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Old 04-03-2013, 10:12 AM   #1
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Troubleshooting pilotless furnace problem


Pilotless furnace blows cold air.
If set to AUTO-HEAT, temp increased on the thermostat it will click, and then make a humming sound, and then stop after about 2 minutes. Nothing blows from the vents.
If set to ON- HEAT, temp increased on the thermostat it will click, and then the fan kicks on and it blows cold air.
I let the filter get too dirty, which I'm guessing is the cause of the problem.
I've tried:
new batteries in thermostat
making sure all wires on thermostat were tightly connected
shutting off breaker for 5 minutes
shutting off furnace reset switch for 5 minutes

From what I've been able to read online, it sound like my ignitor has gone out, my wife wasn't available last night to run the thermostat while I watched the unit though, so I can't confirm that. I'm a handy guy, but haven't worked on a furnace yet. What can I do to troubleshoot this thing, I don't want to spend any more time being cold, lol, but I also don't want to just throw parts at it.

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Old 04-03-2013, 11:10 AM   #2
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Troubleshooting pilotless furnace problem


The first step that you should notice with a stat call for heat is the start of the inducer motor.
Since you are not describing it coming on, I'd be measuring if the board is powering the inducer motor or not.
Can you tell us if the humming sound, is your desciption of the ID motor running or the ID motor just trying to run.


Last edited by how; 04-03-2013 at 11:14 AM.
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Old 04-03-2013, 11:21 AM   #3
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Troubleshooting pilotless furnace problem


Quote:
Originally Posted by how View Post
The first step that you should notice with a stat call for heat is the start of the inducer motor.
Since you are not describing it coming on, I'd be measuring if the board is powering the inducer motor or not.
Can you tell us if the humming sound, is your desciption of the ID motor running or the ID motor just trying to run.
I guess I assumed that was the motor running, and then it shut off once it realized it wasn't outputting heat. I'll have to check that tonight when my wife is home to help me man both stations.

In normal operation it makes the same humming sound for about 15 seconds, and then I hear the fan kick on and it starts outputting heat.
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Old 04-03-2013, 12:10 PM   #4
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Troubleshooting pilotless furnace problem


Brand & model of furnace would help. Photos with the cover off are great.

If your inducer is running then the pressure switch (the round disc connected to the furnace by rubber tubes) should engage a safety circuit after verifying the ID assy is doing it's job. Measuring if the PS is allowing the 24V to complete this circuit is the next step.

Do you have a blinking diagnostic led light on your furnace?
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Old 04-03-2013, 12:14 PM   #5
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Troubleshooting pilotless furnace problem


Quote:
Originally Posted by how View Post
Brand & model of furnace would help. Photos with the cover off are great.

If your inducer is running then the pressure switch (the round disc connected to the furnace by rubber tubes) should engage a safety circuit after verifying the ID assy is doing it's job. Measuring if the PS is allowing the 24V to complete this circuit is the next step.

Do you have a blinking diagnostic led light on your furnace?
It is blinking red, but I can't for the life of me find a make/model sticker or a key to diagnose the red light sequence anywhere on the machine, the only thing I've found is the energy effiency sticker, and a guide on the outer cover that says it's a pilotless unit and how to power down and reset it. Maybe I was too frustrated last night to see it. Again, I'll try all of this again tonight when I get home from work.
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Old 04-03-2013, 12:22 PM   #6
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Troubleshooting pilotless furnace problem


Usually the diagnostic legend is on the inside of the furnace doors or on the plastic body of a seperate ignition module.
Furnaces always display some brand and a identification plate.
I think you should send us a photo of this unit.
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Old 04-03-2013, 12:29 PM   #7
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Troubleshooting pilotless furnace problem


I'm going to:
1. Check to see if the ignitor is glowing with my wife tonight
2. If it isn't, pull the ignitor and check the resistance level
2a if 0 reading, replace
3. See if the inducer is running
4. report back

After 6pm, where would I find a new ignitor at?
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Old 04-03-2013, 12:42 PM   #8
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Troubleshooting pilotless furnace problem


Also Consider Verifying that the pressure switch has 24 V going through it when the inducer motor is running. You won't even get power to the igniter until that happens.
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Old 04-03-2013, 12:46 PM   #9
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Troubleshooting pilotless furnace problem


Quote:
Originally Posted by CaptainRogers View Post
In normal operation it makes the same humming sound for about 15 seconds, and then I hear the fan kick on and it starts outputting heat.
I doubt your blower starts blowing heat 15 seconds after the inducer motor starts.

