Gas furnace cycling on/off. Must be air flow problem?


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Old 10-23-09, 02:40 PM
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Gas furnace cycling on/off. Must be air flow problem?

I posted this problem on another DIY board, but I have not gotten any response. I read through 2 threads on this board about this problem, and I'm pretty sure I have the same problem. The blower motor on the furnace turns on for about 1 minute and then goes off for about the same amount of time. The pilot flame is always on and the burner turns on for about 30 seconds and then seems to stay off about 90 seconds.

I found the air filter was filthy with dirt and I removed it, but the problem is still the same. Could someone confirm that the airflow pressure switch is the unit in my first picture here? There is 120VAC across the purple and black wire when the blower motor isn't running. When the blower motor runs this drops to 0VAC, and there is continuity. I'm not sure if I'm looking at the right terminals. There is about 45VAC across the top 2 terminals and about the same across the bottom 2 terminals when the unit is running. There is always continuity between the 2 red wires whether the blower motor is running or not.

The 2nd picture is the info on top of the switch. I just wonder if ya'll can confirm that I'm looking at the right switch and what terminals I should be looking across to confirm proper operation.

Thanks,
Endzone





















 
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Old 10-23-09, 03:12 PM
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The part is what it says it is on the rotating cam --- a fan/limit switch.

That performs two functions.

The limit switch portion turns the burner off if the furnace overheats.

The fan switch turns the fan on and off.


I can't really make sense out of what you are describing in your first paragraph.

A furnace has a sequence of events that happen in order to produce heat and circulate it through your house.

You need to describe that sequence beginning what happens when you turn the thermostat up, in the order that it happens. You may need to observe this ignition sequence several times before you notice all the details of what happens.

If you'll describe that you can probably get some help with figuring out what the problem may be.

Good pictures. That helps a lot to understand what you have.
 
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Old 10-23-09, 03:27 PM
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You will notice that there are three adjustable pointers on the fan limit switch rotating cam.

When the switch warms up to about 130 degrees, the middle pointers turns the furnace fan on.

Were the furnace to get to a temperature of about 200 degrees, the pointer for the limit switch would turn the burner off to prevent further overheating.

Once the fan turns on, the pointer at the 90 degree position turns the fan off after the burner is shut off by the thermostat and the furnace cools to the 90 degree level.
 
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Old 10-23-09, 03:32 PM
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This looks like an old furnace. It has the older fan-limit switch and older standing pilot design and older 120 volts to the pressure switch rather than 24 volts used today.

When the fire goes out, where does the dial point that has the numbers on it? Just before the fire goes out, do you see the dial inch upward, and then suddenly jump(rotate) further toward 200, and then the fire goes out?

When the burner fire goes out, does the pressure switch go from 0 when running, back to 120 again immediately? If so, what does the volts measure from one terminal to ground(any good metal on the furance), and then also from the other terminal to ground. If say that both terminals say 0 then I'd suspect high limiting or thermostat shutting off.

Is there a separate flame sensor for the burners, not counting the thermocouple for the pilot?

What kind of thermostat do you have, and have you replaced it since the last time the furnace was running properly?

Need to know if the thermsotat is shutting off on you on too low an anticipator setting or small temp swing or short cycle times.
 
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Old 10-23-09, 04:18 PM
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Thanks for your response fellows! Much appreciated. I will make a detailed report tomorrow of exactly what happens in the sequence, but I can tell you that it is cyclical. It just keeps repeating the same pattern. I have seen the metal dial move, but I will have to more carefully observe what happens with the burner and blower motor when it moves.

Seattlepioneer my last job was in Seattle. I've worked there off and on for the past 10 years. I lived in Kent just off Pacific Highway and worked in Renton on the south side just off Grady Way.
 

Last edited by Endzone; 10-23-09 at 04:19 PM. Reason: post wound up in wrong place.
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Old 10-23-09, 07:34 PM
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If your air filter was very dirty, you should inspect the inlet side of your evapoator coil.

 
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Old 10-24-09, 08:06 AM
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Alright, can you tell if anything in this sequence obviously seems out of whack?

0:00 Dial for fan/limit switch is at 70 deg position. Furnace is cold. I turn T-stat to 80 deg to start furnace.
1:00 Burner comes on and dial begins to move slowly up.
2:30 Dial at 140 deg and fan motor comes on. Dial never goes above 150 deg.
3:00 Burner turns off. Dial slowly moves back towards 90 deg.
4:45 Burner turns on and dial is at 110 deg.
4:50 Fan turns off with dial still at 110 deg.
5:30 Burner off. Dial at 120 deg.
6:15 Fan on. Dial at 140 deg. Burner is still off.
7:00 Burner on. Dial at 120 deg.
7:40 Burner off. Dial still at 120 deg.
8:30 Fan off with dial at 110 deg., and it quickly snaps back to the 80 deg. position.
8:30 plus, cycle continues.....

