This furnace is a multi-stage variable speed condensing gas furnace, from the infomation I've found on it --- sounds like the Escalade of condensing gas furnaces.

I have run into this issue probably better than a 1000 times at this point (29 years in HVAC). If I had to guess it's the blower motor and those don't come cheap on these kinds of furnaces.

The manual is about 13 years old. So if this motor made it past 8 years, consider yourself lucky.

Could it be something other than the blower motor? yes, but that's not all that common.

I service the Katy, Texas area.

Thank you for the reply!

The furnace was installed in 2004 - at my last service I was told the heat exchanger was developing pinholes so I guess this will force the issue on replacement. Apparently Carrier tried a coated exchanger and the coating doesn’t hold up - planned obsolescence I guess!

I’m hoping this issue isn’t critical so I can do some research and shopping for a new unit.

Again, many thanks for your help on this!

I believe I have the identical furnace but with the Bryant name on it instead of Carrier. Mine was installed in 2000, and I have had significant issues with the inducer fan and motor. The furnace brings this fan up to a higher speed when the furnace first starts to purge the firebox of any combustibles, and then it slows down to the requires speed of either the high or low fire. The first issue I have had with it was kind of a ticking noise that I could hear from outside the house where the furnace vent is (at sidewalk level). This turned out to be the inducer fan which is made out of plastic, but has a steel bushing in the center which goes over the motor shaft. The bushing had come loose from the plastic fan and was causing it to wobble and rub on the housing. I repaired it with epoxy but it was not in balance and the fan became noisy. In Canada they will not sell this fan as a separate item, but I was able to locate one in the US and replaced the fan with a new one.

But, I have had other issues. The seal between the fan and the motor had gotten damaged and was not sealing well. I also neglected to annually clean the condensate trap, and water backed up into the fan housing and the motor. The motor bearings seem to have gotten water into them and were stiff. This was causing the fan to be slow in coming up to speed and generated the 42 code, so the furnace will not run. With some hesitation I decided to replace the motor bearings. It was not an easy job, and not for the faint of heart. This motor I believe is a permanent magnet variable frequency design, and the control board is soldered right on the end of the motor. With careful desoldering you can get it off and the bearings can be replaced ($16 for new ones at a bearing shop). It worked for a few years and I had to revisit the motor and seal issues.

However this year it failed to start up again with the 42 code and I decided to bite the bullet and replace the unit. At least for the design of motor I have they do not sell the motor or fan, but the complete unit. It cost me $1520 CDN to replace it, with me doing the work. I had a really hard time finding the replacement unit. Carrier claimed that they were right out of stock right across North America, but I found a one man furnace repair guy that had one to sell me. I did it myself as I have had this thing out a few times now and know how to do it. He wanted $200 to do it. Here is a link to the kit instructions. I see your furnace model is listed as being suitable for it.

https://www.shareddocs.com/hvac/docs/1009/Public/07/IIK-340793-762-01.pdf

Bottom line is that if you have this unit you may want to address it sooner rather than later and get the complete replacement unit, and not be caught in a jam with cold weather and no parts. The other alternative is to buy a new furnace, but that will be a lot more. For what it is worth the new replacement unit has worked very well since I put it in. Perhaps there are many more years of life left in the furnace now, but the cost to repair it is not minor.

The other issue I have had with this furnace is the two differential pressure switches. I had to replace them. And since the replacement fan motor is a different design you are supposed to replace these switches when you change to the later design motor.

I have also replaced the original ignitor with a hot rod ignitor. Other than this, the furnace has been "prefect!" When it comes to replace it, I think I will look for a simpler design.

The furnace was installed in 2004 - at my last service I was told the heat exchanger was developing pinholes so I guess this will force the issue on replacement.

D. Klees, If this furnace was registered I would think that heat exchanger should be under part warranty. Usually 20 years is the norm for heat exchangers of this 2004 time period.

All other parts of the furnace are only under 10 parts typically if the unit is registered. It's not likely you will be able to do this work yourself thru a distributor, you'll need a licensed contractor in your area to handle warranty exchange of the heat exchanger.

If you're not the original purchaser of the equipment then there may be a clause that limits warranty periods for second or more owners of a specific property.

There was a class action lawsuit raised on the heat exchanger issue. I have a file about an inch thick on this problem furnance. Somewhere in there, I recall that Carrier extended the warranty if one really wants to go down that rabbit hole. Keep in mind that the firebox in these furnaces operates under a negative pressure. Any leakage through pinholes, if they exist, will be fresh air into the exhaust stream which goes outside the house.

Thanks very much for all the replies!

My son has a friend who is in HVAC and these are his comments FWIW!

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That furnace has a variable speed ECM blower motor. There is no capacitor or belt like your dad mentioned.

1 of 2 things.

1) Either the bearings are failing on the motor. Nothing you can do about it. They're permanently sealed, lifetime lubricated. Would need to replace the motor.

Or...

2) these motors can sometimes

"hunt" at start up. They are looking for a reference point. Kinda like they have a built in tachometer to measure the rpm. Sometimes nothing can be done to stop it from happening. They always do this at start up, but it can become noisier and take longer as they age. If the blower wheel is dirty and causing an imbalance it may contribute to this. If it's dirty and you clean it, the noise may stop. My theory is that the dust that accumulates inside the motor also contributes to it struggling to find the reference point, but that's just a guess.

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Based on all the information, I’m reasonably confident this isnt a short-term crisis - I figure when the weather cooperates, I’ll pull the blower assembly and check/clean everything - but based on the age I’m fairly certain the wife will have me researching a new furnace for our golden years!

If you have the original motor in your furnace that I had in mine, which is 4 years older, there is another more remote possible cause. This motor is very heavy and has multiple steel balance plates on the end of it which I think is to try and keep it from vibrating at higher speeds. On top of the weights, there are fairly soft rubber mounts for the motor, that if worn can let it move enough to cause noise. I thought for a while that was the source of the noise. They can be replaced and it did it. But, it didn't really help. By just taking the cover off the bottom of the furnace you can reach the end of this motor. Jiggle it around and see if it could be hitting something to make the noise. You can also stick your fingers into the end of the motor cooling fan and see how hard it is to turn, or if there is any roughness to indicate the motor bearings have failed. Contrary to what your son's friend says, the bearings can be replaced, but it is a difficult job and the average furnace repair guy is likely not brave enough to try it, and you may not want to pay them for the time it takes.

I don't recall the inducer wheel being dirty and that causing unbalance. It is quite doable to take out the whole motor and fan housing. Then you can reach in and check if the impeller wheel is loose on the shaft. That is probably the most likely cause.