predicated on central furnace and boiler installations, and it should be recognized that generalized procedures cannot anticipate all situations. Accordingly, in some cases, deviation from this procedure is necessary to determine safe operation of the equipment.

(a) This procedure should be performed prior to any attempt at modification of the appliance or of the installation.

(b) If it is determined there is a condition that could result in unsafe operation, the appliance should be shut off and the owner advised of the unsafe condition. The following steps should be followed in making the safety inspection:

1. Conduct a test for gas leakage. (See Section 406.6 IFGC)

2. Visually inspect the venting system for proper size and horizontal pitch and determine there is no blockage or restriction, leakage, corrosion, or other deficiencies that could cause an unsafe condition. (This will require removal of the vent from the chimney in most cases.)

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

4. Inspect burners and crossovers for blockage and corrosion.

5. Applicable only to furnaces: Inspect the heat exchanger for cracks, openings, or excessive corrosion.

6. Applicable only to boilers: Inspect for evidence of water or combustion product leaks.

7. Insofar as is practical, close all building doors and windows and 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. If, after completing Steps 8 through 13, it is believed sufficient combustion air is not available, refer to Section 304 of the code for guidance.

8. Place the appliance being inspected in operation. Follow the lighting instructions. Adjust the thermostat so appliance will operate continuously.

9. Determine that the pilot(s), where provided, is burning properly and that the main burner ignition is satisfactory by interrupting and reestablishing the electrical supply to the appliance in any convenient manner. If the appliance is equipped with a continuous pilot(s), test the pilot safety device(s) to determine if it is operating properly by extinguishing the pilot(s) when the main burner(s) 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(s), test for proper operation of the ignition system in accordance with the appliance manufacturer's lighting and operating instructions.

10. Visually determine that the main burner gas is burning properly (i.e., no floating, lifting, or flashback). Adjust the primary air shutter(s) 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.

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

12. Turn on all other fuel-gas-burning appliances within the same room so they will operate at their full inputs. Follow lighting instructions for each appliance.

13. Repeat Steps 10 and 11 on the appliance being inspected.

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

15. Applicable only to furnaces: Check both the limit control and the fan control for proper operation. Limit control operation can be checked by blocking the circulating air inlet or temporarily disconnecting the electrical supply to the blower motor and determining that the limit control acts to shut off the main burner gas.

16. Applicable only to boilers: Determine that the 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.

1. To properly burn 1 Ft3 of natural gas, 15 Ft3 of air is needed for combustion and to meet excess air requirements.

2. If the gas appliance is a natural draft appliance, an additional 15 Ft3 feet of dilution air is vented through the draft hood.

A. A mid efficiency furnace will consume 15 Ft3 of air/1 Ft3 of natural gas. (1 Ft3 of natural gas âäˆ 1,000 Btuh)

B. A natural draft appliance will need 30 Ft3 of air/1 Ft3 of natural gas burned.

This means a 100,000 Btuh furnace would require 3,000 Ft3 of ventilation/combustion air for each hour of continuous operation. If no make-up air provisions are made, the air must be replaced through infiltration air openings.

Equipment Efficiency

Telling the exact equipment AFUE efficiency is impossible in the field as installed. There are several types of efficiency including AFUE (annual fuel utilization efficiency), combustion efficiency, and actual system operating efficiency. An old furnace can have 85 percent combustion efficiency, yet have an AFUE of 60 percent because of the amount of heat going up the stack. A 95 percent AFUE furnace can be operating in the 60 percent range due to poor installation or operation. In general, furnace efficiency (AFUE) can be generalized by the following data and guidelines provided the installation is adequate.

60-70 percent AFUE range: Standing pilot, draft diverter, belt drive blower, and a single upshot burner. Single wall flue pipe.

70-78 percent AFUE range: Intermittent pilot direct spark or HSI, draft diverter, with/without flue damper, direct drive blower, multi cell construction with ribbon, slotted, or ported burners. Single wall flue pipe.

80 percent AFUE range: Intermittent pilot direct spark or HSI, induced draft, direct drive blower, could be multistage and/or variable speed, jet or inshot type burners, single or double wall flue pipe. If vented in masonry chimney, it must be lined.

90-97 percent AFUE range: Intermittent pilot direct spark or HSI, induced draft, direct drive blower, could be multistage and/or variable speed, jet or inshot type burners, secondary heat exchanger, plastic flue pipe.


The furnace venting should be disconnected from the appliance and checked for blockage from the appliance to the exit of the chimney on all atmospheric and induced draft appliances, including inspection of the chimney liner (tile or metal) as equipped. The vent should be pitched uphill at 1/4-inch per foot of horizontal run. If the connector or vent pipe has any holes or evidence of rust spotting on the outside of the single wall connector, it should be replaced, as rust normally occurs from the inside out. The flue can contain a maximum of four 90 degree elbows and the flue must establish draft within five minutes of operation. Any evidence of staining around the combustion chamber or the draft hood is probably an indicator of a combustion or ventilation air deficiency and warrants further inspection/correction.

