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  1. #1
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    Jun 2012
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    Default HVAC options for addition

    building an addition on house and looking for HVAC options-suggestions. house has oil fired convectors for heat and central air but the addition will be independent of main systems. i thought i would use the mini splits but now am unsure.....

    addition is 20' x 40', it consists of 3 rooms, 1- living room 20' wide x 22' long (roughly 40ooobtu?), 2- bedroom 15' wide x 18' long (36ooobtu), 3- bathroom-closet 5' wide x 18' long (96ooobtu)

    thoughts?

  2. #2
    Join Date
    May 2011
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    Default Re: HVAC options for addition

    Good grief how did you come up with those BTU's? Yikes. I can heat my whole 3200sqft home and full basement with 80-90k BTU's.

    Depending on where you're located, 800 sqft shouldn't need more than 1.5 Tons cooling (18000BTU) and 40k BTU's or less heating.

    IF you have oil now and no natural gas, a conventional heat pump will be most economical. No need for a mini split. You probably only need a 1.5 or 2 ton heat pump for that space with maybe a set of 5KW heat strips for auxiliary heat. Smaller system will be quieter and need less ductwork and fewer supplies.

    Put the unit in the attic and make it a sealed attic with the roof deck spray foamed and use ice shield across the whole roof as a vapor barrier. or better yet use a steel roof and a radiant barrier on the roof deck.
    1925 Two-Story Stucco Beaux Arts Neoclassical

  3. #3
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    Jun 2012
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    Default Re: HVAC options for addition

    well i fell slightly better reading the btu's may be off.... i used an ****** btu calculator.
    20'x22' room catherdral cieling 12' tall. 44' exterior wall
    15'x18' room cathedral cieling 12' tall. 33' exterior wall
    5'x18' room. 23' exterior wall
    the building is conventially framed with joists not truss so there isnt room for ductwork.

  4. #4
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    Jul 2012
    Location
    LA, CA
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    4

    Smile Re: HVAC options for addition

    On the other hand, mini splits would be more economical, they have EER up to 20 versus window AC that have EER in the low teens. Why were you hesitating about mini splits. Let me guess, the upfront cost?

  5. #5
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    Jun 2012
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    Default Re: HVAC options for addition

    i havent been able to get a good read on the size units i would require......

  6. #6
    Join Date
    Dec 2007
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    Tennessee
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    Default Re: HVAC options for addition

    You can run ducts. You can run them under the slab, you can go for the industrial look with exposed ducts (ugh), or you could build a fake ridge beam down the length of the addition. I like the ridge beam myself because you could incorporate it into fake exposed 4x8 rafters. The fake ridge beam could be made from cabinet grade plywood or PVC lumber. Because it is inside the insulated envelope, it will not need insulation itself.

    There are even ultra light, fake foam beams in a 4x8 size on the market and these could be used to create the fake rafters and since they are hollow, they could duct the heat/AC from the ridge beam to the bathroom.

  7. #7
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    Jul 2010
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    SoCal
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    Default

    Quote Originally Posted by gwilkrrs View Post
    i havent been able to get a good read on the size units i would require......
    I don't know why you're still agonizing over "what size".
    Motoguy128 gave you the answer yesterday.

    Quote Originally Posted by gwilkrrs View Post
    i havent been able to get a good read on the size units i would require......
    i don't know why you're still agonizing over "what size unit"...Motoguy128 gave you the sizes yeaterday, in his reply.

    Quote Originally Posted by gwilkrrs View Post
    i havent been able to get a good read on the size units i would require......
    I don't know why you're still agonizing over "what size" you need. Motoguy128 gave you the answer.

    Quote Originally Posted by gwilkrrs View Post
    i havent been able to get a good read on the size units i would require......
    What size you need?

    Motoguy128 gave you the answer yesterday.

    Quote Originally Posted by gwilkrrs View Post
    i havent been able to get a good read on the size units i would require......
    i don't know why you are still agonizing over the size of the unit you need.
    Motoguy128 gave you the answer.

    Quote Originally Posted by gwilkrrs View Post
    i havent been able to get a good read on the size units i would require......
    Motoguy128 just told you the unit size you need, based on the info you supplied.

    gwilkerrs,

    Motoguy just told you the unit size you need, based on the info you gave.
    Last edited by JLMCDANIEL; 07-31-2012 at 11:27 AM.

  8. #8
    Join Date
    Jun 2012
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    81

    Default Re: HVAC options for addition

    my slab is prepped already- duct in slab is out.
    the fake ridge idea is a good one but i dont know if i could sell the look to her- and thats a lot of fake beams to incorporate into the project..... i will look for them premade as you say....... i will look into this option as sized above- thanks.
    i thought and was told that the mini split was my best option while i was drwaing this up originally. i am now having a hard time sizing mini splits......

  9. #9
    Join Date
    Jul 2012
    Location
    LA, CA
    Posts
    4

    Cool Re: HVAC options for addition

    I don't see why mini splits would be sized any differently than any other air conditioner.

    That is, if under "size" we mean "BTU/hr", right?

    You could use a single outside compressor unit and 3-4 inside units with some models of mini split systems. One inside unit per room.

    This site might be useful to find different types of mini splits

    splitunit.org/mini_split_unit.html

  10. #10
    Join Date
    Dec 2007
    Location
    Tennessee
    Posts
    1,381

    Default Re: HVAC options for addition

    To save money on the fake rafters, you can space them on 48" to 60" centers because they are not structural. To give you an idea of the look that I am thinking about, I had a house in Virginia Beach that used a 6x12 ridge beam with 3x8 rafters on 32" centers sitting on top of the ridge beam and 2' wide composite roof panels on top of the rafters, tar and rock on top of the composite panels. The ridge beam and rafters were structural in this case. While it had cathedral ceilings and clestory windows, the roof pitch was very shallow. The ridge ran across the short dimension of the house so the pitch was about 2/12, otherwise the house would have been ridiculously tall.

    But this is your addition. If you do choose to do a fake ridge beam, it probably would be wise to run it all the way up to the ceiling so that you could suspend it from the actual rafters, otherwise you would have to have something inside the box to support its length. The fake rafters would also set into the beam and that would help if you want to use them as ducts too.

    I had another house once where the contractor built a false ceiling in the main hallway that was 6" lower than the actual ceiling. Since this hall shared a wall with every room in the house, he cut vents into the cavity formed between the ceilings and that was the ducts for the house.
    Last edited by keith3267; 07-31-2012 at 02:08 PM.

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