What is the CO/CO2 Ratio (Poison Index)?

This ratio has been proven to be a reliable indicator of the health of a boiler/burner. It allows a simple flue gas test to be used to determine whether or not action needs to be taken to service or repair an appliance.

  • A new or clean domestic boiler will display a ratio of less than 0.004
  • A boiler in need of cleaning between 0.004 and 0.008
  • A boiler in need of repair/overhaul a reading greater than 0.008

The POISON INDEX (PI) expresses the CO/CO2 ratio as a percentage:

PI% = 100 x (CO/CO2 )