Appliance safety should be the primary goal of all technicians. It’s up to the technician to make sure that all reasonable measurements are taken, with the appropriate instruments.
[Editor's note: This is part four of a five-part series.]

Before you test any equipment at an existing installation, before you even perform a clean and check or other maintenance, you need to check ambient carbon monoxide (CO) levels before entering the home. Run the equipment through a complete cycle unless you are there for a no-heat call.

If at any time during this test, ambient CO levels exceed 100 ppm, evacuate occupants and ventilate the dwelling immediately. For lower CO levels, consult the "Ambient CO Limits" section of the Testo Combustion Guide or consult a local authority with jurisdiction over these matters.

Perform the test before entering any home, boiler room, basement, or crawlspace where an appliance is located. CO levels in the home should always be verified before you enter the space, and before you zero the analyzer for an appliance combustion test. If you do not use an ambient combustion air temperature probe, the analyzer must be rezeroed when the probe is surrounded by air similar in temperature to the air that is used for combustion if CO is present.

Appliance safety should be the primary goal of all technicians. The amount of CO in flue gases should be kept below 100 ppm air free, even though the allowable limit in the stack is 400 ppm air free.


Whenever CO is rising and not stable at any level from 100 to 400 ppm during the combustion process, the burner should be shutdown and/or immediately repaired. A burner with rising CO is far more dangerous than a stable reading because the CO can continue to rise far above dangerous levels.

Levels above 400 ppm are not permissible and require the immediate repair and/or shutdown of the appliance.

The ambient measurement of CO does normally result from a cracked heat exchanger. More often than anything else, it is the result of auto exhaust from an attached garage, and/or depressurization of the home, resulting in sufficient air for combustion.

If CO is detected, all possible sources should be checked, including, but not limited to, water heaters, gas ovens and stoves, the furnace, nonelectric space heaters, and vented or unvented appliances, like gas logs and fireplaces.


All appliances are required to have and maintain draft during operation. According to American National Standards Institute (ANSI) standards, an appliance has 10 minutes to prove draft. A spill switch must open within 10 minutes in a completely blocked flue.

During study-state operation conditions, the draft should be stable. Draft will increase as the flue warms until it reaches maximum flue temperature and stabilizes. Some analyzers will record and store the draft reading for the flue gas measurement screen and printout.

(Note: If the appliance and chimney are cold, it can take up to 10 minutes to establish draft. Carefully monitor conditions in the room where appliance testing is being done; CO produced by the appliance could overcome you without warning.)

A system that has been under operation should have or establish draft very quickly. If draft-induced and atmospheric appliances are common-vented, always verify that the atmospheric appliance is not spilling when both appliances are operating, and that flue gases are not spilling through the draft hood on the atmospheric appliance when it's operating by itself. Both appliances, if common vented, should be draft tested.

If multiple draft-induced appliances are common-vented, ensure that spillage does not occur through the heat exchanger and out the burner openings of the adjoining appliance while it is in its standby position (off).

Kitchen exhaust fans, doors opening or closing, etc., should not affect draft.

Getting an accurate picture of an appliance’s combustion efficiency means taking multiple measurements and knowing what to compare them against. Knowledge is a technician’s best means of protection.


Efficiency:It's not possible to measure the theoretical temperature of a combustion process in the field, due to the dilution of gases and absorption of heat by radiation to the surrounding areas. That's why the combustion equation has been developed - to determine whether a combustion process is being properly handled.

The efficiency calculation is based from the theoretical heat value of the fuel being burned minus the stack losses.

O2 reading: The oxygen (O2) reading is by far the most important reading a combustion analyzer measures. The oxygen level in the atmosphere remains constant (20.9 percent); it's the only true constant in the combustion process. The O2 reading should be monitored to produce a flame with the lowest excess air reading possible while maintaining a safe level of CO in the stack.

Excess air readings should always fall within the manufacturer's published guidelines. Always make sure all burner shields are in place to avoid the entry of excess secondary air. Also, excess air lowers the dewpoint of the flue gases by dilution, lowering the probability of condensation in the stack.

CO2 reading: The CO2 level in the flue gas indicates the efficiency of the burner. If the production of CO2 is as high as possible with slight excess air (complete combustion), flue gas heat losses are at their lowest. The CO2 reading is calculated from the CO2 reading by the analyzer.

Maximum CO2: For each fuel there is a maximum possible CO2 level ( CO2 max), determined by the chemical composition of the fuel, which is never reached in practice. CO2 max values for different fuels include: Light fuel oil - 15.4 percent by volume CO2 and natural gas - 11.8 percent by volume CO2.

Ambient air temperature: The ambient air temperature is measured at the burner inlet. Often this measurement requires an additional probe to measure inlet air temperature when combustion air comes into the burner directly from the outside as in the case of a sealed combustion furnace. The ambient air temperature is used for the efficiency calculation and will not affect other combustion gas calculations.

