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  #1  
Old 10-30-2010, 08:58 AM
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Furnace pilot won't light

My furnace pilot won't light--not stay lit, but won't light. It's a standing
pilot. I've lit this pilot before so I'm pretty sure I'm doing it right. It's been
suggested that it could be the thermocouple but it doesn't seem like that
would be involved in the initial lighting. Is that right?

This summer I let the electric company install an energy regulating thermostat
to cycle my AC on hot days. I didn't have the tech light the pilot since it was
summer. My AC and furnace share the same blower unit. The furnace blower
kicks on, there's just no flame. If the tech overlooked something (bad
electrical connection), would this affect the pilot not lighting? I wouldn't
think so.

Is the thermocouple involved or is there a blockage in the pilot tube? All
my other gas appliances are fine.
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Old 10-30-2010, 09:44 AM
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The thermocouple is very involved in the process. Read about it here:
http://homerepair.about.com/od/heati...ot_light_2.htm Good luck, it seems to be a straight forward repair
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Old 10-30-2010, 10:31 AM
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This is completely guessing- but I don't think the thermocouple is involved in lighting the pilot light.

While you've got the pilot lighting button pushed down, gas should be escaping without any flame present for as long as you are pushing that button. Right? Once you let go, that would be when the thermocouple takes over the gas control.

If you think it's a clogged tube- you might be able to check it with bubbles.

My uneducated guess would be something with the control valve itself if it's not a blocked tube.

Stupid question...the tech didn't shut off the gas to the furnace did they?

A couple years ago, I replace the board on my furnace- but that was because the blower wouldn't kick in on in the heating cycle. Pretty straight forward for an old-school furnace. Last year was the thermocouple. At this rate, I will have built a new furnace in a few years from now.

Last edited by parnelli; 10-30-2010 at 10:43 AM.
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Old 10-30-2010, 10:45 AM
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I think you are correct. Even if the thermocouple is bad, you can still ignite the pilot, but it just will not stay lit. The thermocoupler is cold initially anyway. It doesn't even work until it gets hot enough.

What board did you replace if you don't mind me asking.
I have an older gas furnace and AC as well.
I tried replacing the two relays earlier this year because the blower would not cycle by itself. We had to do it manually for a few weeks. I should have already repaired this or had it repaired.
I have worked on these quite a bit before, so no newbie.
I have also had to replace the thermocoupler 3 times in 15 years.
Not so bad. Just a $5 part.

These furnaces sometimes have an additional switch. If that switch is off, nothing will energize either, so then the pilot would not ignite.
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Old 10-30-2010, 11:42 AM
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Quote:
Originally Posted by Stytooner View Post
What board did you replace if you don't mind me asking.
Here's the thread from 05. (includes link to manuf of replacement boards)
http://www.bt3central.com/showthread.php?t=16184

My furnace is about 1990 or so, so it's not high tech like the new ones. There's one control board inside about 6x8". I too had thought about trying to replace the relays but since I found the whole thing for 60 bucks and it was cold outside I just replaced the whole thing. No problems with it in the five years now.

At this point, I only have the blower motor as original equp. Someone else replaced the gas solenoid control valve before I lived here. Hopefully I didn't just jinx myself...
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Old 10-30-2010, 11:55 AM
Daryl Daryl is offline
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The tc is very involved in lighting the pilot, remember, you are holding that button for a minute while it gets hot. If you release it too soon the pilot will extinguish.
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Old 10-30-2010, 12:16 PM
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Quote:
Originally Posted by Daryl View Post
The tc is very involved in lighting the pilot, remember, you are holding that button for a minute while it gets hot. If you release it too soon the pilot will extinguish.
The pilot won't even light. How is the TC involved?

Gas is on. I'm going to see about cleaning the pilot tube-with gas off.
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Old 10-30-2010, 01:17 PM
LinuxRandal LinuxRandal is offline
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The thermocouple has to be a minimum temperature. The oil filled tube site above the pilot light, and when it gets to hot, it shuts off the gas to the burners. Too cold, and it shuts off the gas to the pilot light (at least in the blodgett ovens I have rebuilt). A gas man I work with on them (he went through all the training on it) told me to use a propane torch on it INITIALLY, as the oil that is in the tube of the thermocouple, will gel and block the warm oil from getting back to the sensor part of the thermocouple. Once we got it to cherry red, and let it sit, the pilot stayed lit as the oil worked its way back.


The thermocouple controls the gas flow to the pilot and burners.
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Old 10-30-2010, 02:18 PM
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Quote:
Originally Posted by LinuxRandal View Post
The thermocouple controls the gas flow to the pilot and burners.
sorry, the thermocouple does not control the gas flow to the pilot and burners, at least not on the on the natural gas systems i have worked on. its sole purpose is to let the gas valve know that the pilot is lit so that when a call for main burner ignition is recieved at the furnace or boiler, the gas valve doesn't open to no pilot flame and begin sending gas into the air. even with defective thermocouples, i have gotten pilots to light, but they don't stay lit once the pilot button is released.

atgcpaul noted that his thermostat was replaced prior to his noticing the problem. did the tech cut the power to the system before he commenced the replacement? the gas valve , or more likely a transformer somewhere in the low voltage wiring for the gas valve, may have been shorted based on something the tech inadvertently did. i've been advised by a good friend who is a licensed master electrician to as least turn off power to the furnace or boiler before working on the low voltage control systems. his reasoning, although not real scientific, is that when you think something can't happen, it usually does.

atgcpaul...when the pilot button is depressed, can gas be heard escaping from the pilot light?
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Old 10-30-2010, 03:18 PM
cwsmith cwsmith is offline
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The thermocouple needs to be heated to the proper temperature in order to open the gas line to keep the pilot lit... BUT, it has no importance on the initial lighting! When you turn the knob on the gas valve to the proper position (usually marked on the knob) and then push it in, that allows the gas to flow to the pilot so that it can be lighted.

Normally, you turn that knob about 90-degrees, hold it in (which allows the gas to flow) and while holding it, light the pilot and continue holding it until the flame heats the thermocouple's probe wire to the necessary temperature. Only then can you release the knob and the pilot will stay lit, while you then continue to turn the knob to it's final position. (At least that is how it works on the gas valves in my experience.)

From your description, you are NOT getting enough gas (if any at all) when you hold in the knob on the gas valve. So, one of two things may be happening... either the pilot feed line is plugged (possible spider or other insect) or the gas valve itself is no longer working properly. (There of course, can be a blockage to the line that feeds the furnace (like someone actually turned the furnace's gas valve off.)

Insects are often a problem and these internal gas lines can be favorite nesting places, especially on wall furnaces and gas grills. The other possibility may be corrosion of the valve itself. I actually had that happen last fall. Earlier that summer, we had a problem with some damaged flashing, which we had fixed on discovery... but apparently not before moisture found it's way past the vent flue and into my wall furnace (an Empire) where it corroded the gas valve to a point where it would not work. It looked good on the outside, and it took the technician almost an hour to find the problem. Unfortunately the valve ended up being replaced, to the tune of about $140, plus labor.

So, first thing would be to check to make sure that someone hasn't turned off the main gas valve that feeds your furnace. Then, check the pilot feed tube from the valve to the pilot, to ensure it isn't blocked. If both of those prove to be okay, check the line feeding the gas valve to make sure that there is actually gas flowing to that valve. If all that proves positive, then you may well need to replace the valve itself. (If you're not comfortable with working on any of this, please call your local technician... I did, I don't mind electricity, but I don't mess with plumbing or gas lines.)

I hope this helps,

CWS
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