Furnace Blower Won't Turn Completely Off?

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  #41  
Old 11-17-10, 07:11 AM
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When you remvoed the red wire from BFC, did you remove just one or did you remove both of them?
 
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  #42  
Old 11-17-10, 08:27 AM
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I've done it both ways. But I think we're really over-thinking the BFC...it's really just a switch. A circuit that contains an open switch is the exact same as a circuit that contains no wire to either or both ends of the switch.
 
  #43  
Old 11-17-10, 10:15 AM
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Talking F/ L Controller

ecman51 & Handy To Be,
A lot of what I'm reading on this thread had me confused so I pulled out my fan/limit (blower) controller (equivalent to Camstat-Henry F560) and tested with my Voltage-Ohm Meter. It has 6 wire connections, White,Black, Red, Red, Yellow & Brown. I have a 1987 Heil hi efficiency 125,000btu furnace (open burner design).

(BTW-I also have a newer Tempstar furnace with an enclosed burner, it has a completely different fan-limit sensor and the fan-delay function is handled by the circuit board not by the sensor. The description that follows only applies to the older (Heil) open burner design.)

White & Black are connected to circuit board in burner (upper) cabinet.
One Red is connected to the Flame Roll-out sensor, the other Red goes into the Blower (lower) cabinet.
It looks like the Yellow & Brown go into the Blower Cabinet, probably to the Blower relay?

After testing the Fan/limit Controller with the VOM I found;

Internally there are 2 independent switched circuits.

Red wires are connected to Hi-limit side of the control (circuit normally closed below 170 deg F), non adjustable bi-metallic strip opens the switch above a pre-determined temp, 170 deg.

Yellow and Brown wires are connected to the fan-delay side of the control (adjustable from 95 deg F to 120 deg F on mine, this switch has a normally open circuit below the temp setting selected on the front of the control). This control will keep the fan running after burner shutoff until the air temp in the heat exchanger drops below the setting on the front of the controller. (This switch closes the circuit when the heat exchanger temp is above the setting on the front of the controller.)

Black & White wires appear to be 24v & ground

How Red circuit effects the blower;
-if a red wire is disconnected (red circuit open) before start-up the furnace will not start.
-if a red wire is disconnected after the furnace is running the blower will not shut off even after the burner shuts down and the heat exchanger is cool.

Conclusion;
The red circuit needs to be closed at start-up (thermostat calling for heat) as a safety precaution before induction fan will start & burner will light etc. (closed circuit=heat exchanger below 170 deg)

If the red circuit is open when the induction fan & burner shuts down (thermostat NOT calling for heat) the Blower will not shut off (Red circuit open= heat exchanger above 170 deg) until the circuit closes.

But the red circuit can be operating normally and the Blower will not shut down if the fan-delay circuit (yellow-brown) isn't working properly.
If the fan-delay circuit (yellow-brown) doesn't open when the heat exchanger temp drops below the temp set on the front of the controller the blower will continue to run indefinitely (brown-yellow circuit open= heat exchanger temp above setting on control). The fan-delay keeps the blower running after the induction fan & burner shuts down (thermostat NOT calling for heat) to transfer the latent heat in the heat exchanger and cool it down before the next furnace cycle.

It is the fan-delay (yellow-brown) part of my controller that is malfunctioning and causing the blower to stay on indefinitely so I am replacing the control (Camstat-Henry F560), cost w/shipping is $91.00.

I'm not sure if this is the cause of your problem, if not maybe this info will help you eliminate the fan/limit (blower) controller as one of the possibilities.
 
  #44  
Old 11-17-10, 02:46 PM
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Originally Posted by Handy To Be View Post
I've done it both ways. But I think we're really over-thinking the BFC...it's really just a switch. A circuit that contains an open switch is the exact same as a circuit that contains no wire to either or both ends of the switch.
Not "overthinking" when I did not know you disconnected both wires at once. Because we know that when it say trips out on high limit, that a circuit "opens"(it even says so, to the right of the BFC) yet sends power on it's way. The details as to how exactly is not shown within the BFC diagram. When you disconnected both red wires, did the blower shut off then?
 
