Furnace Blower Won't Turn Completely Off?

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Old 11-06-10, 07:34 PM
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Question Furnace Blower Won't Turn Completely Off?

Hi All,

I thought I knew some basics about troubleshooting and repairing the furnace, and I've spent a lot of time searching around, but I'm now stumped.

My old Rheem forced air furnace is having an issue where the circulation blower never turns completely off. It runs even if I switch the thermostat to Off and the Fan switch to Auto. If I flip the fan switch over to "On" the blower definitely kicks in more, and then it goes back lower if I turn it off again. The problem doesn't appear to be with the fan and limit switch in the furnace. In fact, I unhooked the load wires from the switch and the blower just kept on running.

Any thoughts?
 
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Old 11-08-10, 07:13 PM
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No replies, eh?

Well, at any rate, the problem stopped yesterday. No idea why, but I'd still like to figure out the cause in case it happens again.
 
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Old 11-08-10, 07:24 PM
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When the fan is running, does it seem to be running at the heating speed? When you switch the stat to "fan on", you are usually kicking it into the cooling speed (usually a higher speed than heating).
 
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Old 11-08-10, 11:04 PM
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It may possible you may have a stuck relay but I know you say you tempory remove the load wires and can you point to us what this load wires are you talking about??

Merci.
Marc
 
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Old 11-09-10, 08:16 AM
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Thanks guys.

Grady - Hmmm, yes I guess it was probably running at heating speed. I didn't realize there was a higher cooling speed that kicked on when switched to "fan on" mode.

Marc - On the fan and limit switch, there are 4 wires coming in. Two are marked "line" and two are marked "load." I removed the load wires, and the fan kept running.

But, like I said, now the problem has stopped and the fan is only running when there is heat to blow into the house, as it should.
 
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Old 11-12-10, 11:35 PM
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Well, same thing started happening again. Any thoughts?
 
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Old 11-12-10, 11:43 PM
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I am pretty sure that you have stuck limit switch however before you do anything on this one turn off the power and DO NOT disconnect any conductors at all { this is crictal not to get them hook up wrong for your safety sake } loosen up two screws and slide the limit switch and check the debries on the other side of limit switch.

This part only apply to older furnaces the newer one are wired diffrent and espcally true with oil furnace I know this type of limit switch useally located top of furance on warm air discharge port

Are you refering to this type of this limit switch ?



Let us know.

Merci.
Marc
 
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Old 11-13-10, 09:56 AM
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Thanks for the reply. Yes, I'm pretty sure I have that, but I have no cover on it, and mine doesn't have the manual fan switch.

I did previously pull it out--I didn't see much debris on the back side. I tried moving the spring thing a bit.

Anyway, like I said, I had previously disconnected the "Load" conductors and the fan continued to run. Unless I fail to understand how this works, to me, this suggests that the problem has to be somewhere other than this switch. Any thoughts?

Here is a picture that shows a diagram of these things:

 
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Old 11-13-10, 10:00 AM
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Handy,
If you have a fan/limit like the one Marc showed & the fan is running when it should not be, try smacking the side of the control with your hand. If the fan then shuts off the fan switch inside the control is sticking. DO NOT try to lubricate it. You can try turning the lowest setting up about 10 or so but be sure to maintain at least 20 between the middle & lowest settings.
 
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Old 11-13-10, 06:15 PM
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Thanks for the reply, Grady...I'm guessing you were posting at the same time as I was. Did you see my last post? Unless I don't understand how it works, I don't really think it could be the fan limit switch, since the fan runs all the time, even when I unhook the wires from the fan limit switch.
 
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Old 11-13-10, 07:27 PM
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Woops

Sorry 'bout that part, Handy. I agree, it can't be the fan/limit. If you have 4 wires going to the fan/limit there has to be a fan relay somewhere. Chances are that's where your voltage is coming from to power the fan when the fan/limit has the fan shut off. Do you have a wiring diagram for the furnace & if so can you take a good picture of it & post here or on photobucket or similar photo sharing site?
 
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Old 11-13-10, 08:07 PM
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Let's see if this works:


This diagram is pasted on the inside of the furnace's front panel. Let me know if you have any thoughts. Thanks!
 
