Natural gas furance - intermittant issues - damper does not open

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Old 12-02-11, 06:35 AM
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Question Natural gas furance - intermittant issues - damper does not open

So I've been reading some information on these forums and I've learned quite a bit about how my furnace works. It's a two-zone gas furnace with a damper. Over the past few months I've had nights that it doesn't kick on, which I can usually fix with a few power cycles or a hard tap on the control box the damper is attached to.

The issue is that the damper does not begin to open when the thermostat triggers. Is this likely the control box, or the damper itself? It seems to open very slowly when it does open, but I've not seen it open in the past so that may be normal.

Thanks!
- Kyle
 
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Old 12-02-11, 10:25 AM
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You should be able to measure the voltage being applied to the motor opening the damper. It should probably be either 24 VAC or 120 VAC.

If the voltage is good then you have a defect in the damper mechanism. You would need to determine what that defect is.

Is this a Honeywell damper? If so, it's a 24 VAC system. It's famous for failing.
 
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Old 12-02-11, 02:22 PM
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I'll take a look when I get home. I do recall seeing "24V" somewhere on the control box (perhaps the inside of the cover). I'll get a voltmeter on it today and let you know.

Thanks,
Kyle
 
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Old 12-02-11, 08:52 PM
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Took a look this evening. 4 wires enter the damper. Measuring AC voltage on the black to all others yields one pair that is consistently 24 volts, another that is a variable amount as I tap on the control box (That's a good sign right?) and a third that is 0 VAC. With these 24V dampers, is it typical to pass one 24V line through that will return to the gas valve once the damper is open and another to control the damper itself?

What I found a little more odd than all of that is that when I flip the hold-open switch it made noise as if it were trying to open, and the odd voltage pair dropped in half (too much load?). If it were already trying to open (due to low tank temperature) why did it not already sound like that?

If I'm leaving out anything let me know I can go take a look. I managed to get the unit to turn on for the night (power cycling, poking around, luck, etc.)

EDIT: Almost forgot. The two control boxes are both labelled Honeywell, the boiler itself is a Peerless and I did not see a branding on the damper unit, just some model numbers (which I didn't write down - doh!) and warning information.
 
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Old 12-03-11, 04:19 AM
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the flue damper should be replaced as an assembly, not just the motor. It is unlikely that you will find just the motor. To get by you can switch it to hold open and then manually open the damper using pliers to turn the shaft from the opposite end of the motor.
 
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Old 12-03-11, 06:26 AM
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So I played around some more this morning. Each time I start the unit voltages around the device with the temperature selector have completely unpredictable values. Am I way off in assuming replacing the Aquastat(L8148E-1166) is the correct repair? The damper reliably opens when it receives 24V, which no longer happens long enough for it to open *and* start the gas.

I will try using the hold open and pliers now just to get the tank back up to a normal temperature.
 
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Old 12-03-11, 06:32 AM
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depends, is that where the damper plugs in? Some go to the ignition board which can send bad voltages to the damper.

Once its open and the switch is kept on hold open it should continue to run for you. the damper is an efficiency item, but not required for your system to work.
 
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Old 12-03-11, 06:40 AM
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Thanks for the quick reply. The damper wiring directly enters the aquastat and is wired into the circuit board there. I'm heading down to see if holding the damper open will allow the gas to fire now.
 
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Old 12-03-11, 06:42 AM
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ok, report back and let me know
 
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Old 12-03-11, 06:47 AM
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Well, that worked perfectly (Thanks!). House is heating back up now!

I'll admit my electrical knowledge is touch and go, I'm much better with DC/Digital circuits. How is it that there isn't sufficient voltage to open the damper, but when the damper is held open the gas valve gets a nice ~24VAC immediately?
 
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Old 12-03-11, 06:52 AM
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there are micro switches contained within the damper motor housing that sense when the damper is open.... This allows voltage to flow through the switch and to the GV. The micro switches are safety devices because if the damper were closed and the boiler fired it would put all the flue gases into your home.
 
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Old 12-03-11, 07:08 AM
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Ah, makes sense. Should I be looking at replacing the damper or the aquastat in the future? You say the damper is an efficiency item, is it saving me enough money that I should rush to get it fixed?
 
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Old 12-03-11, 07:22 AM
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no, its not saving you that much money! With knowing which wires you are getting voltage to and where you are checking the voltages I can't say which one is the problem. Often I find that the connector on the aquastat board is what is bad and this can be tested by moving or wiggling the connector on the board to see if the damper operates, but you would have to put the damper back to automatic position in order to test it.
 
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Old 10-29-12, 08:44 PM
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I'm looking at this thread and I just joined the site because my mom has the same problem with the damper not opening. If I jiggle the wires in the box, the damper opens and the burner fires.

The damper was replaced by a repairman a year ago, but the issue of the burner intermittently not firing was never resolved.

So should I look for a new aquastat relay L8148E or could it be a part in the box that can be replaced?

Thanks, Gary
 
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Old 10-29-12, 08:46 PM
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likely the whole aquastat needs replaced
 
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