Furnace not working this morning

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  #1  
Old 11-27-20, 08:21 AM
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Furnace not working this morning

Good Morning,

Woke up this morning and the house was cold to notice the schedule was on for the thermostats. Went down to the furnace and I could hear the furnace fan running and could see on the zone control a call for heat from certain zones. I could not see the flames however.

I have run through some what i believe to basic checks. I checked the fuse on the board and that looks good. I checked to see if i could hear the inducer fire and I believe I can. I never see the ignitor glow. I don't know much about the multimeter but did try and disconnect the ignitor and poke the ends in to check for voltage when a call for heat but I don't see any.

I have attached an image of the ignitor and I think it looks ok or not?

Also i checked to see if any voltage being sent to the ignitor and you will see that below. The image however is the voltage where only the fan on. When the call for heat comes in it will quick spike to about 1.1 then drop to between .6 and .7.

Any ideas why this wouldn't be coming on?

Thanks for the help in advance.



 
  #2  
Old 11-27-20, 08:42 AM
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Model number of the furnace?
What error was the furnace displaying?
No voltage will be applied to the ignitor when it’s sitting idle. And looks like you also had the door switch open when testing.
Does it call for heat and attempt ignition when you put it back together?
 
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Old 11-27-20, 08:57 AM
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Thanks for the reply

Model TRANE TUC100C960B7
I don't see an error. Where might i see that?
Yes same issue when I have the door on. I had taped the little door sensor switch to trick it into believe the door was on just to examine what might be going on.

Does that help?
 
  #4  
Old 11-27-20, 09:02 AM
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Codes displayed on the board. Code schematic should be on the door.
Is it trying to light? What status is the board showing now?
 
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Old 11-27-20, 09:18 AM
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oh i see. I see a red light and it flashes 4 times then pause. On the back of the cover it says something different than what is on the main board.
On the board 4 flashes then a pause says "Thermal Protection Device Open"
On the back of the cover it says "Open High Limit Device"
 
  #6  
Old 11-27-20, 09:34 AM
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Sounds like your limit is open then. Test it and see if it’s manual or automatic reset.
You’ll also have to see why it tripped.
 
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Old 11-27-20, 09:48 AM
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Sorry I am not sure what that is. What is the limit and how do i check it?

Edit:
Based on what I think i found it might be the blower to move air to the ducts. If that is right that part is working as i can feel air come out of the ducts in addition, feel the fan kick on with that door open.
 
  #8  
Old 11-27-20, 10:38 AM
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No, the limit isn’t your blower. It’s a temperature switch in the airstream.
It should be mounted in the burner compartment.
Test it live with your meter. Should see 0 V across the switch when closed.
 
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Old 11-27-20, 11:08 AM
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Is the below it? Is there a visual way of telling if not working right or it needs to be replaced?

Can you tell me a bit more how i would test it with a multimeter? red and black to each wire?


 
  #10  
Old 11-27-20, 11:26 AM
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That should be it. Test across the switch with your leads with live power. Or turn power off and ring the switch out with polarity/resistance.
You’ll have to figure out why it tripped. Take heat rise across the furnace when it’s running again. May be over fired. Are you using a very restrictive air filter?
 
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Old 11-27-20, 11:58 AM
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Hoping this helps a bit. Here is the reading and my settings without power doing what i believe to be a continuity test. If i touch the black and red together i get a near zero reading so i believe this to be broken? Would that be correct?

No the filter is not very restrictive and not that old. we put it in maybe 6 weeks ago. Even with it out it still does not trigger the heat.

If i were to again connect this sensor, turn the furnace on move the multimeter to VAC and touch it in the same fashion would that give you the volt reading needed?
Sorry I am not sure what you mean by
Take heat rise across the furnace when itís running again. May be over fired.
?

If this was tripped or broken how does that happen?



 
  #12  
Old 11-27-20, 01:01 PM
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Often a furnace running too hot causes these switches to fail.
Power applied to a working switch will measure 0V across the terminals. An open switch will measure 24V.
It would appear your issue is with another limit.
 
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Old 11-27-20, 01:14 PM
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Looks like you may have found the problem. That sensor should read 0 ohms, shorted, have continuity, when it is cool.
There may be some temperature readings on the sensor itself, if you looked.
When call for heat is activated, this sensor is cold gas valve to operate. Once burners are working, heat in the chamber will begin to rise. When temperature within chamber reaches the upper limit of this sensor, the sensor will open, removing signal to gas value and shutting off gas. Burners will go out. Blower will continue to run, cooling chamber down. When chamber reaches lower limit of sensor, sensor will once again short. This allows the gas valve to operate and ignition process to once again happen. The burners come back on, heating chamber again. This process will repeat itself, providing warm air throughout the house. Without the the chamber becoming to hot and causing a fire....

Disclaimer: This is only a test, DO NOT run you furnace with this in place!!!!
You could take the sensor out of circuit, by connecting the two wires together and see if furnace will fire up. If the ignitor glows, burners come on, than the sensor is faulty and needs to be replaced. Once you have verified this operation: DO NOT operate the furnace until new sensor is installed.

