Harry Bloomfield wrote:
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%.

My immediate response is that the meter is obviously working the wrong
way, if CO2 levels go down with combustion. The instrument is suggested
to be clearly marked that it reads 'CO2'.

What is the normal level of C02 in the atmosphere as a percentage and
will the instrument be designed to detect the additional amount of CO2
(above the atmospheric normal), or to measure the overall level?

If it's oxygen, that might be broadly plausible.
Oxygen levels are 20% in normal atmosphere, and 9% would not be utterly
unreasonable (though maybe a bit high) for exhaust.

For CO2, well, there is a fault.
IIRC normal range is 10-20ppm.