draft inducer fan running backwards


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Old 01-07-09, 02:55 PM
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Question draft inducer fan running backwards

While helping a friend replace the main fan motor of his gas furnace, I noticed that the draft inducer fan was turning the wrong direction. The unit had been operating like this for approx 6 years. we switched the wires, and the fan ran in the correct direction (verified by checking the airflow at the flue). what are the issues with this condition? was it pushing the exhaust into the house? are their efficiency issues? how could it have operated all this time with that condition?
thanks!
 
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Old 01-07-09, 03:32 PM
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Ummm. I'm not sure I'm understanding you.

You say you were changing out the main fan motor. I presume you mean the motor which circulates air around the house.

It's not unusual for these motors to have a connection which will cause the fan to run the opposite way, and for that reason finding them running the wrong way happens from time to time. Is that what we are talking about?

You also say that you found the inducer motor fan running the wrong way. That would be VERY unusual --- and I've never seen such a condition. Were that to happen the pressure switch shouldn't close and the furnace shouldn't operate. Furthermore, inducer motors are AC motors and connecting the wires "backward" shouldn't cause the motor to reverse.

There is therefore no way to "switch the wires" as you suggest to get the inducer motor to change direction ---not in my experience anyway.


Furthermore, you seem to be saying that your noticed the inducer motor turning the wrong direction while you were installing the circulating fan motor --- but there would be nothing to attract your attention to the inducer motor if you were changin the fan motor.

So the way I am reading this it makes no sense to me. Perhaps you can explain what I am misunderstanding.
 
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Old 01-07-09, 03:40 PM
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I agree with SeattlePioneer. Your question is confusing.
Anyway the inducer motor has an intake side and an exhaust side both of these should be vented to the outside of the house. That's how these high effeciency furnaces are designed. This way when the furnace is running it is not pulling in cold air into the house through windows and doors which eventually end up going out the exhaust pipe.
 
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Old 01-07-09, 07:47 PM
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We replaced the main fan motor, the one that pushes the air through the house. When we had everything put together, we turned the power back on and pushed the access panel switch so that the unit would start the heat cycle. The draft inducer fan started, the hot surface igniter started glowing, the gas kicked on and ignited, and then the main fan kicked on. So far so good. We released the panel switch and the unit turned off, as it should. it was then, as the draft inducer fan was slowing down, that I noticed that it was turning clockwise (I just happened to be looking at the draft inducer motor as it was slowing down). There is a label right on the draft inducer fan unit showing that the direction of rotation for it should be counter clockwise. Also, the vent (outlet) is on the upper right of the housing, where I think it should be for CCW rotation.
When we reversed the wires, the draft inducer fan ran CCW, the correct direction. We ran through several heat cycles to be sure, and it was turning CCW each time.
This is not a high efficiency furnace. there is only one vent, and that is the exhaust vent to the roof. The ductwork joins with the exhaust ductwork from the water heater, and then goes directly to the roof.
 
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Old 01-07-09, 07:59 PM
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Very strange. I have never heard of an AC motor that could be reversed by swapping the wires. Are you sure you are not mistaken? Also I don't see how the furnace could have operated with the fan running the wrong way because the pressure switch should not have closed. (Unless this one had a centrifugal switch for fan proving. Come to think of it I do recall a seeing a mobile home furnace set up that way.) Still confused because, like I said, switching the power wires on an AC motor doesn't switch the rotation. This isn't in a motor home or RV is it and running on DC?
 
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Old 01-07-09, 08:14 PM
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Extremely strange. Very occasionally I have seen a single phase motor run backward and only when the windings are damaged and only when on a 3 phase appliance. Hmmm. Dont see how this furnace would even light like that. What were you guys drinking that night?
 
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Old 01-07-09, 08:16 PM
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Wow! VERY strange.


Good catch there, lbarness. Sorry for the additional questions but I thought I must be missreading or missinterpreting something. But your additional explanation makes it clear that I was not.


Now I wonder how many of those I missed....
 
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Old 01-07-09, 08:18 PM
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This is a single family home. Standard, builder grade furnace, not running on DC.
I am 100% certain that the draft inducer motor was spinning CW (the wrong direction), and after we swapped the two (the only two) wires leading to it, it spun CCW. I am absolutely certain.
As for the pressure switch, I was told that it was could be a pressure differential switch, and operates only on a change in pressure, positive or negative.
 
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Old 01-07-09, 08:21 PM
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Originally Posted by thermofridge View Post
Extremely strange. Very occasionally I have seen a single phase motor run backward and only when the windings are damaged and only when on a 3 phase appliance. Hmmm. Dont see how this furnace would even light like that. What were you guys drinking that night?
We were sober as a judge. and not like the judge from my home town... he had his own bar just down the street from his office! Beer 4U2
 
  #10  
Old 01-17-09, 03:04 PM
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inducer motor running in reverse.

