Bryant FA fan running, tstat unhooked, no heat


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Old 10-15-09, 05:29 PM
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Bryant FA fan running, tstat unhooked, no heat

HI, I am trying to diagnose a Bryant Forced Air Gas Furnace for my parents. Here is what I know

Fan runs constantly

no heat

pilot is lit

thermostat not the problem

unhooked t-stat, fan continued to run, and not talking for just a couple minutes like they tend to do. It ran for a long time and only finally stopped when I shut off the switch at the furnace.

Jumped red and white at t-stat, no voltage, no spark, nothing.
I am tempted to jump them at the board, but look further down, there are two sets of red white on the control board. I guess it doesn't matter what colors as long as I jump W RH I am good.

I traced the tstat lines to the lower panel of the unit. I am no professional, but I was confused when I saw two control lines. One was a red white, and the other is the 4 lines. Now the bundle with 4 lines is the one that goes to the t-stat, and that has a red white in it as well. Why another bundle with just red white? and I don't know where that was going to.

We have never had a problem in 23 years. never changed anything, just went out today.

Thank you we are cold.

PS. I am not getting any other sequence of events, no clicking etc. However I imagine since the furnace is not geting the 24 volt signal to start up, it is just sitting around waiting. I am however wondering why the blower is constantly on, even without any wires connected to the tstat. weird wouldn't you say?
 
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Old 10-15-09, 05:42 PM
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What is the model and serial number of the furnace? This would be on a sticker in the burner compartment.

Using a jumper wire between Rh and W would be worth a try.
 
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Old 10-15-09, 05:53 PM
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It looks like the model is 395baw048095aaba I couldn't get much of a view of the serial without moving a bunch of junk around the basement to get a better view.
 
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Old 10-15-09, 06:28 PM
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Which fan is running? Quick. I have to get off the computer soon.

If it is the blower that blows normally hot air out registers, then you have a few things to do right off the bat.

One is to make sure that the blower circuit is not stuck on high limit and that there is a reset button to push. Look and see if you can find anything like that on your furnace parts inside the big door. This would be on a small device that has at least 2 electrical wires connected to it. That alone, if you find one of these that clicks in when you push it, may solve the immediate issue at had, although it may crop up again until high limiting problem is solved.

The other thing to do is to make sure that you have not lost 24 volts, and test for that in the furnace.

If you have 24 volts, you could have a relay that changes the 24 volt signal to 120 volts for the inducer fan to run, that is stuck. But the fact your blower is stuck on leads me to believe that rather than pursue this angle at this time, that you either have that limit issue or some relay is stuck on your control board. As a last ditch effort if that starts to be suspected, you could perhaps try lightly rapping with, non-conductive tool, on the relay looking devices on the control board to see if that jars the blower to quit and the inducer fan to start in. If you have an older furnace, they sometimes mounted this relay separately, sometimes near where the power enters the furnace. If you have such a relay that looks to be clear plastic, or brown or black bakelite, with say 6 wires coming out of it, try rapping on that.

Another possiblity you really need to check right away is if your inducer motor is froze up or burned up. If it were froze, you should hear it hum. No hum, the windings may be burned. You can test to see if 120 volts is trying to get to the inducer during a call for heat.
 

Last edited by ecman51; 10-15-09 at 06:57 PM.
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Old 10-15-09, 06:40 PM
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it seems to me that the main fan, it gets pretty loud, and also definitely blows some air all the way to the second story, from the basement. ima post this then edit it with some updates, here, to be quick!!

UPDATES:

Tested both limits, continuity but no voltage from either posts to ground.

Roll out limit was not tripped.

W to R at the board, no voltage.

However, I did my voltage testing immediately after pressing in the safety shutoff where the panel is removed. Didn't give it time, but motor was blowing. I would think at least I should have voltage at the W to R at the board.
 
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Old 10-16-09, 07:00 AM
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mjj, is aware of this from a quick PM last night.

