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What flue pipe to use for oil furnace?


Summary of questions is at the bottom. The rest puts everything in context.

I currently have an L.P. Furnace (Miller M1MB 056A BW) but I have an abundance of heating oil and want to install a used oil furnace (Miller cmf 80 po conv) next to my existing unit. I understand this is strange but I want to keep my existing unit because it houses the coils for the AC and I want the option to switch back to L.P. in a couple years when I run out of oil. 

I’m not sure what type of flue pipe to use. I’ve read that single wall (C vent) is commonly used for oil furnaces to connect to a Masonary chimney but requires a clearance of combustibles of 3x the diameter of the pipe. So 15” of clearance for a 5” pipe. But then I saw something else that said 6” clearance is required for C vent which can be reduced to 2” with the use of 28 gauge steel between the vent and combustibles. However I don’t have a masonry chimney. So I’m not sure if C vent can still be used. 

I also read that double wall (B vent) can’t be used but double wall (L vent) can. This doesn’t make much sense to me but I’m sure there’s a reason. I just need to know what to use or what my options are, and what the clearance to combustibles needs to be.  

I’m installing the furnace in a 7 year old double wide so the pipe would be leaving the top of the furnace, traveling straight up about 2’ to the drywall ceiling, then about 6” through an insulated space, than through the roof. No 90°s or 45°s will be needed but I understand there will need to be a damper just above the furnace.

I believe I also read that the pipe should be a minimum of 5’ tall above the flue collar but 10’ is recommended for better flow. 5’ would put it right where it needs to be on my roof (2’ or so above the shingles) but 10’ would look ridiculous on my roof and I would like to avoid that if possible. 

 Please don’t tell me to hire a professional. I’m not the homeowner who fixes things with duct tape and flex seal, I’m an electrician, I’m very good with my hands and I’m a bit of a perfectionist. I will be the one doing the work no matter any response suggesting otherwise. I’m more than capable of doing the work, I’m just not familiar with the materials and I understand the importance of doing it right. 

There is limited information online and the information I do find is very vague and inconsistent or it just says “this should be done by a licensed professional”. I’m doing this to save money. The cost of a professional and the materials combined might not be  worth the savings since I don’t know how much longer I’m going to be getting free heating oil.

I only know one HVAC guy I could ask but he’s kind of **** and I wouldn’t trust him to change a flat tire let alone give me advice on how to install something that could burn my house down if done wrong. 

If you think I might be over looking something, feel free to mention it. 

Thank you 

Summery of questions

What flue pipe to use for an oil furnace without a masonry chimney?

What clearance should the pipe be from combustibles? (I understand code might change from state to state but your answer will at least give me an idea of what’s safe, which is what I’m most concerned about)

How tall should the vent pipe be above the flue collar for proper exhaust flow?


  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,834
    edited October 2022
    Mobile home furnaces have different rules for installation in manufactured housing. The M1MB 056A BW venting requires a pipe with in a pipe "roof jack". Roof Jack is what Miller calls their chimney vent system.

    The air for combustion enters thru the outer pipe and the exhaust vents out the center pipe. Everything from the top

    The CMF 80 uses a different roof jack because the combustion air comes from below the manufactured home. Notice the single pipe vent on this furnace illustration.

    And if you do not want to change the roof jack, the M1 furnaces all have 5" exhaust pipes and the CMF use 4" exhaust.

    When you change out the furnace, there will be no opening in the floor for combustion air for the burner. You will need to purchase the floor base, attach it to the existing duct work under the floor and then install a sleeve to the crawlspace for combustion air. This is a quote from the installation manual of the CMF80.

    "The installation of the furnace must allow for an adequate supply of combustion air.combustion air opening of the furnace must be designed and located to prevent blockage by snow.When drawing the combustion air from underneath the home, ensure that a vent or duct of at least 18 square inches of free area is provided from outside. Check to ensure that the combustion air opening is unobstructed. When Using the combustion air duct, ensure that it extends through the floor. When using the direct vent kit, the combustion air opening must be located in the same pressure zone as the flu exit of the roof jack or chimney. Refer to the instructions provided with the direct vent kit for more information. "

    It can be done, but you need to understand how these systems operate. You can't just swap out these different furnaces as if they are the same.

    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
    rick in Alaska
  • vtfarmer
    vtfarmer Member Posts: 101
    Don't forget to put a backdraft damper under both furnaces - I learned this when helping my father in law parallel a wood furnace with his existing oil one, you need to prevent the hot air coming out of the furnace that's currently running from short circuiting and going backwards through the furnace that's off then back into the intake of the operating unit. You have a MH setup so you probably don't have ducted returns, but the same principle applies.
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,834
    @vtfarmer has a good point. Although you are probably not going to install both furnaces next to each other. If your home is equipped with a packaged AC system connected to the same ductwork, you will want to read this from the IO manual and purchase the damper with the appropriate floor base.

    Damper Installation
    An automatic shut-off damper is available (see
    replacement parts list). An automated shut off
    damper is required when the home is air conditioned by a self-contained unit. This damper
    is designed to fit in the feeder duct cavity,
    directly under the furnace. A damper is required
    to prevent chilled air from flowing over the
    furnace heat exchanger. For proper installation, refer to the instructions provided with the

    This is the part number. 902095 Automatic Shut Off Damper**

    Here is an illustration of the CMF 80 that shows the duct connection and combustion air provisions.