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Old 11-14-10, 01:42 AM
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Before I can confirm which part fail but can you tell me which colour conductor you did yank it out if it was black that for the gaz burner valve { that is on 24 volts } but if you did yank out the red conductor that is blower source that is on 120 volts and there at least two relays in there.

And I am sure you are aware that you have induction fan motour in there as well when the furnace is done with heating cycle did you see or hear the induction fan motour stop.

{ note do not open up the blower door compartment at this time otherwise the whole test will not work due interlock switch set up }

And I am sure Grady will add more comment in this one due this is pretty new furance { accourding the drawing the recent reverison was in 05 so there will be diffrent part than I expected }

Merci.
Marc
 
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Old 11-14-10, 11:02 AM
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Wiring Diagram

I'm going to have to defer to Marc on this one. My eyes aren't good enough to read the diagram at all.
 
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Old 11-14-10, 11:19 AM
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Do you see the HCR (heat-cool relay) in the center of your drawing, which is drawn as about a 1 inch square? Can you decipher what every letter is, what they mean, and where the power comes from and where it goes? Power exits the relay at the right side (of that relay drawing), heads downward, then to the left towards the blower, as wire color R (red), which is the low speed wire on the blower motor. (While the B (black) wire, goes to the highest speed winding in the motor, for cooling(a/c). Follow that all in the diagram?
 
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Old 11-14-10, 01:54 PM
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Originally Posted by french277V View Post
...this is pretty new furance { accourding the drawing the recent reverison was in 05 so there will be diffrent part than I expected }
Just a quick note on this: It's actually a drawing from '85, not '05. The furnace is at least 22 years old.
 
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Old 11-14-10, 01:58 PM
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Originally Posted by Grady View Post
My eyes aren't good enough to read the diagram at all.
A quick note on this as well: if you zoom in with your browser (try holding ctrl and pressing '+' repeatedly to zoom), it is actually quite readable.
 
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Old 11-14-10, 02:20 PM
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Originally Posted by ecman51` View Post
Do you see the HCR (heat-cool relay) in the center of your drawing, which is drawn as about a 1 inch square? Can you decipher what every letter is, what they mean, and where the power comes from and where it goes? Power exits the relay at the right side (of that relay drawing), heads downward, then to the left towards the blower, as wire color R (red), which is the low speed wire on the blower motor. (While the B (black) wire, goes to the highest speed winding in the motor, for cooling(a/c). Follow that all in the diagram?
Thanks for your help. Yes, I can follow all that in the diagram. You describe the red wire coming out of the HCR and going down and to the left, where it goes through the TDC. Are you sure I have the TDC (I notice it says it is an option for downflow models--do I have a downflow model)? That same red wire also goes up to the BFC, and comes out the top of the BFC and back down to the blower's low speed motor. In other words, the BFC is in parallel here with the TDC.

If I did have a TDC, where would I look for it? Here is a picture of the furnace with the front panel off:

 
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Old 11-14-10, 02:39 PM
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Well, the TDC I see is tied in with the BFC located in the drawing at top center. See how the red wires get wire nutted together in middle of drawing,to then go to the TDC?

A downdraft furnace usually has the CIRCULATION blower at the top of the furnace, with the plenum for warm house air at the bottom, such as in mobile homes or in house attic applications. But when CB is at the bottom, and the plenum for warm house air is at the top, that is an upflow furnace.
 
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Old 11-14-10, 03:13 PM
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OK, thanks - so I have an updraft furnace, and as such, I'm not sure I probably have a TDC in there.

So, I can follow the diagram OK, but what are you suggesting I look for to figure out what's keeping the fan motor on all the time?
 
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Old 11-14-10, 03:34 PM
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French277V had a good question. Did you remove the red fan wire or the black gas valve wire from your fan limit?



Only two relays control fan operation. The fan relay in the fan center for high speed and the fan limit for a lower speed. It almost has to be the fan limit if you can hear the fan speed increase when you call for "G" at the stat.

 
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Old 11-14-10, 03:53 PM
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Thanks for posting second photo with all the interal layout and I can able see what the issue but can you tell me on the limit switch what tempture you set where the left most slot { lowest tempture } is at ?? if set too low the fan will stay running as well. { on BFC and look at the dail but do not move any thing or change the setting until one of us will confirm this first }

The relay will be on the left side however the HCR can get stuck by default setting it will be on low speed that is red conductor and high speed will be black { used on cooling mode }

Merci.
Marc
 
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Old 11-14-10, 04:41 PM
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Wow, thanks everyone for all the help!

