pilot light going out on furnace?

Question

since october, our pilot light has been going out on the furnace
replaced the thermocouple 3 times, pse&g have been here 4
times in the last week, they replaced the valve that the thermocouple goes into, today we lost the heat again, when trying
to light the pilot light, when you get a flame going and release
the knob the pilot light goes out again.

Answer 1

First off the thermocouple is NOT filled witha gas. It is just as it says a THERMOCOUPLE. this is a joint between two dissimilar metals. when heat is applied it produces electricity, that electricity creates a magnetic feild and holds the safety valve in the gas valve open. that is why you push down the button when you light the pilot. when the pilot goes out the thermocoupletakes a bit to cool down and the current gets weaker till the magnet can no longer hold the valve open. it is possible that the thermocouple is installed improperly, getting just warm enough to hold the valve open under minimal circumstances. if you can see the thermocouple and the pilot flame the flame needs to just wrap around the tip of the thermocouple. not around the middle and not to close to where the flame originates. the tip of the flame is the hottest point. Also when the furnace is heating it is possible that the airflow moves the pilot flame off the thermocouple allowing it to cool enough to drop out the valve.

Answer 2

Youve changed the thermocouple multiple times, and your utility has been out, and changed the valve..... has anyone thought to check for a backdraft? Your pilot light could be getting 'blown out' like a birthday candle. This could be caused by the vent stack being too short, and catching the wind as it blows across the roof, or, it could be caused by another appliance actually pulling too much air out, and pulling fresh air back down your exhaust pipe. (Ive seen some bathroom exhausts do this, and attic fans)

IF this is the case, you NEED to get it fixed ASAP -- you could be getting a carbon monoxide buildup in the home before that pilot actually goes out.

You may have just had an extremely unlucky streak of getting bad thermocouples.... there may have been 'air' in the gas line, if there has been any repairs to the lines nearby... but this is just one more item that you can check. The utility tech SHOULD have caught it, but if it's intermittent, it may not have been drawing air in thru the exhaust at the time he worked on it.

Good Luck (and invest in a carbon monoxide detector, be safe)

Answer 3

How old is the furnace? Your gauges may be faulty and not deliver the right amount of gas.
Is the thermocoupling the right type and connected correctly.
Is there a draft where the winds blow out the pilot light.
Did you troubleshoot the filters and the fan.
Usually when that happens, the problem is in the delivery of the gas. Have the gas fitter troubleshoot . They might have to replace the pilot assembly, maybe it is corroded and maybe the holes are plugged with soot and dirt.

Answer 4

The other places you can look at are the draft down the chimney, is it blowing to much. I used to have trouble with this on windy days.
The draft through the furnace, the front. something is creating enough wind to blow the pilot out. You've replaced the main valves, probably not having to. It's coming from somewhere else. Have you changed anything lately, over the last year like the chimney vent, etc.

Answer 5

That's the thermocouple.
Just so you know: the thermocouple either works or it doesn't. there is no in between.
They are filled with a gas and operate on pressure. when the flame is on the tip it causes the gas in the thermocouple to expand and push on a valve to keep the pilot on.
if it leaks out then it doesn't work and you have to replace it.
Make sure it is in the pilot flame when you light the pilot so it heats up.

Answer 6

If you have replaced the thermocouple that many times, then your control valve is the next likely culprit. I can't believe they didn't test the thermocouple, before changing it that many times.

Answer 7

There may be particulate matter of some sort blocking the small line right at the orfice end.
Replace that small line.
The gas line going to the furnace should have a small "nipple" line extending down from pipe that goes into the pressure valve. The nipple line should be capped and is there to "trap" any particles that might get into the unit.
Check to see if you have a galvanized pipe going into the unit. It should be a black pipe and not galvanized. Galvanizing can come loose inside the pipe and "blow" into the unit, clogging the orfice(s).
Here's a link to Ask This Old House (.com):
https://www.timeinc.net/toh/secure/asktoh/mailbox.html

Answer 8

Just set it to on it will light itself.