Is it normal for the burner to turn off at the 3 minute mark? 150 degrees is as high as the dial ever goes.

With the fan motor on there is 0VAC across the purple and black wires, and each wire has 120VAC to ground.

With the fan motor off there is 120VAC across the purple and black wires, and the purple lead has 0VAC to ground, and the black lead has 120VAC to ground.

As far as I can tell there isn't a flame detector going to the gas controller. I've taken every cover off that I can, but I don't see one. I could be missing it though.

I know this unit is kind of ratty looking, and I also know what a tech is going to tell me if he comes out and looks at it. In this economy I'd like to keep this piece of junk going if possible. Any comments appreciated.

Furnace off and cold.












 

Last edited by Endzone; 10-24-09 at 08:21 AM.
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Old 10-24-09, 10:56 AM
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Probably the heat anticipator on the Honeywell thermostat is out of adjustment.

If you tajke the cover of the thermostat off, you'll see a tiny coil of wire with a pointer on it.

If you observe the thermostat when you turn it up, you are probably going to see the mercury blob cycle back and forth, turning the furnace burner on and off as you describe.

The coil of wire is a heater coil to cause the thermostat to respond faster to changes in room air, and it allows the thermostat to be matched to the furnace.

If it's turned too high, it heats up too fast, shutting the furnace off, then cooling off and turning the furnace on again ---- repeating that cycle.

That's what I'm supposing you will see happening.



Seattle Pioneer
 
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Old 10-25-09, 06:20 AM
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Originally Posted by SeattlePioneer
Probably the heat anticipator on the Honeywell thermostat is out of adjustment.

If you tajke the cover of the thermostat off, you'll see a tiny coil of wire with a pointer on it.

If you observe the thermostat when you turn it up, you are probably going to see the mercury blob cycle back and forth, turning the furnace burner on and off as you describe.

The coil of wire is a heater coil to cause the thermostat to respond faster to changes in room air, and it allows the thermostat to be matched to the furnace.

If it's turned too high, it heats up too fast, shutting the furnace off, then cooling off and turning the furnace on again ---- repeating that cycle.

That's what I'm supposing you will see happening.



Seattle Pioneer
Thanks Seattle Pioneer. I will look into this soon. Appreciate the info.
 
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Old 10-25-09, 09:02 PM
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You nailed it Seattle Pioneer. That is exactly what is happening. Even with the temp set at 90 deg. in this picture, the coil and the mercury glass thing goes hard to the right after a few minutes of being on. Is this DIY adjustable/replaceable or should I get a new thermostat? Thanks a lot for your help.


 
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Old 10-25-09, 09:06 PM
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Double post.

The quick brown fox jumps over the lazy dog.
 
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Old 10-25-09, 10:42 PM
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If you look at the lower right hand portion of the thermostat, you'll see a pointer on a calibrated dial. This adjusts the number of cycles per hour by energizing a small heater coil that speeds up the thermostat's response to the air temperature.

Yours is set too high. You need to select a longer cycle time.

It's probably way over on the "shorter" portion of the dial.



Seattle Pioneer
 
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Old 10-26-09, 07:56 AM
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Problem solved. Normal operation restored. I don't know how that got adjusted up (or down) like it was. But, thanks a lot for walking me through this problem.

Just thought I'd post a few of the pics I took when I worked off and on in the Seattle area.

Thanks again, Endzone.

There are about 10 from the Seattle area:
My Best Shots Photo Gallery by Craig Coffaro at pbase.com
 

Last edited by Endzone; 10-26-09 at 08:27 AM.
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Old 10-26-09, 08:11 AM
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Nice. But you'd better hurry up and erase those pics before you get erased. Seriously.
 
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Old 10-26-09, 08:25 AM
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Originally Posted by ecman51`
Nice. But you'd better hurry up and erase those pics before you get erased. Seriously.
Ut oh, I didn't mean to violate any board rules. My thanks to all for their help.

Craig
 
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Old 10-26-09, 08:59 AM
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What a really nice collection of pictures. You've got a good eye. A trained eye, perhaps?

That was a treat.
 
 

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