FWIW Department:

Many people have traditionally adjusted their furnace by opening and closing the burner shutter until they get a blue flame.

We’ve always thought a blue flame is a good flame, an orange flame is a dirty flame, and a yellow flame is a flame producing excess carbon monoxide.

Those of us who use carbon monoxide meters have learned otherwise.

Closing the shutter down to reduce air being mixed with the gas can cause the flame to produce large amounts of CO without significantly changing the color or shape of the flame.

I’ve enclosed 2 photos (CO_High and CO_Low)  from a CO analysis I did a couple of days ago showing the effects on CO readings by opening and closing the shutter.

The CO meter probe is in the top of the heat exchanger (outside of the image area) and you can see the large difference in CO with the shutter (i) opened and (ii) mostly closed.  (On this furnace, the shutter is a sliding tube to the left of the CO meter.)

CO levels greater than 300 parts per million are generally unacceptable – under the BPI CO Analysis protocol, if CO is >400 ppm the gas appliance must be disabled until properly serviced.

The CO level in the CO_High image exceeds 1800 ppm.

If anyone cares to use these images, please respect my copyright and give credit to Paradigm Inspections, LLC)