Combustion Control on Natural Gas Furnace: Air-Fuel Ratio, Excess Air, and Combustion Analysis

Combustion Control on Natural Gas Furnace: Air-Fuel Ratio, Excess Air, and Combustion Analysis

Combustion control is the most overlooked area of gas furnace operation. Most gas furnaces run with too much excess air, wasting fuel and reducing efficiency. A well-tuned burner with the correct air-fuel ratio delivers 5 to 15 percent fuel savings and 20 to 50 percent reduction in CO emissions. The tuning is straightforward, but it requires attention to detail and regular verification. Here is the framework I use when reviewing a gas furnace operation.

Combustion Chemistry Basics

Natural gas is mostly methane (CH4) with smaller amounts of ethane, propane, and inert gases. The complete combustion reaction with pure oxygen is:

CH4 + 2 O2 -> CO2 + 2 H2O

In practice, the combustion air is a mixture of oxygen (21 percent) and nitrogen (79 percent). The stoichiometric air-fuel ratio for natural gas is 9.5 to 10.5 cubic meters of air per cubic meter of gas, depending on the gas composition. The actual air-fuel ratio in a furnace is always higher than stoichiometric, because some excess air is needed to ensure complete combustion.

The excess air is the air supplied beyond the stoichiometric requirement. It is expressed as a percentage of the stoichiometric air. For example, 10 percent excess air means the actual air-fuel ratio is 1.10 times the stoichiometric ratio.

Why Excess Air Matters

Some excess air is necessary to ensure complete combustion. With too little air, the combustion is incomplete, and the exhaust gas contains CO and unburned hydrocarbons. The CO and the unburned hydrocarbons represent lost fuel and a regulatory issue.

With too much air, the excess air absorbs heat from the flame and carries it out the exhaust. The exhaust gas temperature rises, and the fuel efficiency drops. The optimum excess air is the lowest value that gives complete combustion with low CO.

For natural gas burners, the optimum excess air is 5 to 10 percent for high-velocity burners and 10 to 20 percent for low-velocity atmospheric burners. The CO emissions should be below 50 ppm (corrected to 3 percent O2) at the optimum.

How Excess Air Affects Efficiency

The relationship between excess air and efficiency is significant. Going from 20 percent excess air to 10 percent excess air improves the fuel efficiency by 3 to 5 percent. Going from 10 percent to 5 percent improves efficiency by another 1 to 2 percent. Below 5 percent excess air, the CO emissions rise rapidly, and the risk of incomplete combustion becomes a safety and quality issue.

On a 5 MW gas furnace running 6000 hours per year, a 5 percent efficiency improvement saves 250 kW of fuel input, which is 1.5 million kWh per year of natural gas. At 0.40 USD per cubic meter, that is 150,000 to 200,000 USD per year in fuel cost savings.

Combustion Analysis Procedure

The combustion analysis is done with a portable flue gas analyzer. The analyzer measures the O2, CO, CO2, NO, NO2, and temperature in the exhaust gas. The analysis takes 30 to 60 minutes per burner, and the procedure is:

1. Start the furnace and bring it to operating temperature.

2. 2. Insert the probe into the exhaust gas stream near the burner.

3. 3. Record the flue gas composition at the current firing rate.

4. 4. Adjust the air-fuel ratio to bring the O2 to the target value.

5. 5. Verify the CO is below the limit (typically 50 ppm corrected to 3 percent O2).

6. 6. Repeat for each firing rate in the operating range.

The target O2 for a high-velocity recuperative burner is 3 to 5 percent (which is equivalent to 15 to 25 percent excess air). For a low-NOx burner with FGR, the target O2 is 5 to 8 percent. The CO should be below 50 ppm at the target O2.

If the CO is above 50 ppm at the target O2, the burner needs service. The most common causes of high CO are: worn burner ports (replace the burner), dirty burner ports (clean the burner), and incorrect air-fuel ratio (recalibrate the ratio controller).

Common Combustion Problems

Problem 1: Excess air is too high. The symptom is high O2 in the exhaust (above 5 percent for a high-velocity burner), high exhaust temperature, and low fuel efficiency. The cause is usually a misadjusted air-fuel ratio, a leaking air damper, or a misaligned burner. The fix is to readjust the ratio, repair the damper, or realign the burner.

Problem 2: CO is too high. The symptom is CO above 50 ppm in the exhaust, smoke in the exhaust, and possibly soot on the workload. The cause is usually a low air-fuel ratio (too little air), a worn burner, or a clogged air inlet. The fix is to increase the air, replace the burner, or clean the air inlet.

Problem 3: NOx is too high. The symptom is NO above 100 ppm in the exhaust. The cause is usually a high firing rate with insufficient FGR, a misaligned burner, or a high excess air setting. The fix is to install or adjust the FGR, realign the burner, or reduce the excess air.

Problem 4: Inconsistent firing. The symptom is fluctuating furnace temperature, fluctuating O2 in the exhaust, and uneven heating. The cause is usually a worn ratio controller, a leaking air damper, or a fluctuating gas pressure. The fix is to replace the controller, repair the damper, or install a gas pressure regulator.

Air-Fuel Ratio Control Systems

Modern gas furnace control systems use a cross-limited ratio controller to maintain the air-fuel ratio at the optimum value across the entire firing range. The cross-limited design interlocks the air and gas valves so that the air is always slightly ahead of the gas (or slightly behind, depending on the burner type). This prevents the burner from going lean during a firing increase or rich during a firing decrease.

The ratio controller uses a fuel flow meter (or a gas pressure signal) and an air flow meter to compute the actual air-fuel ratio. The computed ratio is compared to the target, and the air damper is modulated to maintain the target. The controller also has a trim input from the O2 analyzer, which corrects for long-term drift in the air-fuel ratio.

A well-tuned ratio controller maintains the air-fuel ratio within 2 percent of the target across the entire firing range. A poorly tuned controller can drift 10 to 20 percent, with corresponding losses in efficiency and emissions.

Annual Combustion Tune-Up

MONTE INTELLIGENCE recommends an annual combustion tune-up on all gas furnace installations. The tune-up is done by a service engineer and includes: combustion analysis on each burner at three firing rates (low, medium, high), ratio controller calibration, igniter and flame rod inspection, and air damper and gas valve inspection. The tune-up takes 1 to 2 days per furnace, and the typical energy saving is 3 to 8 percent on the next 12 months of operation.

The cost of the tune-up is recovered in 1 to 3 months through reduced fuel cost. The tune-up also identifies burner and control issues that can cause unscheduled downtime if left uncorrected.

Talk to MONTE INTELLIGENCE About Combustion Tuning

For buyers interested in a combustion tune-up on an existing gas furnace, MONTE INTELLIGENCE engineering can schedule a service visit and provide a written report with the test results and the recommended adjustments. Visit www.cnlymonte.com/products-gas-furnace.html for product specifications. For a service visit request, email helenxu@cnlymonte.com with subject line combustion tune-up and details on your furnace type, burner model, and current fuel cost.