In article ,
Harry Bloomfield writes:
Hi,

I'm not personally involved with this problem so supplying more
information will be difficult...

A permanently installed flue gas instrument (LED display) measuring CO2
levels indicates 20% with no combustion gases passing up the flue. With
the burner system in operation this level declines to an indicated 9%.

As others have said, this sounds like the oxygen reading.
Combustion analysers often don't measure CO2 directly because it's
hard to do continuously in that type of unit. Instead they measure
O2 and C0 levels, and then calculate the CO2 from that based on the
O2 input assumed to be 21% and type of fuel in use.

--
Andrew Gabriel