Blower motor slowing down and speeding up - help please

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Old 11-04-07, 04:40 AM
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Blower motor slowing down and speeding up - help please

A couple of weeks ago when it was still warm and we had the A/C running, I noticed one night that the A/C would bog down from normal operation speed to almost a stop and then come back up to normal speed and do the same thing over and over again. I could first hear it through the vents and then went downstairs in the basement to check the unit. Not liking they way it sounded I just shut the A/C off. It basically sounded and acted as though it were experiencing a power surge where it wanted to shut off but it would just bog down anywhere from one second to several seconds and then go back up to normal speed again. The temperature of the air being blown out remained cold. That was a couple of weeks ago. After that we had a period of nice weather where no A/C or heat was needed, but during the past week I turned the heat on a couple of nights just to raise the temperature to about 68 or 69 with no problems but then a few nights ago I again noticed the blower motor making the sound again, this time with the heat on. So, my layperson conclusion is that there is an issue with the blower motor - maybe a fuse or something? The unit is only 8 years old. It is a Lennox WhisperHeat. Any helpful advice would be greatly appreciated. Thanks.
 
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Old 11-04-07, 05:09 AM
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I'd say your motor may be on it's last leg.

Turn power off after it does this for awhle, and put your hand on the motor, is it HOT?

If so, the bearings maybe going out on your and the motor has to work harder to turn..

When you do get a new motor, be sure you get a new matching capcator.
 
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Old 11-04-07, 05:46 AM
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maybe an odd question but:

does your air handler utilize a variable speed motor? If so, the controls that vary the speed may be a problem.
 
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Old 11-04-07, 09:53 AM
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Originally Posted by nap View Post
maybe an odd question but:

does your air handler utilize a variable speed motor? If so, the controls that vary the speed may be a problem.
WhisperHeat does not use VS motor.. Just the basic PSC motor.

Heat, Has any looked at your heat exchanger? That style of heat exchanger in that model has been know to crack over time. Hope you got CO detectors in your home.
 
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Old 11-04-07, 10:22 AM
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Have you checked the filter and blower compartment at all?

If you catch it in the act again, you could shut off the furnace and try to spin the blower motor by hand to see if there is a drag, or if you notice that upon shutting off the furnace, the blower wheel starts slowing down really quickly.

Any house lights dim at same time?

Could indeed be the motor as stated. If this be me, I'd be ohms testing the blower motor wires and even wire up the blower motor to some other power source to rule out some bad connection in furnace, before buying the new motor.

How often does this do this? The reason I ask is that: One would actually hope it is frequent so you can make the tests I mentioned, because you can't find anything wrong when it is not doing it.
 
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Old 11-04-07, 03:47 PM
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[QUOTE=Jay11J;1253197]WhisperHeat does not use VS motor.. Just the basic PSC motor.
QUOTE]thanks jay. that is why I posted with absolutely no confidence. It was merely a thought.

So, with that, sounds like the motor is in the throws of death barring any odd problem like power supply.(unlikely but not impossible)
 
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Old 11-04-07, 05:14 PM
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I agree. You're probably looking at a bad motor. However, look at the blower and make sure the screeching noise isn't the blower wheel rubbing against the housing. Sad to say, Lennox parts are not cheap. What is the motor horsepower and the capacitor rating? Some Lennox units use a very high mfd run capacitor on the blower motor. You can replace with a generic motor but you have to be careful about the motor selection.
 
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Old 11-04-07, 05:44 PM
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I had a fairly busy day and just checked my post and was happy to see so many helpful responses. Thank you all.