Just to confirm, I again went and unhooked the wires from the fan limit switch. I unhooked both the red and the black, and the fan continues to run. So, it simply cannot be the fan limit switch at this point.

Any suggestions on what to look for next?
 
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Old 11-14-10, 05:30 PM
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I did read the recent reply and I know you did pull the red wire out to rule out the limit switch for a moment but just hold it right there for a second due this furance have two blowers one for induction draft blower that is a small blower right front of the furance and the main circuation blower now which one is still stuck due there is a big diffrence how to elemated this one.

The quick way to elemated this is take the Red conductor off from the transfomer if stay quiet then the main fan relay is stuck if not then you have induction draft blower relay is stuck so just make sure we all understand this one before we get too far and get the wrong item.

Merci.
Marc
 
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Old 11-14-10, 09:38 PM
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Thanks Marc,

I'm sure it is the circulation blower, as it blows air out the registers throughout the house. Also, I can see when the induction blower turns on and off when the gas is getting ready to fire. So, I know it is the circulation blower, but I am happy to pull the red conductor off the transformer to do your test if you can tell me where to find the transformer .

But figuring it is the circulation blower that is staying on, and it stays on even with the fan limit switch unhooked, anybody have thoughts on the next thing to try?
 
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Old 11-14-10, 10:01 PM
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Transformer terminals are circled green.

If the fan runs with R removed, turn off power and remove the 2 screws (circled white) and check for melted wiring behing the fan center.

The fan relay just pulls out, and could be the culprit if we are always running high speed.

The TDC (optional) appears to be above the fan center along the left side.
It would also energize low speed. When you remove the fan center you might trace the wiring to find out.

 

Last edited by Houston204; 11-14-10 at 10:36 PM.
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Old 11-14-10, 10:23 PM
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Thanks - I'll check this out tomorrow. I did already try pulling that top red wire on the transformer, and the fan still ran. However, there are more red wires below, so maybe I should try pulling those?

I did look behind the fan center, I guess it's called, and I didn't notice any melted wiring. It was warm back there, but then transformers usually make a fair amount of heat.

What's the blue box in your picture for? I don't think you wrote anything about that.
 
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Old 11-14-10, 10:29 PM
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The blue box is the low voltage terminal board. You can remove "R" ther also.

If the fan runs with the fan relay removed, it would point to the TDC.
The diagram shows 2 red wires to the time delay relay, wired parallel to the fan center, bypassing the fan limit.
 
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Old 11-14-10, 10:52 PM
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I agree with Houston just try yank the R wire from the transfomer that should stop everything in the furance { it will be front of the transfomer as Houston hightlighted } if so then next step is take the C off from the bleu box where houston marked as well so I can able rule it out and soumd more like Fan relay is stuck.

But just one warning any other conductors and relay just don't touch them it have intermixed of 24 and 120 volts circuits so just give you a head up.

Merci.
Marc
 
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Old 11-14-10, 11:37 PM
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Get yourself an AC multimeter if you want to diagnose the problem. Horsing around by disconnecting wires isn't an especially reliable method of testing things.

You want to test things as they are, not by changing the conditions by which things are operating.
 
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Old 11-15-10, 12:13 AM
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Originally Posted by SeattlePioneer View Post
Get yourself an AC multimeter if you want to diagnose the problem. Horsing around by disconnecting wires isn't an especially reliable method of testing things.

You want to test things as they are, not by changing the conditions by which things are operating.
Well, I don't entirely agree with that, SeattlePioneer. We do a lot of electronics troubleshooting at work, and coming across an issue that stumps everyone for a time is not uncommon. As our experienced and well-regarded senior engineer likes to say, sometimes the best path to a solution is to change the problem.

That said, I DO have an AC multimeter (DC too...), and I've probed around in there a bit. But not knowing what is going on in this furnace, how it works, or what the parts all are by looking at them, measuring what voltage I have at a certain terminal doesn't do me much good. Knowing whether my fan stops running or not if a certain part is taken out of the circuit, on the other hand...now that is getting me somewhere.

If you can tell me what you would like me to do with my AC multimeter to diagnose the problem, I'd be happy to oblige. But I think the wire-removing method, while more primitive, has its merits as well.
 
