The heat is on but it’s blowing cold. That is why I’m in the crawl space below. I’m cross legged in front of the errant furnace. The pilot light is out the burners are dark. I view my immediate cool damp shadowed surroundings and observe mouse droppings on top of the unit case. Hmmmm, must be a warm spot where they like to hang out. I wonder if they are off in the black at safe distance tiny little eyes watching silently as I ponder the problem. I stike a match for a re-light and the pilot begins to hiss it’s usual burn pattern but refuses to stay lit. By process of elimination I narrow the fault(s) down to two. The entire burner assembly must come out so that I can repair the thermocouple and or R & R the valve assembly to which it transmits.
The thermocouple is a fail safe device that monitors the pilot 24 and 7. Should the pilot fail it would be unsat for the main gas valve to open even though commanded to do so by the thermostat. Obviously closed spaces would begin to fill with the poisonous explosive mixture. The part of the thermocouple which actually senses the flame is really technically called a thermocouple junction. It is two dissimilar metal wires which are fused (or welded) together at the end where they are (enclosed and protected by an outer metal case) immersed in the hottest part of the pilot flame. When heated, the thermocouple junction actually produces D.C. voltage from the energy of the flame similar to the way a solar panel cell produces electricity from the energy of light. The output of a thermocouple is a very minute voltage (30 millivolts) but it is sufficient to operate a very small electro-magnet latching switch on the main valve.
It is this latch that is safely defaulting to OFF. This hardware is contained and sealed. This prevents anyone (mice included) from messing with or trying to defeat the mechanism. I will replace this unit as well as the thermocouple — the end of which has been exposed to burning flame for the last 10 years or so. This last inch of tube may in fact be the culprit. I’m not sure how the heat transferring qualities of the copper have been transformed over the length of usage. That would be an easy fix indeed. Simply nip the dead ending and feed through a fresh length. But, as it turns out, a new component is only $5 and change. Gimme two! (one extra for the next time)
Mission completed I exit the dark confines. Your heat is restored, stay warm meeces!