Well, the heat has been off all day and I just tried troubleshooting after reading all your advice. Upon turning on the heat, the blower motor never got up to full speed. That is, instead of it operating at normal speed with hot air coming out of the vents and then bogging down as I first posted, the unit turned on but just made that bogging sounding noise without reaching full speed. This makes me feel as though the problem is worsening because it usually at least runs normally for awhile before it starts slowing down, but this time it never got up to proper speed. As it was on, I pulled the panel off the furnace, which automatically shut it off, and quickly shined a flashlight into the blower. The blower wheel was spinning freely and showed no signs of resistance, so I don't think it's the bearings. However, I did touch the side of the motor itself and it was VERY HOT and this was after being turned on for only about 7 or 8 minutes at most. So I assume it's the motor? The best way I can describe any noise I hear is that while the blower seems to be rotating at a slower than usual speed, I can also hear the lower pitched electric hum of what sounds to be a strained motor, which again seems to suggest that the motor is on its way out.

Someone asked if any lights dim while this happens. No, the problem seems to be strictly isolated to the furnace. Assuming I need a new motor, does anyone have any ideas how much I'm looking at? This is not something I would attempt to do myself. I do know someone who owns an HVAC business so I think I'll be calling him tomorrow. Also, at this point, is it dangerous to leave the heat on if in fact the motor is failing? What is the worst that can happen? Not trying to be foolish but it's been getting pretty cold these last few nights and I could use the heat.

Jay, thanks for the heads up on the possibility of a cracked heat exchanger. I'll have my guy look at that when he comes. I do have a CO2 detector by the bedroom hallway.
 
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Old 11-04-07, 05:52 PM
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I wouldn't advise running the furnace any more.. You are running the heat exchanger hotter than normal and will be cycling on high limit, That may shorten life on your heat exchanger.

Tuff it out tonight with a few extra blankets, and call for service in the morning.
 
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Old 11-04-07, 06:32 PM
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Originally Posted by Jay11J View Post
I wouldn't advise running the furnace any more.. You are running the heat exchanger hotter than normal and will be cycling on high limit, That may shorten life on your heat exchanger.

Tuff it out tonight with a few extra blankets, and call for service in the morning.
Will do. Thanks for the helpful advice.
 
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Old 11-05-07, 05:49 AM
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If they just replace the motor and capacitor you're looking at $300+ If the blower wheel also needs replacing add another $100. Keep the old motor just in case the repair is sour. With the old motor in hand we can compare old to new.
 
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Old 11-06-07, 01:28 PM
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Daddyjohn, thanks for the info. I'm a bit ticked right now as I just talked to my HVAC guy about an hour ago and he's telling me that his distributors are quoting him $400 just for the price of the motor! That sounds insane, even for a layperson like me. Is this a Lennox-made motor? He told me he was waiting to hear back from one more distributor and that if they came back with a similar price we would just go with an aftermarket motor. That's fine with me. He said if the motor alone costs $400 I would be looking at an $800 - $900 repair bill, which means $400+ in labor, just to replace a blower motor and capacitor? Am I being taken for a ride here or am I just being naive as to how much this stuff costs?
 
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Old 11-06-07, 05:06 PM
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You got a local motor shop you can go thru? We have one in town that not only stocks motors from fractional hp to huge industrial motors that have to be hoisted by overhead gantry block and tackle, and they make motors from scratch. My hunch is you will be hosed going thru company distributor. Landlords love me for putting in blowers for as cheap as $100-150 parts and labor.
 
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Old 11-06-07, 06:26 PM
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Ecman, I wish you were around for hire.

I did a little internet research when I got home from work and I've priced OEM Lennox motors for my exact furnace model number for $240 from a couple of online stores. I told my HVAC guy this and he was blown away since he's in the business and his distributor wants to charge him $407. I emailed him the webpage from the online store and he said he's taking it to his distributor tomorrow. I'm semi-friends with the HVAC guy (we have a mutual friend) and I used him a couple of months ago when I sold my old house. I do trust him in that he told me he would not mark up the price on parts for me. Although that doesn't necessarily mean I'm not paying him more in labor. I just don't know.

Funny thing - I work for a national homebuilder and just found out today that our national heating unit provider is none other than Lennox. Doh! So I might be able to get my motor for either at or below cost. I will inquire tomorrow. I still need my HVAC guy to come out and confirm that it is in fact the motor that is the problem.
 
