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UPDATE - INDUCER MOTOR DIED! Apologies for the long post again but would appreciate anyone's thoughts on my current furnace developments...at a minimum it would make for a good troubleshooting exercise for any experienced HVAC folks :)

As I mentioned before, my furnace stopped shutting itself off due to overheating/high limit switch tripping so I continued to use it when one night I heard a loud grinding sound coming from the furnace which obviously was a motor seizing up. I shut the furnace off for the night and later confirmed it was indeed the inducer motor that died. I'm going to buy a replacement and install it myself, but now I'm questioning the cause of the original over heating problem I had. Could it have been the inducer motor going out that was leading to the over heating? If not, could it have been excessive heat from over heating (caused by something else) on the inducer motor that sped up its demise and killed it?

Since some folks mentioned the AC coils getting clogged with dust as a cause for poor airflow and hence over heating, I bought a camera scope and snaked it inside the furnace through the high limit switch hole to get a look at the AC coils and the furnace coils and as far as I could tell, there doesn't seem to be excessive caked on dust on the coils to be causing air flow/over heating issues in my opinion. What do you all think?

Here's a link of pictures I took of the coils https://ibb.co/album/FkSj6D

So assuming that clogged coils isn't the cause for air flow restriction/over heating, then the next logical assumption I think would be that it was indeed the capacitor for the blower motor going out leading to lower blower RPM's causing insufficient airflow/over heating. This would line up with the fact that the furnace stopped over heating/tripping the high limit switch after the new capacitor was installed by the HVAC company. To rule out the high limit switch being faulty again, I took it out and tested it again in a pot of boiling water and it tripped at the temperature it's supposed to so the switch is still good.

So could it be that the blower motor capacitor and inducer motor unluckily started going out due to age (14 year old furnace) around the same time leading to overheating issues and eventual death of the inducer motor or maybe just the blower motor capacitor going out causing over heating issues and that over heating lead to the death of the inducer motor? This all had me confused because the inducer motor has a temperature limit switch of its own built right into the housing so I would think if the inducer motor was starting to go bad and not blowing enough to sufficiently blow exhaust out I would think that limit switch in the housing would have been tripped but it never was. After I removed the dead inducer motor assembly from the furnace, out of curiosity I removed the limit switch from it and found that the switch still had the black plastic cover on it from the factory, similar to this picture: https://i.ibb.co/z715H36/limitswitchcover.png

This would explain why this switch never tripped because it had that plastic cover shielding it from the heat inside. All these years with that plastic cover on the limit switch! I guess the installers never thought to look if it had it on or not? Maybe they assumed the manufacturer removed them since it comes pre-installed in the housing?

In any case, I'm dumbfounded at the real causes of my furnace issues but also surprised at how much I've learned about HVAC because of all of this lol! My next step is to order a replacement inducer motor and install it and see how everything works after that and go from there and I'll make sure to check if the housing limit switch still has its plastic cover on! I think I may also buy a manometer to double check that the HVAC company didn't screw up the gas pressure and set it lower than what it's spec'd for. I'd love to hear everyone's thoughts and opinions on this latest update. Thank you!

UPDATE: So apparently my furnace now runs without overheating/shutting itself off.

When the HVAC company originally replaced the capacitor for the blower motor, the furnace ran for awhile and then turned itself off normally because my house was already warm so it didn't need to run long to pass the 70 degree temp I set on the thermostat. I shut the heater off and didn't run it until the next morning so I could observe the control board/diagnostic lights and it was at this time that the furnace overheated after about 10-15 minutes, so I shut it off and didn't run it at all until the HVAC company could come back to investigate.

When they came back, they turned the gas pressure down (they said it was set too high for what the manufacturer suggested) and ran it for 20-30 minutes without it overheating/shutting itself off, but they recommended removing a burner to 1. prevent anymore overheating and 2. prevent more problems down the road since they concluded after taking temperature measurements with a digital thermometer that the furnace overall was running too hot internally and over time internal components will fail sooner etc.