But for everyone's benefit - also, if you have a 3-wire pressure switch, the furnace will not start any inducer or ignition sequence, either, if that pressure switch is stuck to where part of it is open at start up. There are 3 terminals NC, NO and C. If the NC -C connection is open, it will not start -no inducer motor, no nothing. I think why they made this change from the 2-wire pressure switch is that in theory, if a 2-wire pressure switch go stuck closed, the furnace would always run, even if there was a safety probelm with the venting! But, with the 3-wire, after the call for heat ends and the furnace has to start over again in it's cycle at the next call for heat, the pressure switch plunger must freely be able to move back the other direction to close tthe other set of contacts. This then proves that the plunger is not stuck somewhere, and can do it's job.

...................................................................

Back to questions raised in the last post. If continuity exists through the 24 volt safety circuitry, that means all these things are closed, which means they are not tripped = good.

R to W testing: It helps to understand that 24 volt current is sent out the R terminal, up the R wire to the stat, back down the W wire out of the stat (when stat calls for heat, and enters the W terminal. The 24 VAC power, does not come out of the W terminal, all by itself, say, without the stat working. That teminal is being supplied the power by the stat. Then once W voltage is supplied, that power then goes from the W terminal to a board or relay off the board, that tells that 24 VAC power to close contacts (in a relay, that may have a coil that delays the closing by a number of seconds) so that a 120 volt circuit can close, to energize the inducer fan. The ignition circuit will not proceed until these things happen, on most furnaces. I say most because only here on this board, I have read some seemingly bizzare things were function is not always in this seequence.

Do you have a 2 wire or 3-wire pressure switch?
 
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Old 10-16-09, 08:48 AM
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I will have to check to see what type of switch I have. Let me get back on this in a bit. I do want to say that I appreciate all the time you have taken to assist me here, it is greatly appreciated.
 
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Old 10-16-09, 09:00 AM
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It is a 3 wire. I tested NC to C continuity and NO to C continuity. Is it odd to have continuity from NO to C? So I unplugged NO and nothing changed. I tested with system off and when it was on.

Obviously that isn't right to have continuity at NO to C correct?
What does this mean?

Also, I assume the inducer motor is the one by the pressure switch, that has a caged fan visible from the access area, this is at the top of the furnace, where the main blower is at the bottom?
 

Last edited by mjjstang; 10-16-09 at 09:49 AM.
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Old 10-16-09, 11:17 AM
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Here's a reaqson the fan can run on this furnace ---- a draft limit switch that has tripped.

Look on the right side of the vent pipe at the inside top of the furnace. Look for a switch usually with a red dot on it.

Push the switch.
 
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Old 10-16-09, 11:37 AM
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Originally Posted by mjjstang
It is a 3 wire. I tested NC to C continuity and NO to C continuity. Is it odd to have continuity from NO to C? So I unplugged NO and nothing changed. I tested with system off and when it was on.

Obviously that isn't right to have continuity at NO to C correct?
What does this mean?

Also, I assume the inducer motor is the one by the pressure switch, that has a caged fan visible from the access area, this is at the top of the furnace, where the main blower is at the bottom?

No, something is wrong. Did you do the test with the wires still on? - or taken off? Take them off, then try testing. I can't even comprehend how you could have continuity to both at once, if the wires are off. That be like having a green light show up at an intersection fom both perpendicular directions at the same time.

Some furnaces have a fanblade type inducer(usually so that the housing sits the way a pancake would) and others a cage-wheel type(usually vertical). Either way, that is it.
 
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Old 10-16-09, 09:52 PM
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As I stated earlier, the limit switches have been tested and are fully functioning. I will retest the pressure switch, I can not remember if I tested them with wires installed or not. So just to be sure, on the pressure switch, when I test NO and NC, am I removing wires, and testing the switch itself, or the lead wires, and I am testing these to C or to ground?
 
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Old 10-17-09, 11:50 AM
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Wink

You make 2 tests with the furnace turned off and the 3 wires off the pressure switch. Touch the male spades on the switch. Test N0-C, then for the next test..... NC-C. When furnace is off you should have full continuity from NC-C, and NO-C should be open (no register of continuity at all).