I am sure there are many things that could cause the sensor to fail, but the most common is age. My understanding of this sensor is it is made up of a bimetal which is sensitive to heat. As the metal heats up, it will bend thus breaking contact and causing an open circuit. As the metal cools back down, it will begin to straighten and make contact, shorting the circuit. After multiple heating and cooling cycles, the bimetal could break, not return to its straight state, of the contacts could fail.

Side note: As the ignitor heats, it will glow red. As it heats, the resistance of the ignitor will drop, in reference to how hot it gets. I have seen these glow, but not get hot enough to lower the resistance enough to activate the gas valve. They are very fragile and a small hairline crack can cause them to fail. If yours glows, but gas valve doesn't open, replace it.

I am not a furnace tech, just a DIYer like you. Do only the things you are comfortable doing and question everything you are told.
 
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Old 11-27-20, 01:14 PM
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I tested continuity and not voltage. not sure if you noticed that or not? Not clear how to test for voltage. If you can help clarify that I can get that for you?

i have another post where I was talking about my returns. Is that anything that might have caused this? There were some adjustments made to the damper to essentially close it since it didn't seem it was really doing anything.

I can go and try and test for voltage?

Do you think replacing this would fixt the issue or is there something that tripped this and that is what we are still trying to uncover?

Edit: Sorry Trying2Help We must have been typing at the same time. Let me try what you suggest too here. Thanks also for responding. A bit chilly in here today but thankfully not a super cold day and we have a fireplace that is keeping up
 
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Old 11-27-20, 01:46 PM
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So the sensor seems to be the problem. I did like Trying2Help suggested and it all fired up.

If I am understanding how this works this is used to make sure that things aren't getting to hot within the system and when it reaches the upper limit (180 degrees it looks if the below is the part i need) then it will trigger a close which closes the gas to the system shutting off the heat. The system is 20 years old and I would bet this is original. We are new to this house as of maybe 2 months.

Do you think the source of the problem is the age or is there something else that possibly caused this to fail?

Is the below the correct replacement part? Looks the same and I entered the part I int23l-2176 that i see on the unit itself.

https://www.supplyhouse.com/Trane-SW...BoC7P4QAvD_BwE
 
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Old 11-27-20, 02:44 PM
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Hi, that limit should open on temperature rise, Shutting down the system, once it has cooled the system should restart, unless it is a manual reset in which case you would be required to press a button on the limit.
Geo 🇺🇸
 
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Old 11-27-20, 03:22 PM
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Yes, I would say that is the correct part.

Found this on another site, it may answer some of your questions, better than I can:
  • FYI (For Your Information):
    • A furnace limit is a safety device that is used to cut the gas burners off if the furnace over-heats.
    • If your furnace is going off on limit you might have an air flow problem like a dirty filter, dirty blower, slow blower motor (might need a new capacitor) stopped up evaporator coil or too much gas pressure.
    • All of these can cause the furnace to over-heat and go off on limit.
    • Many times when the furnace goes off on limit the fan blower will continuously blow because the furnace control board senses that the furnace has over-heated and needs to cool down.
    • The only way to get the blower to cut off is to turn off the furnace with the power switch.
    • You can test a limit with a volt meter to see if it is open or not. On most limits you should get 24 volts from each of the two wires on the limit to a good ground.
  • The specifications on the furnace limit switch are: THERMAL LIMIT, OPEN @ 180 F, CLOSE @ 150 DEG F. This means that this limit switch opens and cuts off the gas burners when the temperature inside the furnace heat exchanger reaches 180 F and closes back to allow the burners to come back on when the heat exchanger reaches a temperature of 150 Degrees F.

I believe, it is normal operation for the burners to cut in and out as the system is running. Just as I described before. I would install the part, run the furnace and see if this happens.

With some further investigation, I came across this:
https://www.youtube.com/watch?v=dPXM8TFMQGw

At about the 25 minute mark he talks about your limit switch.... you may be able to get yours to work...
At about 19 minutes he mentions an issue with a clogged "A" coil, which could be a possibility on your issue, if you do have an issue.

Still trying to determine if I am correct on the burners going on an off during normal operation.... I "think" so, but....
 
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Old 11-27-20, 03:27 PM
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Geochurchi thanks for the reply as well.

The system hasn't been on i don't think since yesterday afternoon and has been off and on most of the day. If i am understanding what you might be thinking the reset of the system like i did should have possibly reset this sensor if working? If so, I don't think it has as the only time i have gotten this to work is bypassing the sensor with a wire.

Where might the switch be to reset?
 
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Old 11-27-20, 03:49 PM
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Trying2Help dude wrapping on it like he mentioned seemed to get it to work as it fired up. WOW I would have never guessed that is awesome. Thanks so much for all your guys help.
 