  #45  
Old 11-17-10, 07:01 PM
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Originally Posted by ecman51` View Post
Not "overthinking" when I did not know you disconnected both wires at once. Because we know that when it say trips out on high limit, that a circuit "opens"(it even says so, to the right of the BFC) yet sends power on it's way. The details as to how exactly is not shown within the BFC diagram. When you disconnected both red wires, did the blower shut off then?
OK, so I don't really want to start a debate here, but I do want to make sure the information is correct for later visitors who happen upon this thread. I haven't dealt much with furnaces before, but I am an electrical engineer, and once I got pointed in the right direction by all you helpful folks here and got a chance to digest the schematic for a bit and figure out what actual parts all the different abbreviations are referring to, it became pretty clear how it works.

First, the one you're talking about that "trips out on high limit" or "opens on temp rise" is the limit control, or LC, as labeled right there on the schematic next to the rest of the words you quoted. This is also just a switch, but it happens to be normally closed. It has black wires going in and out of it.

The one we've been talking about, with the red wires, is the BFC, and it is normally open, as labeled right there on the schematic. Normally open means it is a switch that is open to begin with, then switches closed when the conditions are met (the temperature rising above the set point, in this case). There is no "sending power on its way" if the switch is open or a wire on either end is unhooked. It's not a device that sends power on its own--it's a switch, which means it breaks (opens) the circuit when open and simply connects the two ends together when it is closed. If you unhook a wire on either side, separating the switch from the rest of the circuit, it is powerless to do anything about it, regardless of whether it is open or shut.

To clarify, hopefully, let's look at the schematic in the picture where Houston kindly highlighted things with colored lines. Starting just to the right of his red lines, you can see there where it says the 115V Power Supply comes in. Let's start with the H there, which uses a black wire. From there, it comes down through the main disconnect (also a switch, this one manually operated), through a fuse, through the PBS (another switch, which is shut, thereby allowing the current to pass through, when the blower door is shut), and then to a wire nut that distributes this 115V to several places. For one, it goes to the transformer in the fan center, which steps it down to 24V on the other side.

But, let's follow the Yellow wire up from that wire nut, bringing 115V to the common contact on the fan relay (also in the fan center). Notice that there are normally open and normally closed contacts here. That means, under normal conditions (meaning the fan relay coil is inactivated), 115V is brought through the NC contact, up to the red wire, where it goes clear up to the bottom terminal on the BFC (the fan switch on the fan-limit switch we have been discussing so much), which is normally open. IF this switch is closed, which it shouldn't be unless the temperature has risen above the low set point, then 115V is brought through the switch, to the red wire on the top terminal of the BFC, then follows the red wire down and around to the blower heating lead.

If, and only if, 115V gets to that point, then the blower motor runs at heating speed. Breaking that path anywhere before that point, whether by having an open BFC, pulling a red wire, tripping the HCR relay, or any other means of breaking that path anywhere, and the motor cannot run at heating speed.

We could talk more about how the rest of the HCR/Fan Center setup works if anybody wants, but that should pretty thoroughly explain the BFC portion, and how we can be sure that the BFC isn't causing anything to run if we unhook even one red wire from the fan-limit switch. It's just not possible. And on this furnace there is no feedback that goes to any micro-controller or circuit board that is able to auto-detect some wiring problem with the BFC and trip the cooling fan relay as a backup. It's pretty simple and low-tech, really.

Hopefully that makes it more clear?
 
  #46  
Old 11-17-10, 07:06 PM
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You can unwire (and isolate) the black high speed wire connected to the normally open terminal in the fan center. Restore power and see if the problem persists.

I would verify that the TDC is not used by this furnace while the fan center is open. It does not bypass the fan relay as I thought earlier and would turn off with the fan relay removed.
 
  #47  
Old 11-17-10, 07:10 PM
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Houston, we have a problem!

I suppose you've heard that one before, though.
 