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Old 11-15-10, 12:44 AM
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So, I pulled off the Fan Relay, and that stops it from running.... What next?

I couldn't quite figure out what Marc wanted me to pull. There was no wire on the C terminal where Houston's blue box is...at least not on the front, although it looks like there is on the back side of it.
 
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Old 11-15-10, 04:55 AM
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Pull out the fan relay and ohm across the normally open contacts.
 
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Old 11-15-10, 08:49 AM
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Thanks again - the normally open contacts measured as open, so the relay appears to be OK. I guess that means something is always telling the relay to be closed. Any guesses?

Another question - does this relay serve any purpose other than the high-speed cooling fan function? I mean, could I leave it out for now and still get proper heating operation? I could see it being employed as a fail-safe in case of other issues, so I don't want to just yank it out without confirmation.
 
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Old 11-15-10, 04:14 PM
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Was this ever addressed?: What is your OFF set to, in degrees Fahrenheit, on the fan-limit switch?
 
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Old 11-15-10, 04:21 PM
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Originally Posted by ecman51` View Post
Was this ever addressed?: What is your OFF set to, in degrees Fahrenheit, on the fan-limit switch?
I did ask him about the tempture on the fan limit however I will tempory rule this out for a moment due he did pull the red et black conductors off from fan limit switch and still running so I rule that out so the fan relay is stuck { look at post #22 }

Merci.
Marc
 
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Old 11-16-10, 08:18 AM
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Yeah guys, I think the fan switch has been ruled out for now. However, the fan relay does not appear to be stuck, as the normally open contacts measure open. So, back to my previous post. What to do next?

AND will it cause any problems other than disabling cooling-mode fan operation if I just remove the fan relay?
 
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Old 11-16-10, 09:53 AM
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Quick question; by unhooking the red wires to the fan limit switch aren't you opening the circuit? Many furnaces will keep the blower running if this circuit is open as a safety measure (open= a high temp condition). Could you jump the red wires and see if the blower stops (closed= cool condition)?

Not sure if this applies to your type of switch, PLEASE check a wiring diagram of the switch to see if a closed circuit at these terminals causes the blower to stop BEFORE jumping the red wires together

My Fan & Limit switch looks like this one [IMG] http://hvacmegastore.com/shared/view...1NprME8U1L.jpg [/IMG]
 
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Old 11-16-10, 03:02 PM
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DIY56 might have nailed it. Because - one might wonder if the red wire makes what appears to be a direct path to the blower, from the fan-limit switch (see Houston's red highlighted post), then by that reasoning, how by disconnecting it at the fan-limit switch, could the blower still be running ? - which it is Answer: Through a different speed wire - not the red wire. Via the alternate path going through the fan relay. If this is true, we can't rule out the fan limit switch being stuck where the OFF is not working, or set wrong.
 
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Old 11-16-10, 10:12 PM
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I think I'm closing in on this. Seems to be the fan relay, probably.

First, I'm still sure it can't be the fan-limit switch. (At least not solely that, and not the primary issue at the moment, although it could be bad too by now, since I've been messing with it so much.) For one, we have the schematic, and the fan-limit switch BFC (blower fan control) is clearly marked as "normally open, closes on temp rise." That means it starts off as an open circuit. Once there is enough heat to turn the fan on and blow warm air into the house, THEN it closes to provide voltage to the fan and allow it to turn on. So, all pulling the red wire does is force this to be open, just in case it had gotten "stuck" closed.

Now for the fan relay (HCR). I did some investigating with the multimeter. I'm a little confused, because when I pull the fan relay out, the fan definitely stops. However, measuring across the contacts on the relay, it certainly doesn't appear to be "stuck" shorted. Furthermore, measuring the points on the fan center where the relay coil contacts plug in, there is no voltage there trying to activate the coil. Still, when I plug the relay back in, the fan turns back on. It's strange. All I can conclude is that the relay is bad enough in some way that it is unable to stay open when there is line voltage on the input terminal. Furthermore, I did measure the voltage going into and out of the relay. It is 110V going in, and 75V going out. Dropping 35V across the relay explains why its contacts are so warm when I first pull it out!

So, unless someone has a better idea, I think the plan will be to replace the fan relay and go from there. Any thoughts?
 
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