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Old 04-09-2012, 11:31 AM   #16
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Pilot is ON, Furnace won't fire


If your pilot does light and stays lit on it's own then can you test if the gas valve is getting 24V ac when there is a call for heat?

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Old 04-09-2012, 11:34 AM   #17
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Pilot is ON, Furnace won't fire


I can - if you tell me EXACTLY how to do it.
Pretend you are speaking to a small child though.
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Old 04-09-2012, 11:37 AM   #18
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Pilot is ON, Furnace won't fire


The picture shows your gas cock off and the gas valve lever in the pilot position. Obviously both of these need to be on.
If you are able to light the pilot and have it stay on with the red button allowed to rise back up on it's own.... then test if the gas valve is getting 24V ac when there is a call for heat.
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Old 04-09-2012, 12:13 PM   #19
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Pilot is ON, Furnace won't fire


Sorry for the duplicate posts.
You have not yet confirmed that the control on your gas valve that is pointing to "pilot" in the picture has now been turned to the "on" position>

Take your electrical meter, turn it on & set it to measure Volts ac above 24V, unplug the white & orange wires from the gas valve (remember where they were connected so you can plug them back in the right spot afterwards) and put your two meter leads on the ends of those wires & turn up the thermostat.
Let us know what you read.

Last edited by how; 04-09-2012 at 12:17 PM.
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Old 04-09-2012, 05:59 PM   #20
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Pilot is ON, Furnace won't fire


This has been going on way too long. You must have a volt-ohm meter to test with. If you have a standing pilot, lets go from there. Turn off the power to the furnace and test the valve circuit. Picture 3 shows the gas valve and the wires leaving it going back to the control board. The orange wire goes from the valve to a pair of terminals on a black component mounted on the side of the burner compartment. This is a secondary limit switch, it must be closed. Take both wires off, set your meter to ohms and test across the terminals. Your meter should read at or very near zero Ohms, like when you touch your test leads together. If you get infinity or no reading the limit is open. If the limit has a little red button in the middle, press it. If it clicks, it was tripped and you have just reset it. Reconnect the wires and try a call for heat to the furnace. You may need an assistant for this to turn up the stat while you watch it light. If it lights, watch the entire process through the startup and running of the blower. It may work but the problem has not been solved. The issue is why did it trip? I don't remember if this unit has a draft inducer or if it is natural draft. Either way the main flame should light smoothly and evenly and have little to no blowback. Be especially watchful of the flame when the blower starts there should be no deflection of the flame, no lifting off of a burner. These are ribbon burners and the flame should be even and steady the full length of the burner top and a nice blue color with bright light blue centers, no or very very little yellow at the top. If that does not do it turn off the call for heat, switch your meter to read in the 24 volt range, pull the leads off of the gas valve and hook your test least to the wires, bring on a call for heat and read the volts. It should read 24 volts plus or minus about 10%. If you have 24 volts coming in and it doesn't fire the gas valve is at fault. Now, before condeming the gas valve the incoming gas pressure should be tested with a manometer to be not more than 8" w.c. for natural gas and 14"w.c for LP. Values higher than that the valve will lock out. The valve will still likely be shot but that gas pressure will have to be corrected before installing the new valve or you will have the same problem and yes, regulators do fail. Then the manifold gas pressure will also have to be set to the correct value after installation of a new valve. That must be set to whatever is specified on the data plate on the unit itself. Back to flames. If you have any major movement of the flame in any of the chambers the heat exchanger has failed and at the age of that unit it is time for replacement. Do this stuff up to the point that you are comfortable and then stop. If you feel like you are over your head you probably are. Don't just blunder on you are messing with a bomb and your life and house is on the line. If you need help call a company that has NATE or APHH certified service technicians. Yeah, they usually cost more but they know what they are doing and have taken the time and expense to get the certification to prove they have the training and experience to do the job right.
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Old 04-09-2012, 09:37 PM   #21
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Pilot is ON, Furnace won't fire


well said...

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