Ducane 80% Furnace - 3 blink woes - Help please! - Video


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Old 03-03-21, 01:24 PM
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Ducane 80% Furnace - 3 blink woes - Help please! - Video

Hi everyone, first post here. Read countless threads regarding my furnace with a three blinking issue.

It is a Ducane 80% efficient furnace. Installed ~6 years ago, the tech never registered it so I do not have a warranty.

Currently it has an issue where the inducer motor will start and most of the time (70%) I'll get a three blinking LED. This indicates that there is a problem with the pressure switch. Here we go again!

What's been done:

- 3 months ago the main board was replaced because it was failing to start (killing the inducer motor after a few seconds, killing gas after one minute, not starting at all, etc and throwing a three LED blink), three LED flashes but the problem behaved differently. It often would flash nearly instantly the three flashes. It worked flawlessly for these three months.
a. This is was done after a tech came out and fully looked it over and said for sure it is the main board.
b. Verified there is no blockage in exhaust
c. Removed inducer motor, lubricated/cleaned and new gasket was made.
d. Verified Igniter has ~58ohm, all temp safety switches pass continuity test.
e. Thermostat is bypassed during troubleshooting (R and W bridged)
f. Thermostat is a Nest. It does have C wire.
g. Replacing or bypassing the pressure switch does not solve the issue

- Three months later the problem has returned but behaves a bit differently.
a. Installed a new main board and the issue seems to be about the same. With this new board I can get it to work sometimes, about 40% of the time. Once the igniter comes on it will work for as long as needed.

Video of the problem in action: https://vimeo.com/519231172
Video of description of what's been done/checked: https://vimeo.com/519228011
Video of it beginning to work, notice igniter glows orange (blooper): https://vimeo.com/519232555

Can someone please suggest what I can do next! I'm pulling my hair out, a tech coming out is just going to charge an arm and a leg. The previous tech that wanted to replace the board wanted over $1000 for the job! The part is about $100.








 

Last edited by PioneerSX1250; 03-03-21 at 03:05 PM.
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Old 03-03-21, 06:08 PM
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Welcome to the forums.

You have a warranty..... just not an extended one. Typically the extended warranty only covers the heat exchanger. It rarely covers electrical parts and doesn't usually extend labor warranty.

I have changed one pressure switch in over twenty years. They are 99% reliable.
The first thing a competent technician should do is to connect a manometer into the pressure switch line. It should be known immediately how much vacuum is present.

b. Verified there is no blockage in exhaust
How ?? I can't tell you how many times I've heard that here only for that to have been the problem.

On a call for heat..... the inducer starts, pressure switch proves and then the igniter will glow.
The pressure switch must prove before the igniter will heat. It should prove in seconds.

27v output for 24v transformer is fine.
Does your furnace have one or two pressure switches ?

Ok..... here's something strange. I don't work on many Ducane furnaces but I see an issue in the wiring. I took and split your diagrams and highlighted them. Looking at the first one.... You'll see two pressure switches. You'll also see two aux limit switches. I don't know of anyone that puts limits in series with the pressure switches and then just logs a pressure switch problem. You will need to trace that loop. You can determine whether you have one or two limits.

The loop between pin 7 and pin 10 (on 12 pin plug) must be closed to not get a 3 fault and for the igniter to start. You will need to find out what is not closed.


 
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Old 03-03-21, 09:27 PM
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I was hoping to hear from you tonight.

I had a hard time spotting devices in your videos. I did see one pressure switch with a blue and purple wire on it. I don't see a second one. At the end of the second video you were discussing thermostat wiring. It looked like you had your hand on the limit switch you need to check.

You seem to know what you're looking for. Trace out the PS loop.

What I actually do is to use my microfine test leads and stick them right in next to the wires on the plug. This way real time monitoring is easier. If you do that...... a normal loop will measure 0v. An open loop will measure 24vAC. Don't forget.... the loop can only show closed during a call for heat.
 
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Old 03-04-21, 07:02 AM
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Pjmax,

Thank you very much for taking the time to review, analyze, and comment on my issue. Your detailed reply is very appreciated! As a hobby I restore vintage Audio equipment including but not limited to full recaps, diodes, transistors etc. I am constantly examining schematics, helping others troubleshoot their mistakes. I also will post my guides online for others to learn from. I really appreciate your response and do not take it for granted.

