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Gas furnace with 3" exhaust vent

neil74 Member Posts: 4
I am replacing my central A/C and heating. I have a gas furnace with a 3" exhaust vent, it is a very old unit. I am being told that all new units have 4" vents and changing my vent all the way out the roof requires a lot of tearing out sheetrock and replacing it. I can go with electric heat but I do not have the correct wire and it would cost $1,200 to get the correct size wire. Does anyone know of a manufacturer that still makes a gas heating unit with a 3" vent? My A/C man says he has looked everywhere and cannot find one. Just checking before I spent money on the sheetrock or running the wire needed. I am in USA, Texas. Thanks


  • STEVEusaPA
    STEVEusaPA Member Posts: 5,966
    edited June 2020
    Might need another contractor who has installed higher efficiency units.
    Which unit are they recommending? Many of the 92%+ (95, 97) gas units are 3", I recently put one in my rental property. Many vent 2".
    Click on any of the gas furnaces with 92, 95, or 96 in the model #.
    EBEBRATT-Ed Member Posts: 13,070

    I am guessing you have a a B vent system with a metal vent. Possibly you could uses a high efficiency unit as @STEVEusaPA mentioned and vent it through the side wall.

    What size is your old furnace (btu input)? Probably not to large in Texas

  • neil74
    neil74 Member Posts: 4
    Thanks Steve and Ebebratt. I am not a A/C person, so not sure what I am talking about, just trying to get a new unit in without spending a fortune. I live in Galveston, so seldom gets to freezing here. I don't need a lot of heat, this past winter, I think I turned it on maybe twice, just to get the chill out of the air. The Unit I have now is a old Lennox Conservator III G16, it says 4/86, not sure if that is right and says 75,000 BTU, sounds like a lot. My A/C person did say the current A/C is 4 ton and I only need 3 tons. It is cooling and heating only about 1,300 sq ft. Since I am on the Coast, the houses here are on pilings and this unit is on the main level (2nd floor ) and is in a closet, not in attic. I forgot to mention that before and that may limit what I can put in. The air handler is stacked on top of the furnace. The contractor has been looking at American Standard, Carrier and Heil. He says none of them make a furnace, like I need, with a 3" vent. Since this unit is in a closet, the vent does not go straight out the roof, it has to zigzag its way up to the third level and then out the roof. It is located in the interior of the house, so cant vent thru a outside wall and it is a metal vent, not sure what a B vent system is. Maybe the contractor needs to look at other manufacturers, but seems to only handle certain brands. I have had 3 estimates and they all have told me the same thing, but I am stubborn and was sure I could find a unit with 3" vent. Maybe not in the stacked system that I need. I appreciate any and all help and will keep looking at other units, since I know more of what to look for now. Thanks again.
  • Ironman
    Ironman Member Posts: 7,059
    How about posting some pics of the system?
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
    JUGHNE Member Posts: 10,278
    What do you have for hot water heater?

    You seem to be in heat pump climate.
    The major expense for the electrical is the 240 circuit for the heat strips. You may not even need the heater strips if the heat pump preforms well. It will blow cool air during defrost.....and you have no back up heat if HP fails. But it sounds like your house would not be in danger of freeze up.

    Usually HP air handlers are such that heater strips can be added later.
    The existing wire for the 120 circuit can easily be switched to 240 for an air handler with out electric strip heat.
  • HomerJSmith
    HomerJSmith Member Posts: 1,914
    edited June 2020
    As jughne mentions--what about the water heater is there any common venting of the W/H and furnace?

    Abandon the furnace b-vent and go to a sidewall vent with either a power vent like a Tjernlund or go with a furnace that allows side wall venting.

    Newer Category IV furnaces use a plastic vent for combustion gases and combustion air. With condensing furnaces, you have to deal with the condensate from the furnace, but you already are dealing with the water from the A/C evaporator.
    JUGHNE Member Posts: 10,278
    OP mentioned this is in an interior closet with no side wall available. Good chance he has an electric WH.
  • neil74
    neil74 Member Posts: 4
    Yes, I do have a electric water heater. After considering all the costs, I think I will run the electric wire needed and go with a electric furnace. Since it very seldom gets to freezing here, I only turn the heat on a few times during the winter, so not concerned about the costs. Thanks for everyone's input.
    JUGHNE Member Posts: 10,278
    Does anyone in your area use heat pumps?
    If you heat only twice a year the elements might be the way to go. That way you would burn the dust off them at least.
  • pecmsg
    pecmsg Member Posts: 3,526
    75K in Galveston Tx is Insane!

    Heat pump with electric back up
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 4,888
    edited June 2020
    Hay @neil74
    The new Mitsubishi mini splits have excellent heating performance and now there is a Unitary Air Handler available. To translate into NON-Tech talk, The air handler is something that can replace the furnace and can be connected to your existing ductwork. The outdoor unit is the variable speed very efficient Inverter type.


    Should be able to use the existing wiring
    Edward Young Retired HVAC Contractor & HYDRONICIAN Services first oil burner at age 16 P/T trainer for EH-CC.org
  • neil74
    neil74 Member Posts: 4
    Thanks, will look into the mitsubishi units. They may be more expensive than running the wire I need for a traditional electric heating unit, but will have contractor check it out. Also, have had him check into a Heat pump . He has only installed one in last several years, says it has worked great. But, if I need back up heat for one, I might as well go with a regular A/C unit with electric heat. Thanks again everyone.