I have a Ruud gas furnace that I am having an intermittent problem with. I cannot find any model number on the furnace but it was new in 1991 when my home was built. The furnace does not have a gas pilot light. It does have an electric igniter and does have a Robert Shaw Model HS780 – 34NL – 306A Ignition Control Unit. The part # on the Robert Shaw Unit is 100-00812.
The problem is that sometimes when the thermostat calls for heat, the electric igniter heats up and glows adjacent to the burner but the main gas valve does not open to allow gas to flow to the burner. This problem first presented itself in the spring of 2006. During the last few days of that spring when I needed heat I observed that the igniter was illuminated but the gas valve did not open, but then the need for heat was over for that spring. In the fall of 2006 I expected to have to face the problem with the furnace but when I tested the furnace, it fired up and worked flawlessly until the spring of 2007. Again, for the last few days of spring 2007 when it would have been nice to have a little heat, the previously noted problem reoccurred. Now, in the fall of 2007, I am testing the furnace for the coming heating season and the furnace has fired up twice out of perhaps eight attempts. But the ignited is still glowing brightly.
Can any one tell me what it is exactly that is supposed to open the gas valve? I expect that there is a sensor that will verify that the igniter is if fact hot enough to ignite the gas before the gas valve is allowed to open but what is the nature if that sensor? Does is need cleaning?
Just what is the function of the Robert Shaw Ignition Control Unit?
Is any diagram available, which will show how this system is supposed to work?