It is not possible to measure the theoretical temperature of a combustion process in the field, due to dilution of the gases and absorption of the heat by radiation to the surrounding areas. Therefore, the combustion equation has been developed to determine whether a combustion process is being properly handled.
The efficiency calculation is based on the theoretical heat value of the fuel being burned, minus the stack losses. The combustion efficiency calculation is a reasonable estimation of the appliance thermal efficiency, but it is not representative of the published AFUE. Combustion efficiency is representative of the combustion process and associated stack losses. It does not factor in how efficiently heat manufactured in the combustion process is used.
Oxygen (O2) reading: The O2 reading is by far the most important reading an analyzer measures with regard to combustion. The oxygen level in the atmosphere remains constant (20.9 percent), and is the only true constant in the combustion process. While the air always contains 20.9 percent O2 by percentage, the amount of O2 by mass provided to the burners varies with the air density.
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 that all burner shields are in place to avoid the entry of excess secondary air.
Residential furnaces often do not provide combustion air adjustment provisions, because manufacturers have determined that the safety gained by providing additional air to ensure complete combustion outweighs the potential savings that adjustment might provide. Also, excess air lowers the dew point of the flue gases by dilution, thereby lowering the probability of condensation in the stack in noncondensing appliances.
Carbon dioxide (CO2) reading: The CO2 level in the flue gas provides an indication of the efficiency of the combustion process. If the production of CO2 is as high as possible with slight excess air (complete combustion), the flue gas heat losses are at their lowest. The CO2 reading is calculated from the O2 reading by the analyzer.
Each fuel has a maximum possible CO2 level (CO2 max), which is determined by the chemical composition of the fuel:
• Light fuel oil - 15.4 percent by volume CO2
• Natural gas - 11.8 percent by volume CO2
This maximum theoretical level is never reached in practice.
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 to determine the net stack temperature; it 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 the CO2 are at the highest level and the O2 is at its lowest level. The primary importance of stack temperature is to provide enough heat in the flue to prevent water formation in noncondensing appliances, and a low enough temperature in condensing appliances to ensure the removal of latent heat from the flue gases.
Condensation on noncondensing appliances can cause chimney deterioration, liner failure, and rusting of the appliance.
Reducing the temperature of the flue gases provides only a small benefit in the appliance efficiency. For every 50°F the stack temperature is lowered, there is less than a 1 percent gain in efficiency. The stack temperature should be 270° to 370° above the supply air temperature (or supply water temperature on noncondensing atmospheric appliances), and 170° to 270° above supply air temperature on draft-induced appliances.
On condensing appliances, the stack temperature ideally will approach the return air temperature and will always be below 125°. The lower the return air temperature, the higher the efficiency will be on a condensing appliance. Until the flue gases are lowered to the condensing range, there is not a significant increase of the appliance’s thermal efficiency. (Remember, the analyzer is looking at a modified equation that considers combustion efficiency and stack losses of the dry gases. The efficiency calculation may not reflect the thermal efficiency of a condensing appliance.)
Dew point temperature: This is the temperature below which water vapor contained in the flue gas will turn to a liquid state (condensation). Below the dew point temperature, moisture exists; above the dew point temperature, vapor exists. If the chimney or venting material falls below the dew point temperature, condensation will occur in the flue. The dew point temperature is a calculated value the tech can reference if he suspects there is condensation of a noncondensing appliance.
Smoke spot number: This number is determined by using a smoke spot tester. A certain quantity of flue gas is drawn through a filter paper by a certain number of strokes. The degree of blackening of the resulting spot on the filter paper is compared to a scale of gray tones with different numbers. The smoke spot number derivative determined in this way (according to Bacharach) is between 0 and 9. The smoke spot number is not measured in gas burners. Ideally the smoke spot number will be a 0 to 1 with a trace of soot. Smoke numbers above this will result in poor combustion and formation of soot on the heat exchanger.
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, and it’s often eliminated by reducing the amount of excess air to the burner.
Fuel pressures: The only two factors that affect input to an appliance are 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; overfiring or underfiring will result, which leads to premature equipment failure.
Nitrogen oxides (NOx): 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 excess air.
High excess air levels (>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 is entering the combustion process in order to limit the amount of extra nitrogen and oxygen entering the flame. This is accomplished through burner design and can be optimized through the use of oxygen trim controls on commercial applications. Low excess air firing is used on most appliances and generally results in overall NOx reductions of 5-10 percent when firing natural gas.
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 through modification of air-fuel mixing patterns, or creation of intentional flame impingement. The lower flame temperature and decreased mixing intensity can result in higher CO levels.
ATMOSPHERIC DRAFT APPLIANCES
Atmospheric draft appliances tend to have higher excess air readings and flue gas temperatures due to less complex heat exchanger designs. Remember, there is a positive pressure in the heat exchanger and a negative pressure in the vent connector and stack. A heat exchanger crack in this style furnace will show an increase in excess air readings when the blower starts, and a decrease when the blower stops.
If the primary air shutters or gas pressure are adjusted, it is imperative that a combustion analysis be performed; operating characteristics of the furnace have changed. For proper combustion and venting, approximately 30 cubic feet of air is required per 1,000 Btuh. This means a 100,000-Btuh furnace would require 3,000 cubic feet of ventilation for every hour of operation.
Draft-induced appliances have similar operating characteristics to atmospheric draft appliances, with the exception of lower flue gas temperatures. The stack draft operates identically, but the heat exchanger pressure is now negative.
The function of the draft inducer is to pull combustion byproducts through the heat exchanger, not to create positive pressure in the vent. If the vent pressure is positive, the flue pipe is clogged.
