 Wall Furnace
If you called the gas company they sent someone out whom you thought was an expert, who really wasn’t, and they told you that you had a millivolt problem. Your system relies on electricity generated by the thermopile (thermocouple or pilot generator) to activate the gas valve. Thermocouples put out about 36 millivolts (the good ones – millivolt means thousandths of a volt) and pilot generators are rated at up to 750 millivolts. It is a pretty lame thing to say that you have a millivolt problem. This would be the equivalent of diagnosing a lighting problem by saying you have an alternating current problem.
The number of problems could be the thermopile is not producing enough electricity and this could be from a poor flame, improperly placed flame or a defective generator. The vent switch, if you have one, could be open. They go bad a lot. The thermostat could be malfunctioning or be the wrong type. A 24 volt thermostat will not properly operate a millivolt system. If the thermostat wire is more than 20 feet long or has wire nut or butt splices in it you could be experiencing an intermittent problem. This voltage is precious and tiny. Any poor link or corroded connection can sap its strength. The gas valve itself could be stuck closed.
Floor Furnace
All the reasons contained under wall furnace plus water damage. We have seen a great number of floor furnaces in low lying areas that have been submerged under water. This destroys the controls. If the pilot keeps going out the vent hood and the vent should be looked at as a buildup of carbon monoxide and other flue gases under the home will snuff out the pilot light. 
Standing Pilot Furnace Forced Air Furnace
If the pilot doesn’t stay lit the thermocouple could be defective, the pilot flame could be improperly placed or too small or the gas valve could be defective. If the pilot light stays on the gas valve could be stuck, the thermostat could be defective, the wiring running from the furnace to the thermostat could be broken or you have an open limit. Above the burner chamber there is a thermo disc or other type of thermo controller that is called the high limit control. If the furnace gets too hot, typically 180-210 degrees, the limit opens up and shuts down the gas valve. These occasionally go bad and stick in the open position.
Electronic Spark Ignition Forced Air Furnace
Electronic Hot Surface Forced Air Furnace
The main difference between hot surface ignition systems and electronic spark systems is the method in which they ignite the flame. A spark ignition system uses a pilot assembly and ignites the pilot. A hot surface ignition system uses a glow plug that operates on 87 to 120 volts. It glows white hot and the gas valve release gas to either ignite the main burners or a burner spread tube.
If your furnace doesn’t come on, no sparking or no glowing, the problem is electrical in nature. It has to be a problem with the circuit board, the wiring or the thermostat. Any other break in the system is going cause the blower motor to operate 24/7. |
