COMBUSTION PROCESS

CH4 + 3O2 = Heat + 2H2O + CO2 + O2

Where:
CH4 = 1 cubic foot of methane gas (natural gas)
3O2 = 3 cubic feet oxygen
Heat = 1,050-Btu energy produced from the chemical reaction
2H2O = 2 cubic feet water vapor
CO2 = 1 cubic foot carbon dioxide
O2 = 1 cubic foot excess oxygen

If there isn't enough air for proper combustion, CO will be generated. CO is a deadly gas. A yellow flame could indicate CO, but a blue flame could be producing it as well. If you don't test with a combustion or CO analyzer, you can't know for sure.

The formula for incomplete combustion in a gas-fired furnace is:

CH4 + 3O2 = Heat + 2H2O + CO (±O2)

CO can be produced with or without excess air. That's why both diluted and undiluted flue gas samples are taken from the combustion analyzer.

What we call air is roughly 78 percent nitrogen, 20.9 percent oxygen, and 1 percent other gases. For every cubic foot of O2 needed, approximately 10 cubic feet of air are required to provide complete combustion. An additional 5 cubic feet of air, excess air, makes sure there is enough air to burn all of the fuel in most residential appliances. Another 15 cubic feet are required if the furnace has a draft hood for dilution air, bringing the total to 30 cubic feet of combustion and ventilation air.

Combustion gases are vented outdoors through vent pipes, chimneys, or plastic exhaust pipes. They must be sized properly to ensure the flue gases can be vented without creating positive (back) pressure within the heat exchanger, unless the appliance and venting materials are specifically designed for this purpose. If the gases back up in the heat exchanger, flames will come out the front of the combustion chamber, creating dangerous conditions called rollout and/or spillage.

Flue gases are produced by burning fuel. They are hot, but have not given up all their heat in the combustion process. Depending on the type of furnace, a certain amount of heat must go out the flue to prevent the gases from condensing. With high-efficiency furnaces, condensing is desirable because of the additional heat extracted from flue gases.

EFFICIENCY OF COMBUSTION

Stack temperature is the temperature of combustion gases (dry and water vapor) leaving the appliance. It shows the energy that did not transfer from the fuel to the heat exchanger. The lower the stack temperature, the more effective the heat exchanger design, and the higher the fuel-to-air/water/steam efficiency. The efficiency calculation considers stack temperature and net losses. It would include losses from dry gas plus losses from moisture and losses from the production of CO.

Each type of fuel has specific, measurable heat content. The maximum amount of heat that can be derived from that fuel is based on the use of pure O2 as the oxidizer in the chemical reaction and maximizing the fuel gas mixture. In the field, O2 is derived from the air. Because the O2 isn't separated from the air prior to combustion, there is an undesirable effect on the chemical reaction.

Air consists primarily of nitrogen. Nitrogen is inert; it plays no role in combustion, but it cools the chemical reaction (burning temperature) and lowers the maximum heat content deliverable by the fuel. Therefore, it is impossible to achieve combustion efficiencies above 95 percent for most fuels, including natural gas, when air is used as the combustion process oxidizer. The fuel's combustion efficiency or maximum heat content, therefore, is based upon the quality of the mixture of fuel and air, and the amount of air supplied to the burner in excess of what is required to produce complete combustion.

Efficiency calculated by the combustion analyzer is a modified equation that considers combustion efficiency and stack losses. It is a part thermal, part combustion efficiency calculation. The equation offers a reasonable estimation of the appliance's steady-state operating efficiency.

Safety is of prime importance to you and your customers. Ambient CO levels should be investigated, no matter how small.

SYSTEM EFFICIENCY

The entire system (furnace/boiler, ductwork, piping) must be evaluated to determine the true efficiency of the system. Combustion efficiency is a valuable part of the system evaluation, but it shouldn't be used as a sole reason or justification for keeping or replacing equipment.

Most furnaces, from low efficiency to high efficiency, can produce measured efficiencies that are higher than their rated AFUE efficiencies if excess air is carefully controlled. The measured combustion efficiencies only determine the quality of the chemical reaction and stack losses. The ultimate thermal efficiency of the appliance is determined by measuring the rate of fuel input, understanding there is a maximum heat content for each fuel type, and dividing it into the appliance's heat output rate. The AFUE equation takes into account the appliance's cycling and associated losses during a typical heating season.