Stack temperature (the hot spot): The flue gas temperature should be measured in the flue gas hot spot. This is the point in the flue where the stack temperature and CO2 are at their highest levels and O2 is at its lowest. The primary importance of stack temperature is to provide enough heat in the flue to prevent water, a byproduct of combustion from condensing in the flue and/or the chimney. Condensation in noncondensing appliances can cause chimney deterioration, liner failure, and rust in the appliance.

Reducing the flue gas temperature provides only a small benefit in appliance efficiency. For every 50°F the stack temperature is lowered, there is less than a 1 percent gain in efficiency. Until the flue gases are lowered to the condensing range, there is no significant increase in the appliance's thermal efficiency.

  • For noncondensing atmospheric appliances, the stack temperature should be 270° to 370° above the supply air or supply water temperature.

  • For induced-draft appliances, the stack temperature should be 170° to 270° above supply air temperature.

  • For condensing appliances, ideally the stack temperature will be approaching the return air temperature and always below 125°. The lower the return air temperature, the higher the efficiency will be on a condensing appliance.

    NOx (optional measurement): Measurement of NOx and other pollutants is required in some jurisdictions. As a safety factor, to ensure complete combustion, appliances are fired with excess air.

    One of the factors influencing NOx formation in a furnace or boiler is the excess air level. High excess air levels (greater than 45 percent) may result in increased NOx formation because the excess nitrogen and oxygen in the combustion air entering the flame will combine to form thermal NOx. Low-excess-air firing involves limiting the amount of excess air that enters the combustion process in order to limit the amount of extra nitrogen and oxygen that enters the flame.

    Limiting the amount of excess air entering a flame is accomplished through burner design. It can be optimized through the use of oxygen trim controls on commercial applications. Low-excess-air firing is used on most appliances; it generally results in overall NOx reductions of 5 percent to 10 percent when firing natural gas.

    CO emissions and NOx: High flame temperatures and intimate air/fuel mixing are essential for low CO emissions. Some NOx control technologies used on residential, industrial, and commercial burners reduce NOx levels by lowering flame temperatures. This is accomplished by modifying air-fuel mixing patterns, or creating intentional flame impingement. The lower flame temperature and decreased mixing intensity can result in higher CO levels.

    Dewpoint temperature: Below this temperature, water vapor contained in the flue gas turns to liquid. This change is often referred to as condensation. In a nutshell, below the dewpoint temperature, moisture exists; above the dewpoint temperature, vapor exists. If the chimney or venting material falls below the dewpoint, condensation will occur in the flue. The dewpoint temperature is a calculated value the technician can use if condensation of a noncondensing appliance is suspected.

    Smoke spot number: The smoke spot number is determined using a smoke spot tester. A certain quantity of flue gas is drawn through a filter paper, leaving a spot on the paper. The degree of blackening is compared to a scale of gray tones. The smoke spot number determined in this way (according to Bacharach) is between zero and nine. It is not measured in gas burners.

    Oil residues: Yellow spotting on the filter paper indicates incomplete combustion due to insufficient atomizing of the fuel. This condition is usually accompanied by high CO readings. It is often eliminated by reducing the amount of excess air to the burner.

    Fuel pressure: Two factors affect input to an appliance; fuel pressure and orifice or nozzle size. Fuel pressure should always be measured and set to the manufacturer's prescribed settings. Under no circumstances should fuel pressures be adjusted outside of the designed range, because over- or under-firing could result, leading to premature equipment failure.


    Every manufacturer should have published standards for the operating characteristics of all appliances it manufactures. The manufacturer should always be consulted before you change the operating characteristics of any fuel-burning appliance.

    The typical operating characteristics listed here are secondary to the manufacturers' published guidelines. This is intended only as a reference to typical operation of the particular appliance being tested.

    Since there is 20.9 percent oxygen in normal air, this can be used as a measure of combustion efficiency. Ideally, a flue gas analysis of 0 percent combustibles would be achieved with no excess oxygen in the flue gas. Manufacturers of residential appliances normally require 20 percent to 40 percent (5 percent to 9 percent) excess air to ensure enough air is available for complete combustion, even if an appliance is dirty and suffering from neglect.

    It is possible for a system to have large quantities of excess air in the area of the flame. This can come from secondary air sources, or it can be caused by running the burners at high excess air rates. This is particularly true for oil power burners. The excess air can cause the flame to be quenched before combustion is completed, forming CO and aldehydes (CH3CHO) in the resulting products of combustion. With oil furnaces, excess air can be lowered by closing the draw band.


    Once the fuel has been burned and heat (Btu) is released, the heat must be transferred to the heat exchanger or directly to the air. There are three basic types of heat transfer: convection, radiation, and conduction.

    Convection is the form of heat transfer whereby the temperature of a fluid (gas or liquid) passing across another object, normally a heat exchanger, results in a transfer of energy (temperature) from the flue gas to the heat exchanger. We normally think of this form of heating as going from a hot gas to a cold, solid object. The more turbulent the flow, the greater the heat transfer by convection.