It was at this point that I was unsure what to do and posted here asking for advice and considered calling another HVAC company to investigate further. I didn't bother running the furnace at all thinking that it would continue to overheat. Yesterday I decided to give it a shot and ran it in the late morning when my house was pretty cold. I monitored the furnace and had eyes on the burner flames and control board diagnostic light to watch if it shut itself off with an overheating error, but it ran for about an hour or so without issue which was surprising and encouraging so I turned it off to give it a break. From what I remember it raised the temp in the house from 64 degrees to 66/67, don't remember exactly.

Since then I've run it few more times during the day so I can monitor it and so far it hasn't overheated and shut itself off, so I'm glad about that but also confused as to why it would have overheated on that one run the next day after the capacitor was replaced but not anymore? Could it be that the capacitor maybe just needed to get tuned in with the blower motor or something like that? From a technical perspective, I'm not sure what factors could be in play when a capacitor is initially swapped. Could it have just been a fluke and it's probably fine now? While I've been running it during the day, I've gone to all the vents in my house to feel for airflow and they're all blowing fine. What do you all think?

Original Post:

Here's the short story:

My furnace is overheating and shutting itself off and the HVAC company is suggesting to remove a burner so that it doesn't run so hot because they think my furnace is over sized for my 2100 sqft home. Is this really a typical thing to do?

Here's the long story with more background:

My York GY9S100C16DH11H furnace (2006/2007 92% efficiency model) shows 4 blinking red lights for high limit switch open. I tested the limit switch myself with a multi meter to confirm that it is opening when the furnace shuts itself off which it was, then I tested the limit switch in a pot of boiling water while monitoring the water temp with a meat thermometer to confirm that the limit switch is not opening prematurely. It opened right at 180F as designed, so the limit switch is not faulty. Furnace filter is also not the problem.

Due to the design of my furnace model, further troubleshooting would require partial disassembly to open up the furnace to get to the blower motor/housing which I'm not comfortable with so I called an HVAC company. They got into the blower housing area and found that the capacitor for the blower motor was going bad, resulting in the blower not blowing enough hot air through to keep the furnace from overheating. I asked them if the blower motor and other internals like the coils all looked good and they said yes. They replaced the capacitor and called it good, but after they had already left the furnace eventually overheated again and shut itself off with limit switch open error.

They came back out and lowered the gas pressure in the furnace and took some temperature readings with a digital thermometer at different parts of the furnace and eventually concluded that the furnace is over sized for my home and they recommended removing one of the 5 burners from the furnace so that it doesn't put out as much heat and hence overheat itself. I found this conclusion perplexing because the furnace did not have any overheating issues previously so if it was really over sized for my home wouldn't it have had over heating issues to start? To me this comes off as a band aid solution that doesn't address the real cause of why the furnace is overheating. Is removing a burner from a furnace really a typical thing done to address overheating issues?

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    Have they checked the duct static pressures? – ThreePhaseEel Nov 19 '20 at 12:44
  • So this is a 2006/2007 model which I assume has been in place for something approaching 15 years. Is that correct? If so, has this problem existed for all that time or has it recently started? – jwh20 Nov 19 '20 at 13:41
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    When caps go out in motors that can be a clue that the bearings are needing servicing or replacing. Since you state this did not happen previously I would be looking at the bearings. Removing a burner is a band aide and not fixing the problem. As the grease drys out / breaks down in bearings the motor rpm is slowed and this is where your problem is since it worked previously. – Ed Beal Nov 19 '20 at 14:08
  • If it worked fine for 10yrs and now they are saying "the burner is oversized", that may be indication that the blower is not blowing as its supposed to (3phaseeel's duct pressure will tell this, and Ed's motor service or replacement). Last summer I replaced the cap in my blower, and i recently had to replace the fan motor as well, the cap was likely failed due to a degrading motor (though it was 30yrs old...) – mark f Nov 19 '20 at 15:53
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If a furnace is too large for the space it's heating then you get short cycles. In other words the thermostat calls for heat, the furnace provides the heat, the temperature in the space rises rapidly, and the thermostat stops the call for heat. This isn't what you're seeing.

