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  1. #1
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    Furnace now, AC later - Sizing help

    Hi everyone, I'm new to posting but have been reading for quite some time.

    My question: I am about to do a home addition and go from a 1,000 sq/ft to an 1,800 sq/ft house (out not up). I already have radiator heat in the existing 1,000 sq/ft house that I would like to keep using and only use the new furnace to heat the addition. However, I plan to have ductwork installed in the entire house in the future for Central AC. What size Furnace do I need to accommodate this?

    For 1800sq/ft, the online calculators I am using are telling me I need about a 75,000 btu unit. But I will really only be heating 800 sq/ft. However, when I upgrade in the future, I am guessing I will need a furnace that can accommodate about a 3 ton AC unit?

    Any help would be great. The furnace needs to be 95+% efficient for rebates. I would like it to be reliable and two-stage but I don't need top of the line.

    Thanks so much everyone.

  2. #2
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    you could find a contractor who offers zoning. he could set it up to cool the entire house but only heat the addition.

  3. #3
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    I was hoping to manually shut off the vents to the rest of the house and avoid needing to setup zoning because I have heard it is expensive. Is that true?

  4. #4
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  5. #5
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    Quote Originally Posted by beshvac View Post
    I don't follow?

  6. #6
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    That was my....lets wait and see who responds to this thread.....

    But since you asked:

    1. You cant get something for nothing. Hvac equipment needs to run and be distributed properly.
    2. You gave just an impossible idea. I can't put a furnace on an addition and hookup an AC on it to cool the whole house.

  7. #7
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    Do you care to share the reasons WHY you couldn't run ductwork for the entire house, then manually restrict airflow to the existing house during the winter, and then release that restriction in the Summer?

    Would love the help.

  8. #8
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  9. #9
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    pecmsg - Thanks so much for that doc. I REALLY appreciate the help. I am just a regular guy trying to install a system that will adequately help keep my family warm without having to pay a ton cause I don't have two nickels to rub together.

    1. If you found yourself in my situation, what would you do? If you bought a 75k btu unit and put a piece of sheet metal to slip in place for the return and the supply lines and made sure they were taped, what would the impact be? Would it work or would the unit cycle constantly? (I get that this isn't something a professional might do, but would it work?)

    2. Is there a way to UNDER-buy the furnace and still be able to have a 3+ ton A/C unit later?

    My father, a retired Marine and former HVAC installer in the 1970's, has made some of these suggestions to me and I am hoping to find a way to implement them, or implement similar things that are updated based on newer technology.

    I appreciate everyone's help.
    Last edited by ericjdaniels; 11-06-2014 at 03:37 PM. Reason: Poor Grammar

  10. #10
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    I apologize, I just noticed this is not a DIY section of the forum. Is there a place where this question would be more appropriate? Sorry about that.

  11. #11
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    Our site has no DIY section.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  12. #12
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    That's unfortunate. I am grateful for any help anyone else is willing to give. I will refrain from posting this type of question here in the future.

  13. #13
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    If you blocked off airflow, you'd make quick work of the heat exchanger in the winter due to overheating. You'd also be constantly freezing the air conditioner up.
    What your not understanding is airflow. You need a certain amount of air to get equipment to run properly. To put a tiny furnace in with a larger sized air conditioner would result in a giant mess.
    Either leave your current system be and install a mini split for the addition, rip everything out after the addition is built and install one system for the house, or put a zoned system in now with the ability to add the addition on later.
    And im asking you this now, do you plan on this being a DIY project?

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