8.2 Electronic Combustion Analysis

SWS Detail: 5.3003.2 Combustion Analysis of Oil-Fired Appliances, 2.0201.2 Combustion Safety, 5.3003.14 Combustion Analysis of Gas-Fired Appliances (LP and Natural Gas)

The goal of a combustion analysis is to quickly analyze combustion safety and efficiency. When the combustion heater reaches steady-state efficiency (SSE), you can measure its most critical combustion parameters. This information saves time and informs both service and installation adjustments.

Modern flue-gas analyzers measure O₂, CO, and flue-gas temperature. Some models also measure draft. Flue-gas analyzers also calculate combustion efficiency or steady-state efficiency (SSE), which are synonymous.

Lesson: Combustion Chemistry Basics

In this lesson you’ll learn about the chemistry of combustion, the products of complete and incomplete combustion, and how those relate to combustion testing. Click here to watch the lesson.

8.2.1 Critical Combustion-Testing Parameters

These furnace-testing parameters tell you how efficient and safe the furnace currently is and how much you might be able to improve efficiency. Use these measurements to analyze the combustion process.

Carbon monoxide (CO) (ppm):

Poisonous gas indicates incomplete combustion. Modern combustion analyzers let you choose between a normal flue gas and air measurement (as measured) or a corrected measurement that calculates the concentration in theoretical air-free flue gases. Adjusting combustion to produce less than 100 ppm as measured or 200 ppm air-free is almost always possible with fuel-pressure adjustments, air adjustments, or burner maintenance.

Oxygen (percent):

Indicates the percent of excess air and whether fuel-air mixture is within a safe and efficient range. Efficiency increases as oxygen decreases because excess air, indicated by the O₂carries heat up the chimney. Percent O₂ may also indicate the cause of CO as either too little or too much combustion air. Technicians used to measure CO₂, but O₂ is easier to measure, and you only need to measure one of these two gases.

Flue-gas temperature:

Flue-gas temperature is directly related to furnace efficiency. Too high flue-gas temperature wastes energy and too-low flue-gas temperature causes corrosive condensation in the venting system.

Smoke number

For oil only, this measurement compares the stain made by flue gases with a numbered stain-darkness rating called smoke number. Smoke number should be 1 or lighter on a 1-to-10 smoke scale.

Draft

The pressure in the chimney or vent connector (chimney draft or breech draft). Also the pressure in the combustion chamber (over-fire draft), used primarily with oil power burners.

Table 8-3: Combustion Standards for Gas Furnaces and Boilers
Performance Indicator	SSE 70+	SSE 80+	SSE 90+
Carbon monoxide (CO) (ppm as measured/air-free)	<200 ppm/400 ppm	<200 ppm/400 ppm	<200 ppm/400 ppm
Stack temperature (°F)	350°–475°	325°–450°	<120°
Oxygen (%O2)	5–10%	4–9%	4–9%
Natural gas pressure inches water column (IWC)	3.2–4.2 IWC*	3.2–4.2 IWC*	3.2–4.2 IWC*
LP gas pressure	10–12 IWC	10–12 IWC	10–12 IWC
Steady-state efficiency (SSE) (%)	72–78%	78–82%	92–97%
Chimney draft (IWC, Pa)	–0.020 IWC –5 Pa	–0.020 IWC –5 Pa	0.100–0.250 IWC +25–60 Pa
* pmi = per manufacturer’s instructions Use these standards also for boilers except for temperature rise. See “Minimum Oil Burner Combustion Standards” on page 272.

Table 8-4: Carbon Monoxide Causes and Solutions
Cause	Analysis & Solution
Flame smothered by combustion gases.	Chimney backdrafting from CAZ depressurization or chimney blockage.
Burner or pilot flame impinges.	Align burner or pilot burner. Reduce gas pressure if excessive.
Inadequate combustion air with too rich fuel-air mixture.	O₂ is <6%. Gas input is excessive or combustion air is lacking. Reduce gas or add combustion air.
Blower interferes with flame.	Inspect heat exchanger. Replace furnace or heat exchanger.
Primary air shutter closed.	Open primary air shutter.
Dirt and debris on burner.	Clean burners.
Excessive combustion air cooling flame.	O₂ is >11%. Increase gas pressure.