The burner flames from your natural gas furnace should look blue or almost completely blue. A healthy natural gas furnace flame is characterized by a roaring blue flame with a light blue triangle in its center. There may be a tiny tip of yellow. Blue flames are hotter and indicate more complete combustion.Click to see full answer. Then, should I see a flame in my furnace?Check your burner flame! This is one of the best signs of a furnace ready to kick the bucket. If it’s in tip-top shape, the flame visible through the opening in your furnace should be blue/blueish green depending on whether your furnace is running on propane or natural gas.One may also ask, what does a yellow flame on a gas furnace indicate? A yellow or orange flame means that too much gas and/or not enough oxygen is being used when burning. Another symptom is a black, sooty residue, as the gas does not burn cleanly. Keeping this in consideration, how do I adjust the flame on my furnace? All you will need to do is to twist the adjustment screw of the flame till the flame goes from one and half inches to two inches, which is about 4 to 5 centimeters high. Look for the adjustment screw on the pilot assembly close to the gas valve if the control valve has this adjustment feature.What does a thermocouple look like?The thermocouple looks like a piece of metal tubing smaller than a soda straw. To find it, first locate the gas control box. This is the box that the main gas line enters, where you turn the gas on in the furnace. The other end of the thermocouple sits in the flame of the pilot light.