All furnaces that operate with the same combustion efficiency will produce the same amount of heat (latent and sensible) during the combustion process with the same fuel input. The combustion efficiency, therefore, has no bearing on how well the appliance utilizes heat produced after the combustion process has taken place. This is determined by measuring the appliance's thermal efficiency. Heat exchanger design and its ability to transfer heat to the room air, both sensible (measurable with a thermometer) and latent heat (available when water vapor in the exhaust gas is condensed out), determines how well heat produced by combustion is being used.

A digital combustion analyzer makes all the calculations and measurements necessary to determine efficiency, safety, dew point, and the amount of pollution the appliance produces. For most technicians, safety (CO) and efficiency (Eff.) will be the most important and frequently referenced numbers. When safety or efficiency is compromised, other portions of the chemical reaction (CO2, O2) are still referenced, along with calculated values like excess air, to determine the cause of the problem.

Other variables (NOx and SO2) are referenced and controlled to keep them at levels safe for the environment and acceptable to local authorities with jurisdiction over these matters. Some states do not regulate levels of NOx and SO2; therefore they are not usually controlled and measured. Usually larger exhaust sources (higher Btu systems) are targets of NOx and SO2 regulations. (Testo Inc., a test instrument manufacturer, also has a full line of emissions products to measure regulated emissions.)

Unless a component has failed, you as a service technician can only adjust four things on a gas/oil appliance to affect the combustion process:

1. Fuel pressure (should be set to manufacturers' specs).

2. Primary air (on newer furnaces this is not adjustable).

3. Draft (vents byproducts and introduces secondary air).

4. Airflow (transfers heat from flue gases into the home; remember, if heat is not going into the house, it's going out the stack).

Other factors (impingement from an improperly placed pilot, additional excess air from a cracked heat exchanger, insufficient combustion air due to tight construction or improper ventilation, an improperly installed venting system, or incorrect orifices) affect the combustion process, but these are considered defect or installation problems that require mechanical correction rather than adjustment.

It is the service technician's responsibility to determine if combustion problems are caused by improper adjustment, incorrect installation, component failure, or equipment defect. Therefore, it is important that the technician completely understands how each of the subsystems affects the chemical reaction called combustion.

Note: There is no national standard for calculating measured efficiency with a combustion analyzer in the industry. Analyzer manufacturers use differing calculations to derive efficiency values. Often this discrepancy is due to values that have been extrapolated into the condensing range. Heat removed from flue gases on a condensing furnace is latent (hidden) heat. A combustion analyzer that measures only temperature and not volume of condensate cannot measure the quantity of heat removed from the flue gas during the condensing process.

Although the terms "thermal efficiency" and "combustion efficiency" are often used interchangeably on noncondensing units, they cannot be used in the same manner on condensing appliances. The thermal efficiency and combustion efficiency will be different for a condensing appliance.

The only way to calculate the actual thermal efficiency is to measure the exact airflow across the heat exchanger, measure the change in air temperature across the heat exchanger, and input the measured values into the sensible heat formula to calculate the heat energy input into the conditioned air.

There will be minimal losses to the furnace cabinet due to radiation and conduction. Depending on how much of the heat energy is extrapolated from water in the flue gas (an average of 970 Btu/pound), the efficiency readings may differ up to 10 percent, assuming that either all or none of the latent heat energy was extracted from the flue gases after they reached dew point.

This calculation does not affect AFUE numbers, which are derived by a different means. The combustion efficiency calculated by the analyzer is a function of the variables used to calculate the combustion efficiency of dry flue gas and not representative of a condensing appliance's thermal efficiency, which is a measured and not calculated parameter.

Testo Inc. has chosen to use a combustion calculation that does not extrapolate thermal efficiency values of flue gases below the dew point, as the values are not representative of the heat removed from flue gases during the condensing process. Although this may result in the appearance of lower thermal efficiency for the appliance, the science used for measuring combustion efficiency is not artificially high.

Once differences in combustion and appliance thermal efficiency are understood, the methodology of scientific measurement (versus extrapolation of measured values) can be appreciated and applied, allowing manufacturers to publish combustion and thermal efficiencies that are representative of the actual efficiency of their appliance, and creating a standard that is based on actual measurement.

Publication date: 01/09/2006