For high efficiency furnaces, inspect the vent system inlet for leaves or other debris, look for evidence of water leakage form improperly sealed joints or connections, verify vent pipe slopes back toward furnace, and no traps are installed in the venting system that could hold water and block the vent pipe.

The compact nature of modern furnaces makes heat exchanger inspection a challenge. With the aid of a fiber optic scope, the tubular style heat exchanger can be inspected back to the first set of bends where failure often occurs due to the higher temperatures.

Three-Part Testing Procedure For Checking A Heat Exchanger

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

The Gas Appliance Manufacturers Association funded a study to determine the most effective method for checking a heat exchanger. That study and the recommendations for the testing procedure were published in RSES Publication 630-92 9/86. It determined that:

A. A three-part testing procedure was necessary to determine the integrity of a heat exchanger.

B. The three-part procedure recommended, in summary, 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 into the inside of the heat exchanger, then use an instrument that can sense that chemical in the supply air stream.)

C. The International and National Fuel Gas Codes say that you should check the heat exchanger on all service calls. They do not specify the method.

The chemical test was performed with a trace gas (nitrogen and methane) and a calibrated detector usually calibrated to 1,200cc of trace gas. The study was performed in 1986, and although validated and recommended by GAMA to become part of the appendix for heat exchanger testing in the National Fuel Gas Code, no method has been officially adopted into the code.

An 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. Leaks below the burner will be evident. 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 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 the analyzer as the calibrated measuring device.

If performing this test method, it should be noted, as of this writing, there has been no formal field study to document the correlation of minimum leakage rates and O2 changes. However, during lab testing, 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. Multiple holes were also tested to simulate pin holing in a heat exchanger. Locations were specifically selected that avoided air blowing directly in 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 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 and further amplify the leakage rate. If cracks or holes cannot be found, the customer should still sign off in writing that they have been notified there is a potential problem with the operation of their furnace.

Performing O2 Test:
A flue gas combustion analyzer can be used effectively for finding leaking cracks or holes in a furnace heat exchanger. Not all cracks or holes will leak. Smaller cracks and holes found only by a thorough visual inspection may not be leaking during the time of testing, but they still pose an immediate danger to your customer, as 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, and 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 a danger to your customer. Leaks out can allow flue gases that may contain CO into the living space, and pressurization of a draft induced appliance heat exchanger can result in a rollout and possible fire. With all combustion, the 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 are indicative of a combustion air, venting, or mechanical problem such as a cracked heat exchanger.

Important notes:
1. Oil furnaces and older gas appliances can have cleanouts leaking that will 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 the manufacturer's recommendations are not available, an industry-approved method should be used to seal the cleanout.

2. No inspection method is foolproof. The three-part method should always be performed to maximize safety of the appliance.

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.

A. For atmospheric draft appliances this would be directly in the heat exchanger cell.

B. For 80+/90+ 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, a piece of cardboard can be inserted and removed during operation to observe if any changes in the combustion readings take place.

5. Corrective action: Attempt to visually find the crack or hole.

A. If you can find the defect, show it to the customer.

B. Write, on the service invoice, that your testing indicates a leak in the heat exchanger (even if you cannot find the leak).

C. Inform the customer, in writing, that the heat exchanger has a defect and poses a potential danger (even if you cannot find the leak).

Using the word POTENTIAL is very important. If the technician tells them that they will get CO poisoning, or that they are going to get sick, the tech can be accused of trying to scare the customer.

D. Explain the potential health risk:

1) The defective heat exchanger is allowing the flue gases to enter the home/building.

2) If poor combustion takes place, there is the potential to allow carbon monoxide into the structure.

3) Carbon monoxide is a deadly gas. It is colorless, odorless, and displaces oxygen in the blood stream. High levels of carbon monoxide can cause brain damage and/or death.

4) Ask the customer if you can shut down the furnace for their safety. (Open the safety disconnect at the furnace.) Record that request on your service invoice.

5) Possible solutions:

a. Replace the heat exchanger.

b. Replace the furnace.

Note: You should NEVER attempt to repair a heat exchanger.

For a PDF of a customer handout that your company may want to use if a heat exchanger defect is indicated, click on the link "Heat Exchanger Problem Sheet" below.

James L. Bergmann is an HVAC/R Technical Specialist with Testo Inc. For more information, e-mail or visit

Publication date: 10/31/2005

CLICK HERE for Equipment Check Sheet.

CLICK HERE for Heat Exchanger Problem Sheet.