I had to jump the leads on the pressure switch because the inducer motor is running in reverse. what is the danger of letting it run until I get a new one?
 
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Old 01-17-09, 03:25 PM
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Death or permanent brain injury from carbon monoxide poisoning.
 
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Old 01-17-09, 03:28 PM
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Hello I think---


It rapidly becomes confusing to try to deal with more than one problem on a thread. My suggestion would be to post what's going on with your furnace on a new thread and you'll probably get some help with it.
 
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Old 01-17-09, 03:30 PM
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You have an open or closed combustion furnace? "Open" would have only one PVC vent pipe and "closed" would have 2. If you really have that problem, at least the incoming and outgoing air would just have vent directions, in each pipe reversed, but could at least allow exhaust get to the outside. If "open" combusion CO exhaust would directly come back into the house, for sure.

Even at with closed combustion, I cannot recommend you run it this way. I am just explaining how perhaps it could work.

I've never ran into this problem and only have what was already posted by the other person, to go on. Sounds crazy and unbelievable, like the old Ann Landers columns, that were off-the-wall that she'd get from pranksters from this one college. But I guess I have to take the posters (and now you) word for it.
 
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Old 01-17-09, 05:00 PM
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Believe it or not - an electric motor can suddenly reverse direction! I know because I have one that did it. Fortunately, it was not on my furnace! Here's the story:
1/3 HP Marathon with lugs on the board for switching rotation.
I got it free because it was laying on the reject shelf where I worked, was out of warranty and no one knew what was wrong.
It ran fine on my small jointer, until one day. I turned on the jointer and it RAN BACKWARDS. I turned it off, took the motor off the jointer, sit it on the bench, hooked a cheater cord to it and it ran in the CORRECT (wired) direction.
I changed it to run in the opposite direction, and it also ran ok, so I changed it back, tested it again, and it ran ok in the correct direction. I put it back on my jointer, and it still runs ok, and that was 4 years ago. I never did find out why it suddenly switched direction!
However, I wouldn't think that the inducer motor mfgr would put enough money into their design to even provide for rotation switching capability.
 
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Old 01-17-09, 05:06 PM
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Believe it or not - an electric motor can suddenly reverse direction! I know because I have one that did it. Fortunately, it was not on my furnace! Here's the story:
1/3 HP Marathon with lugs on the board for switching rotation.
I got it free because it was laying on the reject shelf where I worked, was out of warranty and no one knew what was wrong.
It ran fine on my small jointer, until one day. I turned on the jointer and it RAN BACKWARDS. I turned it off, took the motor off the jointer, sit it on the bench, hooked a cheater cord to it and it ran in the CORRECT (wired) direction.
I changed it to run in the opposite direction, and it also ran ok, so I changed it back, tested it again, and it ran ok in the correct direction. I put it back on my jointer, and it still runs ok, and that was 4 years ago. I never did find out why it suddenly switched direction!
However, I wouldn't think that the inducer motor mfgr would put enough money into their design to even provide for rotation switching capability.
 
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Old 01-20-09, 02:24 PM
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Induced draft motor running backwards

Oh, yes an induced draft motor certainly CAN run backwards... I have one that does. Here is what happens...
House gets cold.. but furnace (Lennox Boiler) is running, only not lit. If you are familiar with the noise, it doesn't sound right. Reason for not lighting is that the pressure switch is not closed. If you shut off the power to the unit and let the induced draft motor coast down you see that it is turning clockwise, not pushing air up the chimney. Once it stops, I notice that it starts to turn by itself very slowly in the clockwise direction (WRONG!!) due to a slight draft coming down the chimney. If you turn the power back on while it is turning in the wrong rotation, it will run in that direction and the furnace boiler will NOT light. If you power it off and start the induced draft motor counterclockwise with your finger or a pencil tip, then turn on the power, it will run the correct counterclockwise direction, and then the burners WILL light.
This seems to me like a defective motor, with not enough starting "oomph" to overcome a slow rotation in the wrong direction, or maybe a bad / weak start capacitor... I haven't gotten around to testing these yet.
The way I got around this problem was to fabricate a hinged cap for the end of the flue pipe (4") that closes when the burners shut off, and thus stops any draft down the flue pipe. This keeps the blower from turning in the wrong direction. Once the blower starts again, since it was stopped, it has enough power to start up again in in the correct direction, and the cap opens from the air pressure and the burners light. Same principle as an exhaust cap sometimes installed on a tractor to keep rain out of the exhaust... albeit temporary until I get the real issue fixed.. this unit is only 3 years old and this is the first time I know of that I have had this problem.
 