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Old 11-08-07, 02:32 PM
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Your guy's distributor is buying it from a Lennox reseller and marking it up. Even the $240 is nuts, Lennox is outrageous on parts prices. If you're guy is worth his salt, he can cross you over into a generic motor much cheaper but he has to be careful and watch the specs. Did you read my previous post in this thread on Lennox motors and high mfd capacitors? For example, some Lennox furnaces use a 3/4 hp motor with a 40mfd capacitor. Normally, a 3/4hp fan motor will use a 10mfd capacitor. Lennox uses a high mfd capacitor to improve the motor's efficiency. If you go back with a 3/4hp generic motor, you'll be in trouble. In this case a 1 hp motor would need to be selected. Have you heard this expression going around?- "don't taze me bro" It came out of that incident a few weeks ago at the Univ. of Fla. Well, don't taze me Heat, but Whisperheats have had their issues with heat exchanger cracks. Have your guy check yours out.
 
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Old 11-08-07, 06:43 PM
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Daddy, interesting stuff. Well, my guy put in a new motor and capacitor yesterday. Total cost of parts and labor = $360 so I am happy with that. He ended up getting a generic replacement motor through his distributor at a price of $142 (that's more like it). Interestingly enough though, the replacement motor is a 3/4 hp just like the old one, but the replacement motor called for a 15 mfd capacitor while the old one was a 40 mfd capacitor, just as you suspected. My HVAC guy didn't know why there was a difference in the capacitors for a motor that had the same HP and rpm rating.

Why am I in trouble for using a generic motor with a 15mfd capacitor especially if the new motor specified a 15mfd capacitor? Please explain. Funny thing is that this "generic" motor had the same name on it as the old one (can't think of the name right now). My guy was complaining how he didn't like Lennox because of the high prices and proprietary parts. But if you looked at the old motor and the new "generic" motor side-by-side you couldn't tell them apart. The only difference we noticed was that the generic motor was ever so slightly longer by mabye 1/32 of an inch, just enough so that we had to bend the bracket that wraps around and holds the motor. Maybe that is Lennox's way of forcing people to buy the $400 motor as opposed to the $142 motor.

As for the heat exchanger, he checked it out and it was fine. He looked for the initial lighting of the burner flame and said there was no anomaly with the flame to indicate it was cracked. He explained that upon lighting, the flame didn't backdraft and that the flames burned steady and clean without flickering.

Daddy, please let me know if you think there's an issue with the new motor and the 15mfd capacitor that this motor called for. Is there any danger in doing this? Thanks.
 
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Old 11-17-07, 06:41 PM
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My furnace, Armstrong, was shutting off. I think it is the motor or your capacitor. When you have a dead capacitor your motor will heat up and eventually die. Or worst your heat exchanger can be ruined. I couldn't understand why power was resetting on the unit. One morning we had a very bad smell coming from the vents. The smell was coming from the almost burn out motor. The unit was running but very slow air was coming from the vents and it smells. The smell is enough to warn any average person that there is something wrong somewhere.

My HVAC guy came over and also wanted $300-$350 for the repair. He said I could do it myself and save some money. I took out the old motor and went to buy a new motor and capacitor(total=$90.22). I am your average weekend handy man so it was not hard for me at all. It took me two hours to get it going again.
 
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Old 11-17-07, 06:45 PM
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If you live in the Southeast then Foxparts.com is the best place for DIY-ers.
 
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Old 11-17-07, 06:50 PM
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I find it odd that your HVAC guy did not understand why the differences in capacitor requirement?