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Old 11-27-20, 04:22 PM
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I'm beginning to believe I may have been wrong on the on/off cycle of this limit switch. If it opens because the heat chamber got to hot, you have a problem.
During normal operation, there should be enough air flow through the chamber to prevent it from overheating. If the chamber is overheating, than you need to check all the items that would prevent air flow or reduce air flow.
Here is another video:
https://www.youtube.com/watch?v=Fr69...19FD4&index=42


Of course, there is a possibility your limit switch is just faulty. I would install/fix the switch and see how the furnace works. If it does "limit out", than start checking for a air flow issue....

 
  #21  
Old 11-27-20, 05:11 PM
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It is not normal for the burners to cycle on and off during a heating call.
Most limits and thermal safety switches fail because they’ve been repetitively cycled, which they aren’t supposed to do.
 
Trying2Help voted this post useful.
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Old 11-27-20, 09:57 PM
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bline22 I have to agree with roughneck77 , This sensor should not open, unless there is a overheat condition in the chamber.
At the very least, I would set the thermostat at a high temp (90) and let the heater run for at least 10 - 15 minutes to see if the burners shut off. If they do, it is most likely because of this sensor, you could verify it, by testing it again.
If this is the case, you need to look at the air flow issues; ensure blower motor is spinning freely and at correct speed, clean filter, clean A coil, etc... If all that is good, check your flames/burners...

If it doesn't shut down, well.... not much you can do. You can't fix something that isn't broke. Just keep a eye on it and see if it happens again.

roughneck77 You got anything to add? I'm thinking this is more in your line of work than mine.
 
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Old 11-28-20, 04:26 AM
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Thanks again roughneck77 and Trying2Help

I don't have easy access to the A coil to check that it seems so not sure how one goes about doing that?

I had another thought, a few weeks ago a zone damper went out. I noticed that there seem to be some play in these (first time i had looked at one) where one could have it partially open on close. We have 4 zones in the house. Do you think this might have something to do with it that zones are close when the heat triggers on or something building up heat down there and tripping this sensor?

I will go buy a new filter here just in case as well but this one sure doesn't seem dirty and like i mentioned was replaced shortly after we moved in here which was around 6 weeks or so ago.
 
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Old 11-28-20, 01:42 PM
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Have you ran the furnace to see if burners shut down?

The damper may have reduced the air flow enough, but hard to tell.

Is your system an air conditioner and heater system? I would think so, but if not, you won't have an "A" coil.
 
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Old 11-28-20, 03:57 PM
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Thanks for following up. Sadly yes it is cycling.

I am trying to revert back what the guy that came and serviced it did as I am wondering if that is causing some issue. I have some return issues that he was trying to resolve.

After further looking too i may have access to the a coil as we do have a/c here as well. It might be some challenge to get this access panel off as there are pipes with putty in addition venting that runs directly in front of it.

I haven't yet replaced the filter yet but maybe I will do that here as well. What would you start with in looking at this?
 
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Old 11-28-20, 04:19 PM
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Replace the filter immediately if it looks dirty. You need to get full air flow thru the furnace.
If you have air dampers.... you may be able to open them manually.
 
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Old 11-28-20, 07:23 PM
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I replaced the filter and the furnace stayed on with one zone for the duration. It seems I have to replace these things every 6 weeks or so is that right?

If so, where do folks buy cheap filters 20x25x4?
 
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Old 11-29-20, 12:43 AM
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Depends on your needs:
https://www.amazon.com/ospublishing/...ilters+20x25x4

Amazon has some decently priced ones, bulk price is high up front but over time it is better way to go (most times).
 
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Old 11-29-20, 12:51 AM
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There’s no single answer to ‘how often you should change your furnace filter (aka air filter). Below are some guidelines to help you know what to expect in terms of frequency. Every home runs their HVAC unit for different amounts of time, and every home has a unique amount of dirt, dust, pet dander and allergens in the air. These are the things that most impact how long a filter lasts.

A good rule of thumb is to change 1-2 inch filters every three months, 4 inch filters every six months and 5 inch filters every 12 months.

Signs your air filter needs changed:
  • The filter is visibly dirty. Certainly if you can’t see the material of the filter itself, it should be replaced.
  • The heating/cooling system is running more frequently than normal.
  • You home is more dusty than normal.
  • You notice strange smells, or a burning smell near you HVAC unit.
 
  #30  
Old 11-29-20, 11:41 AM
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b,
You can try this sight. They have everything and quick service and good pricing. If you have questions you can call them.

https://www.supplyhouse.com/Hart-Coo...iABEgKfw_D_BwE

Hope this helps a little.
 
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Old 11-29-20, 11:47 AM
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Your furnace does sound like it has a return problem. You'd need a sharper tech that could check the static air pressure within the system. Air zoning adds its own set of problems. Since your system is running so close to the edge..... it doesn't take much dirt in the filter to reduce the flow enough to cause the burner cycling.
 
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Old 11-30-20, 10:49 AM
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Could the return problem be part of this problem as well?
 
 

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