Typical sequence is something like this:
-After the inducer motor starts
-pressure switch is verified
-hot-surface ignitor or spark ignitor will energize.
-gas valve will open
-as long as the flame is sensed the burners will continue expel flame.
-Some time or temperature setting later, the fan blower will turn on.

there are some other internal safety control board checks that go on too

Last edited by hammerlane; 04-03-2013 at 01:15 PM.
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Old 04-03-2013, 12:51 PM   #10
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Troubleshooting pilotless furnace problem


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Originally Posted by hammerlane View Post
I doubt your blower starts blowing heat 15 seconds after the inducer motor starts.
You are correct, it starts blowing, but takes a bit for the air to be hot. I was mostly trying to convey that the startup hum that I am currently hearing was normal- but the next step didn't seem to be starting when it should.
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Old 04-03-2013, 12:55 PM   #11
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Troubleshooting pilotless furnace problem


how far away is your furnace from your thermostat? Can't believe you do not have enough time to set the thermostat to call for heat and then get to the furnace location by the time the ignitor is suppose to energize.

If needed set the thermostat and blast it with a few seconds of warm air from a blow dryer. That should give you plenty of time to get to the furnace location before the stat cools to the surrounding air and then calls for heat.

Or you could just jumper the red/white stat wires together at the furnace.
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Old 04-03-2013, 12:55 PM   #12
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Troubleshooting pilotless furnace problem


After watching youtube videos all morning, I think the first startup him I hear is the inducer motor. I agree with "how" that I also need to check the pressure switch as well as the ignitor.

Where do I find these types of parts since the Lowes, Menards doesn't carry them?

Last edited by CaptainRogers; 04-03-2013 at 01:00 PM.
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Old 04-03-2013, 12:59 PM   #13
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Troubleshooting pilotless furnace problem


Quote:
Originally Posted by hammerlane View Post
how far away is your furnace from your thermostat? Can't believe you do not have enough time to set the thermostat to call for heat and then get to the furnace location by the time the ignitor is suppose to energize.
Thermostat is in the hallway of my ranch style home, which is on the opposite side of the house as my garage. The furnace is in the attic. The problem I have right now is almost directly below my attic entrance door sits a 1972 Buick Skylark with a blown transmission- so I have to open the car door, climb to the roof of the car, and then do a pullup to get in the attic, because I can't swing the stairs down because they hit the car.

Also, does the dirty air filter lend itself to one probably cause more than another? I'll also clean my flame sensor while I've got it opened up tonight.

Thank you all for the help, I'm feeling pretty confident in being able to troubleshoot this thing now.

Last edited by CaptainRogers; 04-03-2013 at 01:23 PM.
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Old 04-03-2013, 02:59 PM   #14
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Troubleshooting pilotless furnace problem


is this a condensing furnace? Chances are your trap, or drain lines to your trap, or both, are clogged, and possibly your secondary heat exchanger is clogged with crapola. You need to clean all with warm soapy water and bleach.

Parts: www.pexsupply.com free overnight in most cases. Awesome supplier.
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Old 04-03-2013, 03:03 PM   #15
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Troubleshooting pilotless furnace problem


Quote:
Originally Posted by CaptainRogers View Post
Thermostat is in the hallway of my ranch style home, which is on the opposite side of the house as my garage. The furnace is in the attic. The problem I have right now is almost directly below my attic entrance door sits a 1972 Buick Skylark with a blown transmission- so I have to open the car door, climb to the roof of the car, and then do a pullup to get in the attic, because I can't swing the stairs down because they hit the car.

Also, does the dirty air filter lend itself to one probably cause more than another? I'll also clean my flame sensor while I've got it opened up tonight.

Thank you all for the help, I'm feeling pretty confident in being able to troubleshoot this thing now.
I doubt the filter had anything to do with it. The filter will cause a main limit to go out, but usually if that's the case the blower would run all the time. Also cleaning the flame sensor while you're up there is a good idea, but I also doubt that has anything to do with the problem. If it was the flame sensor it would light but only stay lit for 5-10 seconds.

As stated above pressure switch, ignitor or possibly even a bad ignition module are the likely causes.

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