Results 1 to 7 of 7
  1. #1

    Confused Retrofit 2 floor AC install design best practice

    I have a 2,100sq ft colonial in Massachusetts built in 1985. It has a full basement that is partially finished, open floor plan on 1st floor, 4br & 2bth on 2nd, and spacious attic with fiberglass insulation on the attic floor. I have baseboard heat and no duct work.

    I am looking to have central AC installed and have gotten contractors telling me different things on how the job should get done:

    Option 1) 5 ton (possibly 2 stage) attic unit with ceiling registers in each 2nd floor bedroom, return in 2nd floor hallway, then 2-3 runs down through closets to ceiling registers on the first floor. 1st and 2nd floors as separate zones.

    Option 2) 2.5 ton attic unit with ceiling registers in each 2nd floor bedroom, return in 2nd floor hallway; And 2.5 ton basement unit with runs to floor registers in each first floor room. Not sure where the return would be - I guess another floor register

    What makes more sense from a comfort and air flow perspective?

    I thought that having ceiling registers for AC would be preferable to floor registers as cool air falls. I'm not too fond of floor registers as you can drop things in them, they limit where you can put furniture and shouldn't be walked on. I suspect having 2 units would be more expensive, but might provide less cycling than one larger unit and therefor more comfort and efficiency? I would rather have the right air flow, temperature, and humidity, and cost and wall/floor repair are secondary.

    All feedback appreciated.

  2. #2
    Join Date
    Nov 2006
    Location
    Athens, Ohio
    Posts
    1,372
    The first matter should be a heat load calculation. Without it you (and the contractors) have no way of knowing if 5 tons is the right size.

    Only 2 or 3 ducts run to the first floor cannot deliver 2.5 tons of cooling, provided you really need that much cooling.
    Of the choice you offer, I would favor 2 systems.

    Have you considered ductless heat pumps such as Mitsubishi? They adjust their capacity to the demand, humidify better than a ducted system and are very efficient. They are also in effect zoned because each indoor unit has its own controller.

  3. #3
    heat load was calculated at 57,980 BTU for the house and I asked for a little extra since we are planning a 20% addition to the house.

    Wouldn't the size of the ducts make a difference? if there where 2-3x 8" lines?

    I am familiar with ductless, but think it would equate to the same as running 2-3 larger lines to the first floor w/o the hotel look of the ductless units. I just really don't like the look which is why I'm left with the 2 options I listed.

    additional thoughts appreciated.

  4. #4
    Join Date
    Dec 2010
    Location
    The Quad-Cities area (midwest).
    Posts
    2,479
    A 2.5 ton A/C with the AH (air handler) in the attic for the entire house should do the trick. I've done a lot of homes like yours and it works extremely well.

    As Kdean mentioned, and if you or your wife don't mind air handlers on the wall in every room, the mini-split systems work well. I just can't find many wives that like the look.

    P.S. Five (5) tons of A/C for 2,100 sq. ft. 2-story is ridiculous.

  5. #5
    Join Date
    Dec 2010
    Location
    The Quad-Cities area (midwest).
    Posts
    2,479
    Quote Originally Posted by pblumenthal View Post
    heat load was calculated at 57,980 BTU for the house and I asked for a little extra since we are planning a 20% addition to the house.

    Wouldn't the size of the ducts make a difference? if there where 2-3x 8" lines?

    I am familiar with ductless, but think it would equate to the same as running 2-3 larger lines to the first floor w/o the hotel look of the ductless units. I just really don't like the look which is why I'm left with the 2 options I listed.

    additional thoughts appreciated.
    That "load" number might be what it takes to heat the home. I can not imagine it takes that to cool a home built in 1985. They must have used some insulation when they built the house.

    I cool my 2,250 sq. ft., 1946 Colonial home (no sidewall insulation and single pane windows) with a 2.5 ton when it's 100 degrees outside.

  6. #6
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    5,987
    Size for heating ( ~ 36,000 BTU/HR) and use a Heat Pump with HSPF > 9.5 and electric strip for Aux Heat
    Operating cost < $1,500

    At $0.15 / kW, I suspect you may be using about $3,400 for electric baseboard*
    You may reduce operating cost by > 50%.
    ______________ _________________
    If oil-fire boiler is used for hot water baseboard heat, I would guess > $2,600 at $3.70/ gallon.

    Two 2-ton 2-stage Heat Pumps or use a single unit inverter driven heat pump.
    What are the incentive programs (rebates from various, multiple sources) in your locale?


    http://www.hvacopcost.com/

    http://www.bls.gov/ro1/cpibosap.pdf
    Last edited by dan sw fl; 10-29-2013 at 05:53 AM.
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  7. #7
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    5,987
    Quote Originally Posted by pblumenthal View Post

    heat load was calculated at 57,980 BTU for the house
    and I asked for a little extra since we are planning a 20% addition to the house.

    additional thoughts appreciated.
    Detailed review of heat source operating cost will reveal that 58,000 is no where close to the actual heat loss / gain.

    What are the utility costs over the last 24 months (EACH month)?
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event