Thanks for the welcome!

The warranty comment, this is from the tech that came, said he called and the furnace was never warrantied. I've tried to call Ducane but only get recordings. I've called their parent company (Allied Air), only be forwarded back to the dregs of a recording for warranty support. I'm not sure if the tech is pulling a fast one or not.

I am glad to hear your comment on the pressure switch. I understand my generic comment of there is no blockage in the exhaust is generic and lacking any real substance. Three months ago when I removed the inducer motor I also gave a visual look at the flue to the roof, there was no blockage. I have no data as far as the performance of the suction, simple fact. My only basis is that bypassing the pressure switch does not solve the issue and when the furnace does work, it seems to work until the thermostat is satisfied with the internal temp. I retract my statement and thank you for correcting me.

My furnace has one pressure switch.

You hit the nail on the head. To be honest I did not know until yesterday that this furnace has a simple schematic on it. My fault. Yes, if anything in line of the pressure switch (seems to be a simple 27v loops) fails to complete the circuit it will throw a pressure switch failure. I realized this yesterday as I followed the 27v. It's all in a loop!

Well guess what I found!! The high temp flame sensor, or I belive its called a limit switch (sorry I do not know its name yet as I have not looked it up) is showing signs of arching! Further probing is showing anywhere from around 300mv to 4.5v AC drop! This would make sense why it took a while for the problem to return, as the arching occurred it slowly killed the contact. Maybe it even wore out a component on the control board as the arching could be drawing high current?

I had to use another website to host a new set of videos due to storage limitations. Notice how when I hit the right side terminal the arching starts:

High temp switch arching: https://streamable.com/6r6cnf
Demonstrating voltage drop: https://streamable.com/ifi3gu
Quick Resistance test: https://streamable.com/aeyaoq

I'm actively looking this up now, will update/edit when I find more info. So far in my three times test of bypassing (bridging) this switch the furnace has operated normally. Of course, this will be replaced as bypassing is only acceptable for troubleshooting.
 

Last edited by PioneerSX1250; 03-04-21 at 08:29 AM.
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Old 03-04-21, 08:27 AM
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Update, looked up the part I needed and it is available, model is Lennox 25M60. Still, I wondered if I could solve the problem by cleaning/soldering and creating a healthy contact. Gave it a shot, solder took very well and the numbers are looking great! Furnace is working great too!

Video of measuring resistance/voltage. I realized towards the end there was no voltage drop (problem solved) and I needed to measure reference to ground. https://streamable.com/d0n9cb

Previous voltage drop: ~330mv to 4.5v AC.
Previous resistance across the switch in circuit: ~50 ohms
New voltage drop: 5mv AC
New resistance across the switch in circuit: 1 ohm - (Out of circuit 0 ohm)

I have not tested to verify that the safety mechanism is still performing as designed, of course pushing the disc in will cause it to open the circuit. What are your thoughts?






 

Last edited by PJmax; 03-04-21 at 04:25 PM. Reason: resized pics/removed extras
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Old 03-04-21, 04:17 PM
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Ok.... you've made some progress.
But...we have a problem. That device is a manually re settable limit switch. Based on it's location directly over the firebox..... it is a rollout switch. Rollout switches are typically re settable. If you look at the diagrams.... the rollout switches are not in the pressure switch line. They should stand alone.

That is an extremely important part. I would use what you have now but replace it with a new one.

Did you follow the pressure switch loop all the way through ?
Is that rollout switch in series IN the pressure switch loop.... pins 7 and 10 ?
 
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Old 03-04-21, 04:28 PM
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I removed some of the extra pics and I set your thread to immediate email reply.
You can change that back to weekly at the bottom of the advanced editor reply page.
 
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Old 03-04-21, 06:25 PM
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Hey Pjmax,

You are right, that switch is not in circuit of the pressure switch. It is its own, which appears to have its own error code (5 flashes). I have ordered a new one, no messing around with safety. It really has seemed to solve the issue... Mind boggling. Only thing I can think of is it does use the same 27v as the pressure switch circuit?

