#combustion analysis

Combustion analysis: what the numbers actually mean

An analyzer reads what the flame is doing inside the heat exchanger, which your eyes can't. The short course for furnaces and water heaters:

O2: 6-9% on natural gas. Lower = running rich. Higher = excess air stealing efficiency.

CO air-free: the number that matters. Dilution hides CO, so the analyzer corrects it: CO x 20.9 / (20.9 - O2). 100 ppm measured at 14% O2 is really 300+ air-free. ANSI Z21.47 allows 400 ppm air-free as a pass/fail ceiling, but field best practice calls 100 ppm the action line, and a healthy furnace runs under 50. CO that climbs as the blower starts is the classic heat exchanger red flag.

Stack temp: 270-370°F above supply air on natural draft, 170-270°F above on induced-draft 80% furnaces. A condensing furnace should run around 100-130°F at the flue. If a 90%+ unit shows 200°F, it isn't condensing and something's wrong.

Where to probe: 80% furnace: drill the single-wall flue 12-18 inches above the inducer, before any elbow, and cap it after. 90%+: use the factory test port. Atmospheric water heater: get the probe upstream of the draft hood, because dilution air past the hood makes a sick unit look healthy.

Why bother: a tune-up without numbers competes on price. Before-and-after readings justify the burner cleaning, the gas pressure adjustment, and the heat exchanger referral, and a documented reading protects you when the customer declines the repair.

Interpreting high CO: high O2 with high CO points at flame impingement or dirty, misaligned burners. Low O2 with high CO points at overfiring or starved combustion air. Same CO number, opposite fixes, so check manifold pressure (3.5 inches water column on natural gas) and clock the meter before adjusting anything.

What is combustion analysis?

Combustion analysis begins with the measurement of flue gas concentrations and gas temperature, and may include the measurement of draft pressure and soot level.

To measure gas concentration, a probe is inserted into the exhaust flue and a gas sample drawn out. Exhaust gas temperature is measured using a thermocouple positioned to measure the highest exhaust gas temperature. Soot is measured from a gas sample drawn off the exhaust flue. Draft is the differential pressure between the inside and outside of the exhaust flue.

Once these measurements are made, the data is interpreted using calculated combustion parameters such as combustion efficiency and excess air. A more in depth analysis will examine the concentration of the undesirable products described earlier.

Why do a combustion analysis?

A combustion analysis can help improve fuel efficiency, reduce emissions, and improve safety.

What all is measured?

Combustion analysis involves the measurement of gas concentrations, temperatures and pressure for boiler tune-ups, emissions checks and safety improvements. Parameters that are commonly examined include:

• Oxygen (O2)

• Carbon Monoxide (CO)

• Carbon Dioxide (CO2)

• Exhaust gas temperature

• Supplied combustion air temperature

• Draft

• Nitric Oxide (NO)

• Nitrogen Dioxide (NO2)

• Sulfur Dioxide (SO2)

We have measurements, now what?

Once flue gas and temperature measurements are made, combustion parameters are calculated to help evaluate the operating performance of the furnace or boiler. Typical combustion parameters include:

• Excess Air

• Carbon Dioxide

• Combustion Efficiency

• O2 Reference

• Emissions Conversions

What tool is used for combustion analysis measurements?

They are several types of measurement tools. At Deckard Mechanical we use the Testo 327-1 combustion efficiency analyzers.  We do all the measurements and leave the customer with a printed report with all the measurements.  We'll even explain it if there are any questions! 

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