  #48  
Old 11-17-10, 07:32 PM
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Seems like the test results are pointing to a faulty TDC.

I see two red wires going up the left side of the furnace picture. These two wires may connect to this control.
 

Last edited by Houston204; 11-17-10 at 07:52 PM.
  #49  
Old 11-17-10, 10:18 PM
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Yep, the TDC

Good call, Houston. I finally was able to spend some time digging in and tracing wires. What do you know, there is a TDC installed! For some reason, I didn't expect it to be clear up in the blower compartment bolted to the blower frame. The TDC was stuck shorted. I unhooked one leg of it, effectively taking it out of the circuit, and now everything operates as expected.

Which leads me to a couple of questions:

1. Do I really need a TDC? I mean, would it hurt anything if I run without it in the meantime before I get a new one? It appears to be truly optional...a 2nd way to turn the fan on if the BFC in the fan-limit switch isn't working.

2. What are the ideal settings for the fan on and off points on the fan-limit switch? Now that the fan isn't constantly running anymore, some heat finally builds up and I can finally watch it operate. I was surprised by how fast it moves--the temp zooms up, then the fan turns on, then the temp zooms back and the fan turns off. The result is a lot of cycling the blower on and off at like 30s intervals. Is that typical?

Thanks!
 
  #50  
Old 11-18-10, 04:24 AM
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Re; Ideal fan settings

[QUOTE=2. What are the ideal settings for the fan on and off points on the fan-limit switch? Now that the fan isn't constantly running anymore, some heat finally builds up and I can finally watch it operate. I was surprised by how fast it moves--the temp zooms up, then the fan turns on, then the temp zooms back and the fan turns off. The result is a lot of cycling the blower on and off at like 30s intervals. Is that typical?[/QUOTE]

Handy To Be, looks like you have a Honeywell F/L controller, here's a nifty explanation about how it works and the best settings to use; Honeywell L4064B Combination Fan and Limit Control: How to Set the Temperatures and Limits on the Furnace Fan Limit Switch Control

Curious, you refer to a TDC, is that the same as the Fan-Delay circuit in my Camstat-Henry F/L controller, or something different?
 
  #51  
Old 11-18-10, 04:36 AM
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  #52  
Old 11-18-10, 03:12 PM
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Fan Relay

Sounds like a plan to me. The symptoms are certainly there to justify replacement. Those relays are cheap & easy to replace.
 
  #53  
Old 11-18-10, 05:06 PM
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Handy,

Regarding your long post #45, you are right. It is the black wire between the limit portion of the switch, and ignition module, that shuts down - which then would shut down the gas valve, yet keep the blower running. Sorry I had to put you through unnecesry thinking and typing.

Regarding the TDC, and if your furnace even had one (which now we know it does) - I guess the fooler was because they used the words downflow and optional, to the right of it. But upon better analysis, I guess that would imply it IS on your furnace, but MAY not be in some downflow models.
 
  #54  
Old 11-18-10, 08:50 PM
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Ah, ok...but the gas valve was never a problem. It would come on and off normally when heat was called for, as expected.
 
  #55  
Old 11-19-10, 12:32 PM
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Originally Posted by ecman51` View Post
Regarding the TDC, and if your furnace even had one (which now we know it does) - I guess the fooler was because they used the words downflow and optional, to the right of it. But upon better analysis, I guess that would imply it IS on your furnace, but MAY not be in some downflow models.
Yeah, I've been trying to decide if that's what it actually means. In which case, would it be dangerous to run without the TDC until I am able to get it replaced?

I don't quite understand the point of the TDC, unless it's just a backup for turning on the fan. But then why would it make any difference whether you had a downflow model or not?
 
  #56  
Old 11-19-10, 03:12 PM
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Originally Posted by Handy To Be View Post
Ah, ok...but the gas valve was never a problem. It would come on and off normally when heat was called for, as expected.
Which then would mean that it wasn't limiting. IOW, the black wire circuit was never openng.

Have you priced out a new fan relay yet? Relays do commonly get stuck either in off position, or on position.
 
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