furnacetestingthermocouple
So the pilot on my furnace wouldn't stay lit. I tested the thermocouple with my multimeter and got 29 millivolts which meant it was still working.
Assuming I was in over my head, I called the plumber who proceeded to change the thermocouple and now everything is fine. Family is warm, what more can you ask for?
Is it common for thermocouples to test fine but still not actually work? Everywhere I looked said over 25 millivolts means the thermocouple is fine and there must be a different problem.
I finally found some time yesterday to take a closer look at the furnace.
I started by measuring the incoming gas pressure using a gas pressure gauge. I found it to be 6 WC (inch water column), which is well within the acceptable range specified on the furnace.
Next I took a closer look at the spark gap itself, and watched closely as the spark was generated. I even recorded the spark with my video camera, which allowed me to zoom in fairly close. This is when I noticed a bit of carbon buildup on the electrodes, and wondered if it could be that simple...
I grabbed a bit of 220 grit sandpaper, and cleaned the electrodes up a bit. steel wool is probably the tool of choice for this job, but I didn't happen to have any on hand at the moment (not yet unpacked).
After testing the pilot ignition a few times, the problem seems to be fixed. We had a cold night last night, and the furnace fired up fine all night. As I write this, the furnace has just cycled successfully yet again.
I found this YouTube video from Grayfurnaceman that describes the problem exactly, and says that the only solution is to replace the assembly. I'll be replacing the pilot assembly once the weather warms up, so we'll see if that solves the problem.
This is an edited answer
After reviewing the additional information provided above along with relevant information obtained from this post: What is wrong with my gas furnace? I have entered my answer below.
The "something red hot" in your furnace is apparently a "hot surface ignitor". The red hot glow is produced by passing current through the ignitor and is the intended function.
Now, to my concern for your safety:
I originally stated that there did not appear to be a thermocouple on your furnace. While I maintain that there is indeed no thermocouple, I also add that the hot surface ignitor is likely monitored by the control module in the furnace. While I am not intimately familiar with this furnace nor the application of such an ignitor, I can safely presume that the control module senses the resistance of the ignitor and can tell this way whether the pilot light is lit or not.
One way to verify my assumption about the control of gas flow would be to remove the connector for the ignitor while the unit is off and then see if you can hear the gas flowing (or can otherwise sense the gas flow - temperature, smell) when a demand for heat is made. Just make sure you do this quickly and safely! (i.e. Do not reconnect the ignitor unless the thermostat is off!!!)
Best Answer
Voltage is not a universal indicator of effectiveness. Ask anyone with a dead car battery.
Like many providers of power, it is not uncommon for them to have a reasonable "open-circuit" (no-load) voltage, only to fail when the load attempts to draw its ordinary and reasonable amount of current. That qualifies as "defective" and the voltmeter did not tell the tale.
People use voltmeters because it is easy, not because it is thorough. Examining current is more useful but harder, because it must be detected "in-line", and particularly with DC, that means splicing something in series with the current flow. Or unhooking the load and hooking up a dummy load of known value. Or permanently installing an "ammeter shunt", a resistor of known and low value, and measuring a small voltage drop across it. We just had a $10,000 problem because of not knowing a large engine's normal cranking current, because there is not ammeter in the starting circuit. Putting one in is hard.