In many cases the reason that the burners in a gas furnace will not ignite is that the flame coming out of the pilot light is not supplying enough heat to the thermocouple in order to raise its temperature to a point where it will actuate the gas control valve.
Figure 1 - Pilot light flame
For a natural gas furnace the pilot light flame should be blue for at least 97% of its length, as shown in Figure 1. A small tinge of yellow at the tip of the flame is not abnormal.
On propane furnaces, the flame will be blue with a tinge of green throughout. A small tinge of yellow at the tip of the flame is acceptable.
There are four reasons that the thermocouple may not be getting hot enough to actuate the gas control valve.
- The thermocouple is not placed in the proper position with respect to the pilot light flame.
- The orifice in the tip of the pilot light tube is dirty.
- The pilot light is not receiving enough gas.
- There is a draft entering the burner combustion chamber.
The pilot light flame should be directed at a position approximately 1/2 inch from the tip of the thermocouple and overlap thermocouple tube slightly.
If you are experiencing a yellow flame or a split flame it is possible that the orifice in the end of the pilot tube is dirty. This is a common occurrence at winter season start-up if in fact you have turned-off the pilot light during the summer. The orifice is quite small and it only takes a very small piece of dirt to change the pilot flame characteristics.
To clean the orifice, make sure that the gas control valve is turned to the “OFF” position, as shown in Figure 2. You can use a straight or safety pin, small brad or if you have a wire brush, cut one of the steel needles from the brush.
Figure 2 - Furnace gas valve in "OFF" position
Most gas control valves will have a screw that will adjust the flow of gas to the pilot light assembly. This can make a dramatic difference to the heat given off by the pilot light assembly. Each control valve is different and if the screw is not identified on the gas control valve you will have to read the gas furnace’s instruction manual or contact the manufacturer for assistance.
If you have a flame that dances from the end of the pilot tube assembly it is most likely caused by a draft. The draft may not be there when the cover is replaced on the burner assembly. But in order to light the pilot and make sure the furnace is functioning properly it is best to identify where the draft is coming from and stop it, at least temporarily while you are lighting the pilot light and starting up the gas furnace.