I probed every component in circuit of 7 and 10 (pressure switch) for resistance with no AC and found nothing that raised suspicion. I then left the probes in the socket for pins 7 and 10 and recorded the data.

All measurements with a probe across pins 7 and 10 at the main board:

Voltage:

Pressure switch in circuit, inducer on no gas/blower: 6mv AC drop
Pressure switch in circuit, inducer, gas, blower on: 89mv to 103mv AC drop

Resistance:

Pressure switch in circuit, inducer only on: 4 to 5 ohm drop
Pressure switch in circuit, inducer, gas, blower on: 24 to 31 ohm drop

Voltage drop across pressure switch contacts only with inducer, gas and blower ON:

Current original pressure switch (6+ years old): 15 to 18mv AC drop
New Amazon (same rated pressure) pressure switch: 20 to 22mv AC drop

Here is a rear of the PCB. I probed for continuity between the rolloff switch (11 and 5) and the pressure switch (10 and 7), and they are completely isolated.




Not that it is necessary but here is the simple single rollout switch circuit highlighted in its brown wire color:

 

Last edited by PioneerSX1250; 03-04-21 at 07:00 PM.
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Old 03-04-21, 07:26 PM
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I know the rollout circuit is different. I needed you to see that.

It's hard to check for circuit ohms when the circuit is powered and active.
In a perfect circuit the switches and wire would be a dead short. We know that doesn't happen.

Your AC voltages look good. Keep in mind..... you want them as close to 0v as possible.
I don't know how much voltage leeway that board will allow. It may trip when it sees 500mv between terminals.

I probed every component in circuit of 7 and 10 (pressure switch)
What did you find. Pressure switch and what else ?
 
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Old 03-05-21, 10:38 AM
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I know the rollout circuit is different. I needed you to see that.
Understood, yes all logic points to my finding not being applicable to the error. Because I did not know the name I did not know specifically where that part was on the simple schematic however I knew just by tracing it, it probably was on its own circuit.

It's hard to check for circuit ohms when the circuit is powered and active.
In a perfect circuit the switches and wire would be a dead short. We know that doesn't happen.
Right, that is well put. Especially with aging components carrying (low) AC and exposed to heating/cooling 8 months a year. We also do not know what the tolerance is for voltage drop before the board pulls an error.

Your AC voltages look good. Keep in mind..... you want them as close to 0v as possible.
I don't know how much voltage leeway that board will allow. It may trip when it sees 500mv between terminals.
That is great to hear. 30mv in the audio world can be a simple raise/lowering of a bias, it can also be the reason for oscillation/hiss/sign of leakage etc. I'm hopefull that this circuit is more forgiving.

I probed every component in circuit of 7 and 10 (pressure switch)
What did you find. Pressure switch and what else ?
All switches have little to no resistance (less than .1 ohm). Most of the resistance is in the pressure switch. I did short the pressure switch (power off) and had 0.1ohm resistance for the whole circuit. I found one wire going from the main board to a simple two wire molex. At the junction one wire went in one end of the circuit loop, the other at the other end (hope that makes sense).

Starting from the mainboard it went to a safety switch on the blower motor (very low resistance), all the way down to the pressure switch. From there it went to another safety switch that was installed in the cavity for the flames/exhaust fumes. And then it met with the last wire of the loop from the main board.

I should however check pins 1 and 7. This is the main limit switch (on the blower). I did check the switch itself, but not probing at the actual mainboard connector. Worth a shot, although I do not suspect it. Looks like more of the same, pass along the 27v in a loop = happy main board.

To answer your question more short, I found that I need to read the dang schematic As you can imagine, a lot of this journey was focused on the pressure switch as will as the inducer motor itself, which I have disassembled/cleaned/lubricated.

Thank you again for all your help. I can tell by the way you ask questions that you have been doing this a while. You've probably seen a thing or two, especially DIY homeowners (some who should call a pro) make problems worse (or better!).

Let me know your thoughts and if I should continue probing. It's been working without a single hiccup ever since that switch has been serviced.
 
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Old 03-06-21, 09:34 AM
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You're doing ok. Now if you get the pressure switch code again... you'll know what to check.
 
 

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