These furnaces characteristically do not leak flue gas into the house during heat exchanger failure due to the negative heat exchanger pressure. Combustion ventilation air requirements are reduced to 15 cubic feet/1,000 Btuh. No additional dilution air is required for venting unless a liner is not used.
Note: If the furnace is multistage or modulating, each stage must be checked independently to ensure safe operation through the entire operating range.
High-efficiency appliances are considerably different in operation. The heat exchanger pressure is negative, and the pressure becomes positive. Flue gas temperatures drop below 125°. Often outdoor air is used for combustion, allowing these furnaces to operate without indoor air ventilation requirements. If used with a two-pipe configuration, the combustion air temperature must be referenced to get an accurate combustion test result. All burner shields and doors must be in place.
The high efficiency of these appliances is achieved by removing the latent (hidden) heat from the flue gases by condensing the water from the byproducts of combustion. This additional removal of heat through a secondary heat exchanger lowers the flue gas temperature below 125°. High heat extractions, in conjunction with careful control of combustion air, allow these furnaces to operate with high combustion efficiencies and very high thermal efficiencies.
Combustion air required by these furnaces is reduced to 10 cubic feet/1,000 Btuh. Provisions for condensate removal must be made for condensing-type furnaces and boilers.
POWER BURNER APPLIANCES
Power burner appliances tend to have lower excess air readings and higher stack temperatures due to less complex heat exchanger designs, although some do approach condensing. Remember, there could be a positive pressure in the heat exchanger and a negative pressure in the stack, depending on the design.
A heat exchanger crack in this style of furnace may not show an increase in excess air readings when the blower starts or a decrease when the blower stops. If the primary air shutters/air band or gas pressure is adjusted, it is imperative that a combustion analysis is performed, since operating characteristics of the furnace have changed.
For proper combustion and venting, approximately 20 cubic feet of air is required/1,000 Btuh. This means a 100,000-Btuh furnace would require 2,000 cubic feet of ventilation for every hour of operation. Always set the gas manifold pressure per the manufacturer’s specifications.
OIL POWER BURNER APPLIANCES
Oil power burner appliances tend to have lower excess air readings and higher stack temperatures due to less complex heat exchanger designs. Some high- and ultra-high-efficiency models approach and operate in condensing mode. Remember, these furnaces must operate with a negative pressure in the heat exchanger and a negative pressure in the stack unless otherwise specified by the manufacturer.
A sizable heat exchanger crack in this style furnace will be indicated by an increase in excess air readings when the blower starts, and a decrease when the blower stops. If the primary air shutters/air band or oil pressure is adjusted, it is imperative that a combustion analysis is performed, since operating characteristics of the furnace have changed. For proper combustion and venting, approximately 25 cubic feet of air is required/1,000 Btuh. This means a 100,000-Btuh furnace would require 2,500 cubic feet of ventilation for every hour of operation.
SELECTING THE BEST COMBUSTION EQUIPMENT
Digital combustion equipment provides real-time answers to the dynamic combustion process. Technology in most cases is more reliable and dependable than it has ever been. For analyzers like the Testo 330, long life is expected due to a chemical cell life in excess of six years (compared to earlier models with an expected life of one-half of that).
Modular designs and better filtering help ensure a long pump life, simple service, and can guarantee the instrument will spend more time traveling in your truck than in its way for repair. Today’s combustion instrumentation tends to be more intuitive to operate and easier to maintain.
It’s time to stop making excuses and start testing; if you’re not, you are leaving money on the table and putting your customers and yourself at risk. Combustion and ventilation air testing is part of the reasonable standard of care that should be provided to all customers. Couple with that the additional revenue it creates and it makes very good business sense.
Sidebar: Typical Readings
Atmospheric draft gas fired burners*:
• Efficiency - 75-80 percent.
• O2 - 7-9 percent.
• CO2 - 6.5-8 percent.
• Stack temp - 325° to 500°F.
• Draft - -0.02 to -0.04 inch wc.
• CO - < 100 ppm (undiluted).
Oil-fired power burners*:
• O2 - (cast iron cone) 5-9 percent.
• O2 - (flame retention) 3-6 percent.
• CO2 - 10-12.5 percent.
• Stack temp - 60-79 percent AFUE 400° to 600°
• Stack temp - 80-plus AFUE 330° to 450°.
• Stack temp - 90-plus AFUE less than 125°.
• Draft - -0.02 inch wc overfire.
• Draft (stack) - -02 inch wc/0.04 inch wc.
• CO - <50 ppm (diluted).
• Smoke spot - #0-#1.
• Oil pressure - 100-150 psi (per manufacturer).
Gas-fired power burners*:
• O2 - 3-6 percent.
• CO2 - 8.5-11 percent.
• Stack temp (gross) - 320° to 570°
• Draft - -0.02 to -0.04 inch wc overfire.
• Draft - positive pressure (manufacturer’s specifications).
• CO - <100 ppm (diluted).
Draft sealed combustion induced gas-fired burners (90-plus AFUE)*:
• Efficiency - 88-92 percent.
• O2 - 5-7 percent.
• CO2 - 7-8.5 percent.
• Stack temp - Less than 125°.
• Draft - 0.02 to 0.08 inch wc.
• CO - <100 ppm (undiluted).
Draft induced gas fired burners*:
• Efficiency - 80-82 percent.
• O2 - 7-9 percent.
• CO2 - 6.5-8 percent.
• Stack temp - 325° to 400°.
• Draft - -0.02 to -0.004 inch wc.
• CO - < 100 ppm (undiluted).