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 )