When the furnace runs, then what technically happens is the contacts at NC, which were closed, then open, after the ventor comes on--------- and the NO contacts, that were open, then closes under ventor vacuum, so the ignition can proceed.
 
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Old 10-17-09, 01:41 PM
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Thank you, just what I needed, I know I sound like a complete tool, but I have never touched a furnace before, and after this saga, I don't think I will ever have to call the furnace man again, assuming I DO eventually fix this.

Talked to my buddy at work who did this stuff his whole life, He is basically on the same page as you guys, but he thinks there are roll-out limits around the burner, I am not at my parents now to check, but I remember looking, and didn't see any of these, the only safety type switches I saw were the one up in the corner by the vent that had the red button, and the one directly in front of me in the middle of the access that secures into the guts of the furnace. These both tested good. I don't think I am missing any am I?

Will have to take a short break from this as I am hundreds of miles away now and won't be back for a few days. I really do appreciate the help, this has not only narrowed down my search, but also given me a crash course in general furnace workings and repair. Without a doubt one of the most informative threads I have entered into. Beer 4U2
 
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Old 10-19-09, 10:23 PM
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Ok, I have done a lot of testing, and familiarized myself with the wiring schematic on the panel. This is what I have found.

Where 24 volt comes from the transformer into the limit switch, I am losing voltage on the inline fuse by the limit switch itself. This of course seems like a no-brainer. However, before I try anything I wanted to check with you guys.

The schematic says this is a fusible link, and 1 is required on up-flow, 3 required on down/horizontal. (I have one), but it doesn't look like any fuse I have ever seen. It reminds me of a diode. That is why I didn't want to bypass it just for testing, because I am scared when it comes to electronics and diodes etc. I cant imagine it would be for one, what reason? for two, in plain English, the schematic tells me it is a fuse.

The reason I ask is because it is soldered in between the terminal, and the wire. Why would a fuse, which obviously has the potential to blow, be soldered on to the wire? Why not inline replaceable, or board replaceable? Also the 'potential' fuse is round barrel, completely enclosed, and tapers to one end and the taper is marked red. Can anybody relate to me here and verify it is a fuse? Thank you.

I am aware that potentially this means there is a larger problem, assuming it is the fuse. If it is a fuse, I will replace it and if problem persists, start diagnosing further.

EDIT:

This is it, and I am even more inclined to just go and bypass it. For testing purposes only, of course! I just figured if anyone can verify this for me then I will feel better working on my parents home. I don't want to disappoint them and lose all trust.... actually, then the to-do lists would probably become shorter. HMMM....

Edit again: Geez these fuses are expensive. 15-25 bux so far during my search. Anybody know where to get these cheap. I don't know if there are national chains that would have them, but couldn't I just go get an inline fuse holder and use a buss fuse of proper rating. I assume there is a temperature issue that may limit the type of fuse used?
 
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Old 10-19-09, 10:49 PM
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Ok, it was the thermal fuse. I am an idiot, but again, this journey has taught me a lot, so well worth it. Next comes stage 2. A thermal fuse melting means I need to figure out why it got too hot. What might cause this? I am sure it probably has something to do with being old, but also it had worked for a little while this year. Say there was blockage in the chimney or vents. This could cause it after a few days or so to finally build up in temperature, right?

What are some reasons for this, and also like I was asking, can I use another type of fuse that will work. I want to be safe so I probably should get a new thermal fuse, for the long term. Didn't know if anyone had any tricks of the trade regarding this.

Thanks again for the help guys.
 
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Old 10-19-09, 11:53 PM
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I jumped it temporarily, and it is definitely trying to fire up. I have the red white tied together to test so it is always calling for heat immediately when I turn the safety switch on. However......

I think My inducer is seized. Right when I flip the switch, I get a good humming noise. so I turn it off right away, but sometimes when I turn it back on and off, when I turn it on, the blower fires up as well. I spun the little fan with the cage around it, it spins but it isn't real easy, but it is smooth. I don't know if this is actually spinning the motor, or maybe that visible fan is just spinning on the shaft, and the main shaft is really not spinning. I guess I am going to take it apart, but just wanted to make sure that this is making sense, and the inducer should come on right away from what I understand.
 