    Convection heating is the most commonly used method of heat transfer in the HVAC industry; hot gases to come into contact with most surfaces of the heat exchanger. To transfer energy to the heat exchanger, the heat energy must penetrate multiple layers of air that are electrostatically bonded to the heat exchanger surface. Air, an excellent insulator, makes this transfer difficult. Therefore, the faster that a fluid (gas stream) passes across the surface, the more rapidly these air layers will be swept away to be replaced by hotter gases. Excessive draft will decrease heat transfer by convection and lower the system efficiency.

    Radiation is the transfer of energy (heat) between surfaces at different temperatures without the two being in physical contact with each other. The most common example is the sun and the earth. The amount of heat transfer via radiation is proportional to the fourth power of the temperature difference between the heat source (emitter) and that which is being heated (receiver).

    Only those surfaces that "see" the heat source will receive the heat waves and have their temperatures raised. At higher temperatures, radiation is the most intense form of heat transfer, but only in straight-line radiation. In a furnace heat exchanger, while a significant amount of heat is transferred through radiation, more complex heat exchanger designs have increased heat transferred by convection and conduction.

    Conduction is the mode of heat caused by the increased activity of molecules within a body. An object heated at one end (by convection and/or radiation) will cause the opposite end to get hot by the molecules passing along the heat. The speed at which this transfer occurs is a function of the thermal resistance of the material (the inverse of the conductivity); the mass of the object; and the temperature differential between the energy source and the surface of the body being heated.

    While conduction is generally the slowest of the heat transfer mechanisms and depends on the molecular structure of the material, it is the only way to completely heat an object once the energy has been transferred by convection and radiation to the surface of the object being heated. With newer heat exchangers being low mass, the time required to reach steady-state efficiency or the point at which a constant rate of input produces a constant rate of output has been significantly reduced.

    (Note: Low-mass heat exchangers do come at a cost. Stresses produced on a low-mass heat exchanger due to loss of airflow or low airflow, overfiring, and/or excessive short cycling, can cause premature failure of heat exchange material and/or mechanical or welded connections. Inspection of the heat exchanger is required if a furnace has been cycling on the high limit control, has experienced blower motor failure, or has been overfired, no matter how short the length of time.)


    On furnaces with or without air shutters on the burner, controlling draft can control the combustion air and, in turn, the secondary air to the furnace. This would only be true where excess draft is apparent. The addition of a double-acting barometric damper to a flue that does not properly draft will not correct a low or no draft problem. If there is no or low measured draft, the chimney and vent pipe should be inspected for blockage, shifted tiles, and/or improper installation or height.

    "AGA Laboratories Field Test Program" details a procedure to control draft and increase efficiency. This is a field report and not equivalent to a design certification. The field-testing program suggests the addition of draft controls on draft hood-equipped appliances to improve performance, lower CO production, and improve overall operation. Testo cannot recommend this course of action because of the number of factors involved in making this recommendation that must be determined in the field, but we do believe you should be aware of its findings.

    In the case of an atmospheric burner, the combustion air is drawn in by buoyancy of the heated flue gases and mixes with the gas as it enters the combustion chamber. The fuel-air mixture burned in the combustion chamber quickly releases its heat to the heat transfer surfaces surrounding it, and the hot flue gas escapes through a draft hood into the flue.

    The role of the draft hood is to prevent too great of a flue draft or a back draft in the flue system from affecting the combustion process. As warm air rises and moves toward the vent connector, fresh air will be required to replace it. So long as draft is provided at the vent connector, the low-pressure zone created in the draft hood will direct all of the flue gases and the proper amount of dilution air into the vent pipe and chimney.

    If a negative pressure is created in the appliance combustion/venting zone that is greater than the draft provided at the vent connector, spillage will occur. Flue gases will always move to the area of lowest pressure. This occurs whether a draft hood or barometric damper is installed, hence the need of spill switches on both to improve safety.

    Sidebar: Gas Pressures

    Although not published in a field or scientific study, some industry experts have recommended adjusting appliance input to improve appliance operation. Testo does not recommend adjusting the fuel input outside of the manufacturer's recommended manifold pressure (usually 3.5 inches wc) to lower the excess air reading and/or increase the combustion efficiency on appliances with or without adjustable primary air shutters.

    While this may result in a slightly (2 percent to 5 percent) more efficient appliance due to increased radiant heat transfer, lower excess air readings, and/or lower CO levels, it can and will result in overfiring the appliance. Condensing problems can cause premature heat exchanger failure and leave unnecessary liability for the technician and company by not setting up the equipment to the manufacturer's specifications.

    With the large variance in the heat content of fuel, and factors that affect air density like temperature and humidity, excess air is a necessary evil. Although it should be carefully controlled to a minimum whenever possible, it is a requirement of all commercial and residential burners.

    Publication date: 03/13/2006