If a furnace hits its high limit cut-out it's because there is a problem with the heating system. Think about it this way: a furnace is a box full of fire. The goal is to get rid of the heat from that fire before the box melts. When the high limit is hit this is a clear indication that we're not getting the heat of the fire out of the box fast enough. Thinking about the ways heat is removed from the box, here are some possible conditions to check:

  • obstruction of the exhaust or fresh air intake
  • under-performing exhaust inducer fan
  • obstruction of conditioned air flowing through (blocked vents, dirty air filters, dirty air conditioner coil on top of furnace)
  • under-performing blower
  • excess fire (too much gas being burned)

Your concern is well-founded: a furnace doesn't work well for 15 years and then suddenly become over-sized for the home, causing the unit to overheat. Something is deteriorated and needs to be corrected. I'm not an HVAC professional but I think a modification like disabling a burner would be an uncommon thing to do.

Checking the condition of an air conditioner coil probably requires cutting a hole in the side of the duct where it sits. It's not hard to do nor to patch, but it's important to cut the hole in a place where the coil won't be damaged by the cutting tools.

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  • In my recent case the high limit was tripping due to a clogged heat exchanger, which led to heat accumulating in the wrong places. – isherwood Nov 19 '20 at 19:37
  • Thanks for your reply! I have an update, my furnace is no longer overheating. More details in my original post with the update. Would appreciate your opinion, thanks! – copofop80 Nov 24 '20 at 22:16
  • Have another update, hoping I could get some input from some experienced HVAC folks. – copofop80 Dec 30 '20 at 21:51
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Inducer failure fits the narrative - decreased inducer performance is one of the possible causes for overheating. We couldn't hope to do failure analysis over the internet to assess whether it was pure coincidence or whether the earlier blower failure might have contributed to inducer failure. Coincidence isn't a bad explanation; all kinds of crazy things can happen once.

It is surprising that the limit switch on the inducer motor has a plastic cover. I wouldn't blame that on installers though -- no manufacturer would leave a cover on an internal part like that and expect equipment installers to remove it. Depending on its position in the motor, though, might there be an engineering reason why the cover should be left in place? Maybe the sensor was intended to respond to heat conducted through its housing, for example, and without that cover it could be exposed to the cooling air that blows through the windings of the motor. That could cool the sensor excessively and prevent it tripping when the engineers intended it to trip. That's just a theory -- if the new inducer arrives with a cover over its limit switch and it doesn't have instructions directing you to remove the cover then leave it in place.

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  • Thanks for your reply! The limit switch is in the housing where exhaust is ejected from the furnace. See this picture: i.ibb.co/yRY3z2g/inducer-motor-housing.jpg This sensor is definitely intended to be tripped when the exhaust fumes are too high & I think the plastic cover would prevent the sensor from accurately measuring the temp. I found these videos ( youtube.com/watch?v=BU3XOz1mam0 & youtube.com/watch?v=B29muKKgFR8 ) where someone installed a switch with the plastic cover on. In the first video's comments, people say the plastic cover should be removed. – copofop80 Jan 2 at 5:19
  • Thought I'd come back & mention the HVAC guy that turned down the gas pressure claiming it was "too high" wasn't truthful. I measured the outlet pressure with a manometer & got 1.75 when it should be 3.5. Can only speculate he purposely lowered it as a band aid to prevent more overheating to avoid finding the true cause (likely failing inducer motor) anticipating I would call back & agree to remove burner/s to de-rate the furnace. I obviously can't prove anything, but still shady. I have the new inducer motor installed so going to raise gas pressure to 3.5 & hope it doesn't overheat after. – copofop80 Jan 14 at 20:08
  • @copofop80 Interesting result. Don't forget to accept an answer if you found it correct/useful/etc. :-) – Greg Hill Jan 15 at 17:23

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