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What size furnace pipe/

mgmine Member Posts: 58
I'm extending the back of my house out 14 feet and need to disconnect the original 4" pipe. Can I simply add a few extra lengths 4" pipe or do I have to use a larger diameter pipe? Currently, the pipe is coming about 1 foot off the main rectangular run and then takes a 90-degree turn to the floor above.


  • GroundUp
    GroundUp Member Posts: 1,649
    You're going to need a whole lot more information than that.
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 6,019
    edited June 2021
    What @GroundUp said

    Here is one example why.
    Here are 2 different 14" additions on a home with a 4" extension to the old duct The 14'x 28' room is too large for only one 4" supply duct. That room would need a Manual J load calculation and the entire system would need a Manual D duct design to calculate the new trunk duct and branch duct sizes.

    A 6' wide master bathroom might be OK with a 4" extension but it may not because there are 28 more feet of the outside wall where the cold weather can infiltrate, and conduct thru to the conditioned space. So even that small room is only a Maybe!

    Consider the size of the room that the 4" duct was designed to do. then consider the size of the new space added to the size of the room you are taking the 4" duct from. that space still needs heat also. Add the two spaces together and you will plainly see that just extending an existing duct won't do it.

    It is like trying to fill 2 buckets of water with the same hose. It is going to take longer and when the other buckets (rooms in your house) are full, all the hoses will shut off which leaves the two buckets (the addition and the room it is attached to) somewhere less than full. that equals a cold room.

    Mr. Ed.

    PS recommend you have a pro look at it.

    I have a brother-in-law who is a builder and he has done a lot of additions over his 45+ years of construction... he still doesn't get it! The pro you want is an HVAC pro. Not your builder.
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
    EBEBRATT-Ed Member Posts: 13,750

    Any addition will likely have 3 exposed walls. You need to calculate the heat loss. Trying to balance the air flow to this addition and keep it the same temperature as the rest of the house wil be a problem.

    I would do the heat loss and run larger duct (you can't get much air through a 4") or multiple smaller ducts.

    And then be prepared to add some electric baseboard or some other supplimental heat
  • pecmsg
    pecmsg Member Posts: 4,169
    4" Vent pipe of Duct Work?
  • mgmine
    mgmine Member Posts: 58
    GroundUp said:

    You're going to need a whole lot more information than that.

    You're pretty close on the size. Here is an update of the question

    I'm extending the back of my house out 14 feet (the current room is 12x23 and has 3 registers) it's a one-story open concept house. I need to disconnect two of the original three registers and 4" pipes. Can I simply add a few extra lengths 4" pipe or do I have to use a larger diameter pipe? Currently, the pipes come about 1 foot off the main trunk line and then take a 90-degree turn to the floor above The new space added will be 14x23 making it a 26x23 foot room. The back wall will be all glass. It will have 2x6 construction and meets the building energy code requirements. Once the floor is down it will be impossible to add or change the piping so I only one chance to get it right. Anything else to consider such as pipe insulation or dampers?
  • STEVEusaPA
    STEVEusaPA Member Posts: 6,227
    You're going to have a velocity issue, and probably hear the air coming out of the vents. You should have a pro do some simple calculations, and either provide a new take off of the right dimension or extend the original trunk to make that connection. Otherwise some hack will recommend a duct booster fan.
    EBEBRATT-Ed Member Posts: 13,750

    As I said before it's not going to work with 4" duct.

    I know you don't wan't to here that but you need a lot more air out there than you thing. Especially with nearly all glass wall.

    Have someone size it and put in what they think you need.

    With a room that size I would recommend larger duct work and probably a ductless split or some electric baseboard to act as a booster.

    If you don't your not going to like the end result
    JUGHNE Member Posts: 10,777
    In today's houses, I have seen 4" round pipe feeding a 1/2 bath or small laundry room, usually without windows. Little AC cooling out of these vents.

    FWIW, Way back in the 50/60's Lennox had a system that used 4" (sometimes 4 1/2"--strange) that might go nearly 20' per run. No extended plenum, all connected at the furnace. We had one in our house with a total of 6 runs.
    Ran hot, you could burn your foot on the register. Furnace ran much hotter than today's furnace.

    AC was never considered with this set up, if tried it was always a disappointment.

    I think it was a short lived experiment by Lennox.

    I salvaged all the 4" for dryer venting.