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Old 10-20-09, 12:14 AM
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Removed inducer. Spins freely, but I plugged it back in to watch it try to spin, and sometimes it will spin like half a rev but then freeze up, still pretty confident it is the motor itself, but its strange that I can spin it by hand. It won't spin on its own though if I give it a good whirl, it'll just stop, but I know some motors do that anyway. Its old must be the inducer. So I guess its just a waiting game. Hopefully somebody has these parts cheap.

Just noticed the fan is stamped 3/86. Guess they got a pretty good life out of it.
 
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Old 10-20-09, 05:38 AM
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From your description, the inducer motor is bad and should be replaced.

When the inducer motor quits, the draft through the furnace fails and the flames come out the front of the furnace and cause the thermal fuse to open. Very likely that's what has happened.


That also suggests that the pressure switch is failing to operate properly. The pressure switch should detect that inducer motor failure or slowdown and shut off the burner before the flames back out.

Replacing the inducer motor assembly, thermal fuse and pressure switch are recommended (by me).

Don't try to replace just the motor. The fan seizes up on the shaft and usually can't be removed.
 
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Old 10-20-09, 07:46 AM
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Thanks, this makes sense. I want to do this cheap as possible, so I was thinking about replacing just the inducer and thermal fuse, and seeing what happens. Pressure switch tested fine, I tested it and it switched after it powered up.

When you say don't replace just motor, you mean replace motor and fan, but the inside wheel can be the original? or you think I should replace motor, fan, and wheel?
 
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Old 10-20-09, 08:05 AM
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But.....it is the inducer fan that switches the pressure switch. If the pressure switch switches properly(when furnace runs), and stays switched, it can't be the inducer fan.

Careful with shotgun fixing the furnace. Can be very upsetting.
 
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Old 10-20-09, 09:10 AM
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Originally Posted by ecman51`
But.....it is the inducer fan that switches the pressure switch. If the pressure switch switches properly(when furnace runs), and stays switched, it can't be the inducer fan.

Careful with shotgun fixing the furnace. Can be very upsetting.

I'm supposing that the pressure switch is stuck in the made position. I've seen this exact sequence happen several times on Carrier BAW type furnaces.

So the pressure switch is effectively jumpered out and eliminated because of the stuck switch.

Of course, that can and should be tested to verify my guess.

But if the pressure switch is working properly, it will shut off the burner when the inducer motor starts slow down and prevent the flames from backing out of the furnace and shutting off the thermal fuse.
 
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Old 10-20-09, 09:15 AM
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Originally Posted by mjjstang
When you say don't replace just motor, you mean replace motor and fan, but the inside wheel can be the original? or you think I should replace motor, fan, and wheel?


In theory you can buy the inducer motor only, remove the old fan from the old motor and put it on the new motor.

In practice that usually doesn't work. Very likely the old fan is corroded anyway.

So do yourself a favor and shop for the fan assembly that includes a new fan. Then it's a simply screw on job to install.
 
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Old 10-20-09, 10:43 AM
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Thanks guys, yah I decided to just purchase the inducer and fuse, but opted for the new fan as well because it was fairly cheap, and didn't want to hassle with the corrosion. Recip sawed the shaft off and removed everything. Simple bolt on and assembly just like you said, and for 186 bux in parts, I was done.

Could have spent about 100 on ebay but guess you pay for convenience.


Guy told me fuse was not needed, but being my parents place and for 12 bux I think it was the best idea. Plus, I am thinking without that, the inducer would have tried to run till it burnt out like it tried to do when I bypassed the fuse earlier. Everything is in and working great. Thanks for the help guys!
 
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Old 10-20-09, 11:23 AM
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Originally Posted by mjjstang
Pressure switch tested fine, I tested it and it switched after it powered up.
I based my repsonse on the above quote.

Been answering too many posts too quickly I guess. But now after reading the new inducer is allowing the furnace to run properly I then had to go and read all the relevant posts earlier, and now I can see that had to have been the case.
 
 

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