Miller CMF80

Questions about repairs and parts for other brands of furnaces and air conditioners. Please note that our speciality is Coleman and Nordyne products, so our knowledge of other brands may be limited.

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Paula39

Me again!! :) I posted a couple back regarding our furnace not igniting intermittently, just waking to IDM running only. The centrifugal switch seems to be all in one with IDM (booster motor?). My husband spoke to someone over the phone who suggest we check the settings for the limit switch (combo w/fan). We are not sure if a repairman in the past adjusted them but they were as follows: 80 100 200 (or close to, my husband only saw it- not sure how accurate). He suggested to my husband that the upper limit was too high and should be 160. Upon research I have found that seems to be a factory setting. We have never adjusted these settings ourselves in the past but it seems that the other settings may be off from what I have researched. My husband set upper limit to 170 but touched nothing else. I am not sure what that (limit switch) would have to do with the furnace IDM running at times without ignition with a new cycle. Would the limit switch have any effect on that in your opinion? The gentleman is away for two weeks, and thereafter if we determine it is more involved than the switch he can repair it for us. I am concerned we may have to replace the booster motor (IDM)/centrifugal) in one shot. I would rather pay $65 for fan/limit switch if that is the problem! I have kept track of a few cycles as well before and after change in limit upper setting, if anyone wants the info (it is cycling a bit different with the change to the 170). We have noticed for awhile heating not effective, and furnace coming on every couple of minutes. Any opinions on limit switch and it's effects on our problem with IDM? Thank you all. I have been a lurker for years, posted twice on old forum and now decided to become an official member. This is a great site!! Sorry for long post...
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Robert
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Posts: 6413
Joined: Wed Feb 28, 2007 12:07 am
Location: Tennessee

Hi,

The Limit should have no effect on the IDM. The IDM should reach correct rpm if ok, then centrifugal switch closes to send signal for ignition.


If the air pipe to the IDM is clear, only other possibles are the IDM or Cent Sw. which is a all in one deal.


The settings for Fan & Limit Combo should be around 90 (fan off) / 130 (fan on) / 200 (Limit trip temp).


That's where I would advise , if problems still exist, call a reputable Tech out.


Thanks for inquiring and rejoining us.


Take care and best wishes,
Robert
Some people are Humbly Grateful, while some are Grumbly Hateful.................... Which one are you ?
Paula39

Thank you Robert for your reply. I have watched these forums long enough to know how much expertise you all have, that is why I wanted to post. I had my doubts myself. I had mentioned to my husband, that being intermittent that it seems it may be the IDM itself that is not reaching the rpms it needs (I have learned so much from this site and online in general). When we do wake to just the IDM running(usually on the coldest night), I tell him the motor sound doesn't seem as strong as it should almost as if it isn't spinning as much as it needs to trigger the centrifugal switch. Then for some reason, when we "reset" it it will be perfectly fine for days or weeks. I had also noticed the air shutters on the IDM, it looked as if I could see a line of demarcation where someone may have adjusted them at one time. They measure B and C. From the literature from our furnace it recommends D (the burner is PGB-2) or within that range(?). I understand that would effect the air flow to IDM. Do you think that could be a problem or should that be left alone? The opening from under the home seems clear and I can feel a cold draft when the door is open. The only techs that have worked in the past was our former gas company. Our present one doesn't service or seem as knowledgable (but we saved tremendously on the gas price- oh well). The limit switch settings (incorrect) may be causing our other heating problems, so that may be helped. Well, we may purchase the IDM/centrifugal here if needed as it may be easier to find and cheaper for us, then find someone reputable as you say to install it for us. I was leaning that way, but hoped we may save $. Thanks again!
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Robert
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Posts: 6413
Joined: Wed Feb 28, 2007 12:07 am
Location: Tennessee

Hi,

The IDM will come preset from factory to design specs and should not be changed.

Those centrifugal switches do fail, as does all switches with contact points.


Do you have Direct Ignition or Standing Pilot ?

Natural Gas or LP ?



Thanks,
Robert
Some people are Humbly Grateful, while some are Grumbly Hateful.................... Which one are you ?
Paula39

Robert,
I am sorry I did not receive notification of your reply!! :o I hope it's not too late to respond. Our furnace is direct ignition, LP gas. This problem I have explained about (IDM running but no ignition following) is happening much more frequently now. We have also had problems for awhile with short cycles. Takes so many cycles for temp to reaching setting on thermostat. I think we have found a reputable technician and we will be calling him soon. I hope he can diagnose something as this only happens on occasion. Getting too cold to hope it will keep going. Thank you very much again.
User avatar
Robert
Moderator
Posts: 6413
Joined: Wed Feb 28, 2007 12:07 am
Location: Tennessee

Hi,

As stated, the IDM w/cent switch is all in one assembly. If cent. switch is not closing, it is the switch or motor most likely and for either, you replace both.


A good Tech is best in these cases to keep down cost of unnecessary parts replacements.


Take care and best wishes,
Robert
Some people are Humbly Grateful, while some are Grumbly Hateful.................... Which one are you ?
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