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Miller CMF2 80 Furnace

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josephny
josephny Member Posts: 270

I have a manufactured home (new to me) from the early 1990s with a Miller furnace CMF2 80, oil-fired Becket AF-15.

It works, but lately I noticed that when the tstat calls for heat, the system tried about a dozen times before staying on.

The burner seems to ignite and the furnace gets hot early on in this cycle. The fan, however, definitely starts and stops. It might stay on for 30-45 seconds, then stops/pauses, then starts again. Eventually, it stays running.

I had the roof recently redone and did not personally confirm how the venting was handled.

This is a picture of the vent pipe. Does it look right?



Thank you.

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  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,833
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    Those tended to be not the best furnaces. The heat Exchangers tend to fail. My advice would be to find someone familiar with Miller trailer furnaces and have it checked over. If the furnace is early 90s you are due to replace it
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,166
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    30 years for a Miller is about right. It's time for a new one. That said, not everyone has the dollars to just put in a new furnace at the drop of a hat. As long as the heat exchanger is sound, you can keep it going. The 1980 models and before had a problem like you described. As I recall, the burner would operate and then if the fan failed to start, the auxiliary high limit would shut off the burner. Soon after that the fan limit control would click the fan on, that would cool off the auxiliary limit enough to bring the burner back on. then the cycle would finish normally.



    If you have this Fan limit control on your older Miller you may be able to adjust the fan on temperature or replace the control to solve the irregular start sequence. That may also make the fan blow cold air at the end or the cycle after the burner shuts off. That limit control when paired with the CMF 80 can be tricky to get it just right.


    Newer CFM 80 models use a fan timer and have eliminated the fan-limit control

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

  • josephny
    josephny Member Posts: 270
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    Took some pictures of the furnace this morning.

    It was clearly made after 1996 (sticker says it conforms to 1996 standard), but it sure looks a lot newer -- everything visible (without disassembling) is pretty clean.

    I don't see the fan limit control in the picture above.

    Here is what I have. I wonder if the fan cycling a bunch of times before staying on could be a problem with one of the limit switches (blue and red, I assume).

    Thank you!























  • HVACNUT
    HVACNUT Member Posts: 5,887
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    That furnace isn't too old. It shows the Beckett 7505B Genisys primary. 21st century anyway. I'm gonna say 2017 from the serial number. 

    When was the last time it was serviced? It should be serviced annually. I don't think the roof work had anything to do with it.
    You said "the system tried about a dozen times before it stayed on." Do you mean the fan, or the burner, or both?

    The Blue disc is the fan control. F115 -20F, fan comes on at 115°F, and off at 95°F.
    The Red disc is a limit. At 165°F, the burner will shut down. Fan will continue to run. 

    Does it have that plastic fiber air filter in the door that gets all greezy because the stove is 2 ft away? Measure it and get disposable 1 inch pleated filters. If it's an odd size, you might be able to find then online. 

    Is it burning #2 fuel oil or #1 Kerosene?
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,166
    edited June 2023
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    My guess is that the blue limit control is your problem. I would replace that one first to see if the solves your issue. https://www.supplyhouse.com/Nordyne-626552R-Fan-Limit-Switch-L115-20F. If the problem persists with a replacement fan switch, then your system has different issues that could be related to a very dirty fan or duct restrictions or any number of items. I'm going with the fan switch because it is low cost and easy.

    But a good cleaning will never hurt any mechanical device. Dirt is the biggest enemy to HVAC equipment. Eliminate dirt and keep it lubricated, they will last a long time.

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

  • josephny
    josephny Member Posts: 270
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    October, 2017?

    I had it looked at by a local guy but, despite trying repeatedly, I couldn't find out what he did or what he found.

    It seems like the burner is coming on and staying on just fine. It's burning #2. I dumped some cleaner in the tank (Hot Shot).

    It's the fan that cycles.

    I ordered both the blue and red switches and will install and report back.

    Thank you for explaining the what each is (and what it does: Now I know what a fan control switch does and what a limit switch does (yay!)).

    Any tips/suggestions on how to give it a good cleaning and lube? And how to check for duct restrictions?

    Thank you guys!
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,166
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    From what I can see Your Blower cabinet looks pretty clean. If it ever gets to look like the other three, it's time to pull it out of the furnace and take it apart to clean it



    Other oil burner maintenance items are better left to a professional. Although many of the Mobil Home furnaces do not lend themselves to combustion analyzer probes because the exhaust pipe is in the negative pressure zone of the duct blower inlet. I have used RTV cement with a 1/4-20 bolt to seal the test hole in some cases where the test was needed to verify the heat exchanger was not compromised or I needed to use a vacuum to remove copious amounts of soot. Most times they are best left to the experienced eye of an oil burner expert. Nozzle replacement, oil filter change, and combustion head cleaning is usually all part of a standard "tune-up" from your oil dealer or oil burner pro.

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?