[QUOTE=Heatisgood;1255771]Daddy, interesting stuff. Well, my guy put in a new motor and capacitor yesterday. Total cost of parts and labor = $360 so I am happy with that. He ended up getting a generic replacement motor through his distributor at a price of $142 (that's more like it). Interestingly enough though, the replacement motor is a 3/4 hp just like the old one, but the replacement motor called for a 15 mfd capacitor while the old one was a 40 mfd capacitor, just as you suspected. My HVAC guy didn't know why there was a difference in the capacitors for a motor that had the same HP and rpm rating.
=
=QUOTE]
 
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Old 11-18-07, 11:10 AM
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capacitor

The 40 mfd capacitor made the old motor very efficient meaning the motor had a very high power factor. I've run into this exact situation before on Lennox furnaces, but I only made the mistake once. I didn't even price a motor from Lennox because I knew it would be insane. A new 3/4 hp generic motor I installed drew too much amperage and I knew I couldn't leave it in the furnace. So I took it out and put in a 1 hp generic motor [with appropriate capacitor] which worked just fine. Did your guy take an amp draw on the new motor? Did he check both the heating speed and cooling speed amp draws? During the cooling cycle, the motor runs at a higher speed than when on the heating cycle [IOW- draws more amperage on cooling]. The time to check it is now, not next Summer after any repair warranty you have has long since expired.
 
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Old 11-18-07, 11:31 AM
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Unless I missed something, I can't recall this part of the possible equation being brought up: Woudn't the load factor - the actual resistance load fighting the running of the motor due to the amount of air the blower has to move, have something to do with it?

But you would think a motor manufacturer would sort of have it figured for normal application for a specific hp motor. So, does the 40 mfd make the motor last longer or make the electric bill less, or both? And is the difference going to be something great enough that the higher cost is still worth it?

I have replaced quite a number of motors and got chep -----> reasonable replacements from a motor shop, where the owner/motor fabricator (not some grunt) gave me a comparable replacement that fit, but never have gotten into this mfd capactior issue - and all I know is every motor I put in, I can't recall having to go back and replace the same one due to failure.

This post is NOT for argument or cynisim. I'm just curious about this, myself.
 
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Old 11-18-07, 07:58 PM
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No problemo. A normal cap for a standard 3/4hp fan motor is usually 7.5 mfd to 10 mfd. A motor like that will usually have a power factor in the low 80% range. A 40 mfd cap for a 3/4 hp motor is way out of the usual range for a motor that size. You would be more likely to see a 40 mfd cap on say a 3hp or 3.5hp compressor motor. The 3/4hp motor/40 mfd cap is a unique design to Lennox. It's a very efficient 3/4 hp motor. I won't use the Lennox motor coz, IMHO, they're nuts when it comes to price.

Think about refrigerator compressors. What size run cap do you see on them? 12 mfd? 15 mfd? What size are the compressors? 1/8hp maybe? Normally you might see a 15 mfd cap on a 3/4hp PTAC compressor or on a 1.5hp fan motor.

Go to http://www.fasco.com/fasfacts1.asp and download the fasco facts book. I think you'll find it quite interesting.
 
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Old 11-19-07, 06:47 AM
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The time one gets thru here one might be able to get their doctorate degree in motors/caps.

But three more things: What about the associated voltage of the cap in regard to the smaller and larger size cap?: Does the 15 have one voltage and the 40 another?

And also what about the bottom line on how the poster will fare with his lesser mfd. cap with a cheaper motor? Like, if he got the Lennox one, would he have saved $5 a year on motor run electrical costs and the motor last 30 years instead of 10?, or what?

What was Lennox's objective do you think?: Maintaining their standard for quality (like the perception Maytags have?), where initial cost savings is not really the object?
 
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Old 11-19-07, 08:13 AM
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1. The voltages are probably the same. In HVAC run applications, you only ever see 2 vac levels, 370 vac and 440 vac. Since you can always go higher on voltage, it makes sense to stock only 440 vac caps. Have you seen theTurbo caps? http://www.amradcapacitors.com/index.php

I especiallly like the multi start cap; it was always a pain to stock a lot of start caps.

2. I have no idea about the operating cost difference. I'm more concerned that the new motor's amp draw needs to be verified so make sure it's not overloaded.

3. My best guess is their effort to meet government mandated energy standards. A little wattage saved here, a little there, it adds up and looks good on paper.
 
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