Excess Air Calculator

This tool supports three common approaches. All are shown on a dry flue-gas basis, assuming stable draft and near-complete combustion.

• From O2: Excess Air (%) = (O2 / (21 − O2)) × 100
• From CO2: Excess Air (%) = (CO2_max / CO2 − 1) × 100
• From air/fuel: Excess Air (%) = (AF / AF_st − 1) × 100

Lambda (λ) = 1 + Excess Air / 100. Estimated O2 ≈ 21 × (λ − 1) / λ, and estimated CO2 ≈ CO2_max / λ.

1. Select the calculation method that matches your instruments.
2. Choose the fuel used by your heater or burner.
3. Enter the required reading: O2, CO2, or air/fuel ratio.
4. Optionally add CO and temperatures for extra diagnostics.
5. Press calculate, then export the log if needed.

If readings fluctuate, average several steady measurements before tuning dampers or air shutters.

Example values are illustrative. Field conditions and analyzers can vary.

Why excess air matters in protected cropping

Excess air is the extra combustion air above stoichiometric demand. Too little air can raise CO and soot, stressing plants and workers. Too much air lowers flame temperature and sends heat up the stack. Many greenhouse and garden heaters run best around 5–15% excess air during steady firing, balancing safety and efficiency.

Using O2, CO2, or air-to-fuel methods

The calculator supports three inputs. With dry O2, it uses EA% = (O2/(21−O2))×100, where 21% is ambient oxygen. With CO2, it compares measured CO2 to a fuel-specific CO2max (for example ~11.9% for natural gas, ~13.8% for propane, ~15.6% for fuel oil). With air/fuel, it compares actual AF to stoichiometric AFst.

Reading lambda and estimated stack values

Lambda (λ) equals actual air divided by theoretical air, so λ = 1 + EA/100. The tool also estimates O2% ≈ 21×(λ−1)/λ and CO2% ≈ CO2max/λ. These estimates help cross-check analyzer readings and spot sensor drift when results disagree sharply.

Practical measurement guidance

Sample at a stable location, usually in the flue, downstream of mixing. Let the heater reach steady temperature before recording. If CO is above roughly 100 ppm, treat the result as a warning and correct airflow, draft, or burner condition first. Large flue-to-ambient temperature rise can indicate fouling or poor heat transfer.

Recordkeeping and tuning workflow

Use the on-page log to capture repeated checks across weather changes and crop cycles. Export CSV for maintenance files, and use the PDF report for technicians. Recheck after cleaning burners, changing nozzles, or altering venting. Track changes in excess air alongside fuel use to validate improvements.

For seasonal baselines, note outdoor temperature, fuel pressure, and fan speed. A small change in damper position can move O2 by 0.5–1.0%. When switching fuels, update the fuel selector so CO2max and AFst match. If readings vary widely, average three stable samples taken two minutes apart. Aim for repeatable results, not perfection, always.

What is “excess air” in a heater?

It is air supplied beyond theoretical combustion demand. It improves stability and reduces soot, but too much cools the flame and increases stack heat loss.

Which method should I use first?

Use O2 if you have a reliable flue-gas analyzer. Use CO2 when your analyzer measures CO2 well and fuel type is known. Use air/fuel when airflow and fuel flow are measured accurately.

Why does fuel selection matter?

CO2max and stoichiometric air/fuel depend on fuel chemistry. Selecting the correct fuel keeps CO2-based excess air and estimated values consistent with real combustion behavior.

My O2 is high but heat output feels low—why?

High O2 often means excessive draft, too much fan air, leaks, or a partially blocked heat exchanger causing poor heat transfer. Reduce unnecessary air, then confirm flame stability and CO.

What CO level should trigger caution?

If CO rises above roughly 100 ppm during steady operation, treat it as a warning. Check burner cleanliness, venting, and air settings before optimizing for efficiency.

How often should I recheck excess air?

Recheck after maintenance, fuel changes, vent adjustments, or seasonal weather shifts. For stable systems, a quick monthly check helps catch drift before it affects crop comfort.