    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • Bob Harper
    Bob Harper Member Posts: 1,035
    You can use type 'L' vent or class A chimney for oil. Observe the mfrs. installation instructions including vent height but above anticipated snow level. Most L vent 3" clearance to combustibles. Will need a ceiling support/ firestop/ attic insulation shield, roof flashing, storm collar, rain cap and the pipe. Use a "draft hood connector" even on L vent to connect to the appliance collar. Clearances on single walled pipe start at 18" for unlisted unit/ 9" for listed. The gauge pipe is determined by the diameter but starts at 26 and goes down in number, which is thicker the lower number. You can't use single walled connector in a mobile home- listed venting all the way. Unit must be electrically bonded to the frame.
    BW and B vent are for CAT I gas only.
  • JohnWhite6607
    JohnWhite6607 Member Posts: 16

    Thank you. This has been extremely helpful. So far you’ve given me more information than I could find anywhere else and now I know what parts I’ll need to order for the flue and the base. I’ve spent the weekend researching everything about these units but I’m not clear on the dampers 

    The manual for the oil heater says that if there’s a self contained AC unit; I need to install an 18” automatic shut off damper in the feeder vent at the base of the unit. 

    They say it’s to prevent chilled air from flowing over the furnace heat exchanger. 

    My AC coils are in the gas unit which is connect to the same ductwork the oil furnace will be connected to. 

    (Just a reminder, this is not a replacement furnace. Both units will be hooked up next to each other and operational. But not at the same time)

    Although my unit isn’t a self contained unit (or packaged unit), I think that in this case; it would be doing the same thing as a SCU (causing backdraft into the oil furnace when AC is on)

    I don’t understand why it would matter if cool air flows over the heat exchanger in the oil unit in the summer. But I do understand that you don’t want hot air going through the unit that’s off and short circulating back into the return of the unit that’s on. 

    I’m having trouble finding the damper for the oil unit online for a reasonable price and from a trusted source (I plan on calling the HVAC supply company in my area tomorrow) and i really don’t want to remove my existing gas unit to install a damper unless it’s absolutely necessary.

    If all I need to do is prevent air from short circulating from one unit to the other; could I just replace the air filter in the unit that’s not being used with sheet metal and seal it with foil tape? I think this would prevent air from short circuiting but I understand it won’t prevent cool air from flowing over the heat exchanger in the oil furnace in the summer and it won’t prevent warm air from flowing through the AC coils in the gas furnace In the winter. Don’t know if that would be an issue or not.

    I would never do this if I was installing it for someone else in fear they would forget to remove the sheet metal or try to run both units at the same time or something but once again, this is temporary. Probably only a couple years, 4 at the most. Right now I have around 1000 gallons of stabilized fuel from people who switched over to heat pumps and wanted their tank removed. So whenever people stop doing that, I’ll switch back over to gas or install a wood furnace or something. Maybe a wast oil furnace, but I’m not sure if they’re still legal. I was doing some work for a service shop that told me they had to get rid of there’s because of emissions.

    All I know is that the current cost of energy right now is ridiculous and I’m going to try to lesson the financial burden anyway I can. 

    Thank you for your responses so far. They’ve been extremely helpful. 

    Summary of questions 

    Why don’t you want cool air to flow over the heat exchanger in the oil furnace in the summer when the unit would be off?

    Can I replace the filter with sheet metal (in the unit that would be off) to prevent short circulation back into the unit that’s on?

    Also do I not need a barometric damper on this furnace in a mobile home as you would on a typical residential furnace in a house? 

  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,834
    edited October 2022

    1. Cool air flowing thru the ductwork will take the path of least resistance. Since the CFM 80 is a larger opening close to the M1 furnace, much of the air conditioning will flow up thru the supply of the CFM 80 and short circuit out thru the return.
    2. Condensation may form on the heat exchanger of the CFM 80 during the summer months. This will allow for premature failure of the heat exchanger. Get the damper ordered and installed before you put the CMF80 on the floor.

    B )
    In order to not have the heat from the CMF 80 do the short circuit back thru the M1 furnace/AC unit, You can cut a sheet of metal and lift the blower on the M1 just enough to slide it in place. This will block the short circuit path thru the M1 furnace.

    1. You CAN NOT use a barometric draft control!!!!

    2. The blower inlet is in the same room as the vent pipe and will suck byproducts of combustion thru that opening and distribute them into the duct work. This might make you a statistic that you don't want to be.

    I must say that your idea of 2 furnaces in a manufactured home or a mobile home leads me to wonder how much room there is in that home? Every miller furnace I ever worked on was in a pretty cramped space.

    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • JohnWhite6607
    JohnWhite6607 Member Posts: 16

    Got it. Didn’t think about condensation forming on the heat exchanger and the absence of a barometric draft control in mobile homes is very clear to me now. I’ll call Ferguson HVAC supply tomorrow for the damper and the other materials. 

    As for the space. I’m fortunate to have a broom closet right next to the furnace closet which runs parallel with the ductwork under the floor. I’m going to combine the two closets and put in fully louvered double doors.  I’ll loose the broom closet but I replaced my hot water heater with a tankless when I moved in and turned that area in to a closet as well, so I’ll still have a place to put the items that are currently in the broom closet. I don’t have much around the house anyway. Most of my things are in the out buildings or at the farm. 

  • JohnWhite6607
    JohnWhite6607 Member Posts: 16
    Ferguson HVAC doesn’t carry what I need and RE Michael company won’t help me without an account and HVAC license. Does anyone know a way around this or know any online distributors I could use? Im only interested in what the parts are going to cost at the moment. 
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,834
    Johnstone supply usually carries Miller Parts

    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • JohnWhite6607
    JohnWhite6607 Member Posts: 16
    Johnstone supply is willing to sell me the parts. Thank you. 
    I’m going to start a new discussion about patching a cracked heat exchanger being that I’ve strayed away from the this discussions title