Follow up:
It turns out that the heat exchanger on out 14 year old furnace was going bad. It looked good. This furnace is a 90 something percent efficiency. It is not a closed loop combustion cycle. It used the air from the house as the combustion source air. On each start up it runs the combustion draft fan for a while before it energizes the hot surface resistive heated igniter. After that is run long enough to get hot the gas comes on. There were two things going on with the furnace.
The little crimp rings that hold together the upper heat exchanger at spaced holes fail and fall off with age. That allows separation of the sections that make up the upper heat exchanger. That allows forced draft (at higher pressure than ambient because of draft inducer) to blow back through the combustion chamber and back flow into ambient air intake region. This causes the flame to flash back into the ambient air. This was noticeable when the flame first ignites each time. This puts some CO into ambient air in the cellar. This heat exchanger has a 20 year warranty.
The other thing going on was that the gas valve has a pressure regulator built into it and that regulator had failed in the max pressure mode. This ran too much gas into the burners. This aggravated the flash back during initial ignition, increased the amount of CO in exhaust, and cut down efficiency. This showed up as orange flame instead of clear and blue flame.
To top it all off the furnace main blower motor that moves air through the house failed for the second time for this furnace. The first time one failed ten years ago one of the three mount feet broke. That resulted in excess vibration, that ruined the squirrel cage fan by abusing it with excess vibration, and it bent near the mount. That over stressed the motor and ruined the bearings.
It turns out that the heat exchanger on out 14 year old furnace was going bad. It looked good. This furnace is a 90 something percent efficiency. It is not a closed loop combustion cycle. It used the air from the house as the combustion source air. On each start up it runs the combustion draft fan for a while before it energizes the hot surface resistive heated igniter. After that is run long enough to get hot the gas comes on. There were two things going on with the furnace.
The little crimp rings that hold together the upper heat exchanger at spaced holes fail and fall off with age. That allows separation of the sections that make up the upper heat exchanger. That allows forced draft (at higher pressure than ambient because of draft inducer) to blow back through the combustion chamber and back flow into ambient air intake region. This causes the flame to flash back into the ambient air. This was noticeable when the flame first ignites each time. This puts some CO into ambient air in the cellar. This heat exchanger has a 20 year warranty.
The other thing going on was that the gas valve has a pressure regulator built into it and that regulator had failed in the max pressure mode. This ran too much gas into the burners. This aggravated the flash back during initial ignition, increased the amount of CO in exhaust, and cut down efficiency. This showed up as orange flame instead of clear and blue flame.
To top it all off the furnace main blower motor that moves air through the house failed for the second time for this furnace. The first time one failed ten years ago one of the three mount feet broke. That resulted in excess vibration, that ruined the squirrel cage fan by abusing it with excess vibration, and it bent near the mount. That over stressed the motor and ruined the bearings.