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Old 01-20-09, 05:26 PM
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Can you post a picture of this inducer motor and furnace? I've never heard of reversing rotation on an inducer motor, especially if it would be a shaded pole motor.
 
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Old 01-31-09, 03:10 PM
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Motor turning wrong direction

Odd as it may seem, this is an entirely plausible explanation for the behavior of a damaged induction motor, particularly small fractional HP types. It goes back to the basic operation of induction motors, and can get a bit complex. However, the basic idea in all of them is that it takes a rotating magnetic field to to make them work (get rotational power). In most (not all) you have a rotor that behaves like a shorted transformer winding. You have a stator that can induce current in the rotor (kind of like a transformer primary (stator) and secondary (rotor). If the rotor is stationary, and the stator (one winding) is AC energized, you just have a transformer with a shorted secondary, and nothing happens other than generating heat! This is a static field. In this condition you can start the motor turning in either direction with a spin.
To get it turning on its own requires a rotating magnetic field to create a torque vector. This is done in several ways, but in these small motors it is often done with another winding 60-90 deg out of phase with the main winding. The phase shift is created with a combination of position, reactance and resistance. In order to avoid large capacitors and centrifugal switches, the starting winding is designed to have a low 60Hz reactance with low rotor MMF (not turning) and high reactance once turning. (sort of self-switching starting winding). If this winding is independent, it can be used to control direction.
This form of starting winding is not a high torque method. It is used on fans of many types. What happens is that the normally high impedance start winding gets heat fatigued and gets even higher in resistance, maybe from shorted turns etc. The result is that the starting torque is reduced.
Now, you can see how, in the IDB discussed, if there is a downdraft that gets a bad motor turning the wrong way it will continue that way if the start winding torque is not enough to slow and reverse the rotation.
WOW, am I wordy... sorry. Its a interesting electromechanical problem though. (BTW: get a new motor if it ever does this...)
 
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Old 01-31-09, 04:12 PM
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That may be the best post I've ever read on this site. Thanks for your professional input.
 
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Old 01-31-09, 04:21 PM
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Scholarly indeed -IF this came forth out of his head with no research. As if Kev is an electrical engineer employed by motor factory?
 
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Old 01-31-09, 04:38 PM
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Motor runs backward

As previously stated some motors especially when subjected to a force such as a downdraft or refrigerant running back after the compressor shuts off thus causing the driven device (fan, etc.) to turn the motor, will indeed start & run backward.
In the early scroll A/C compressors this was a common problem.
What I find strange is the draft proving switch actually allowing the furnace to fire. It needs to be checked as well.
 
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Old 03-01-09, 05:11 PM
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I have same problem - See Post "High winds no heat with Lennox Gas Boiler"

Originally Posted by klausmanj View Post
Oh, yes an induced draft motor certainly CAN run backwards... I have one that does. Here is what happens...
House gets cold.. but furnace (Lennox Boiler) is running, only not lit. If you are familiar with the noise, it doesn't sound right. Reason for not lighting is that the pressure switch is not closed. If you shut off the power to the unit and let the induced draft motor coast down you see that it is turning clockwise, not pushing air up the chimney. Once it stops, I notice that it starts to turn by itself very slowly in the clockwise direction (WRONG!!) due to a slight draft coming down the chimney. If you turn the power back on while it is turning in the wrong rotation, it will run in that direction and the furnace boiler will NOT light. If you power it off and start the induced draft motor counterclockwise with your finger or a pencil tip, then turn on the power, it will run the correct counterclockwise direction, and then the burners WILL light.
This seems to me like a defective motor, with not enough starting "oomph" to overcome a slow rotation in the wrong direction, or maybe a bad / weak start capacitor... I haven't gotten around to testing these yet.
The way I got around this problem was to fabricate a hinged cap for the end of the flue pipe (4") that closes when the burners shut off, and thus stops any draft down the flue pipe. This keeps the blower from turning in the wrong direction. Once the blower starts again, since it was stopped, it has enough power to start up again in in the correct direction, and the cap opens from the air pressure and the burners light. Same principle as an exhaust cap sometimes installed on a tractor to keep rain out of the exhaust... albeit temporary until I get the real issue fixed.. this unit is only 3 years old and this is the first time I know of that I have had this problem.
PLease see my post -

" High winds no heat with Lennox Boiler " -
I believe you may have a draft problem causing the inducer fan to spin too fast and prevent ignition of your boiler leaving evrything else running.
I believe this is an engineering design falt.
Ted
 
 

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