Draft-Hood Dilution Ratio
How much room air the draft hood pulled in, from two O2 readings.
Example
You enter
- Appliance O2, before the draft hood (%) 5
- Diluted O2, after the draft hood (%) 12
You get
- Dilution ratio 1.7865
- Dilution air fraction (%) 44.0252
- Co read fraction (%) 55.9748
Checked against published worked examples from: Combustion-analysis practice.
Details, formula, and sources
Scopedilution_ratio = (20.9 - O2_appliance) / (20.9 - O2_diluted), and the dilution-air fraction (20.9% ambient O2 mass balance). A clean appliance at 5% O2 that reads 12% O2 at the vent has pulled in 44% dilution air - so a CO reading there is only 56% of the true flue value, which is why the air-free CO sample must be taken before the draft hood. A diagnostic aid, not a certified combustion test.
Formula or tabledilution_ratio = (20.9 - appliance_o2_pct) / (20.9 - diluted_o2_pct); dilution_air_fraction = (diluted_o2_pct - appliance_o2_pct) / (20.9 - appliance_o2_pct) x 100; co_read_fraction = 100 / dilution_ratio.
Source and editionCombustion-analysis practice; ambient-oxygen (20.9%) dilution mass balance, by name.
Free accessThe dilution mass balance is standard combustion-analysis practice; the analyzer and appliance manufacturer instructions govern the actual diagnosis.
What governsEstimate. AHJ and licensed professional govern.
API field namesappliance_o2_pct, diluted_o2_pct, dilution_ratio, dilution_air_fraction_pct, co_read_fraction_pct
- Ambient oxygen dilution air is ambient at 20.9% O2; the two flue O2 readings drive the whole mass balancecombustion-analysis practice
- Sample points appliance O2 before the draft hood, diluted O2 after it; dilution can only raise the O2combustion-analysis practice