The flame tells a story the thermostat can't
A burner flame should burn steady and blue. If it's lifting off the burner, rolling out, or tipping yellow at the edges, that's incomplete combustion, and it means carbon monoxide is forming somewhere it shouldn't. None of that shows up on a thermostat reading or in how warm the house feels. A technician pulls the burner compartment apart to look at it directly, because from the vents it looks like any other furnace running fine.







